HomeMy WebLinkAbout20131126-SBPF emergency certification appl to ConCom_201404041218344869PROJECT SITE VICINITY MAP:PREPARED BY:LOCATION MAP:BAXTER ROAD TEMPORARY STABILIZATIONNOI SUBMISSIONBAXTER ROADNANTUCKET, MASSACHUSETTSPREPARED FOR:TOWN OF NANTUCKETNANTUCKET, MASSACHUSETTSNOT TO SCALENOT TO SCALEOne Financial Plaza1350 Main Street, Suite 1012Springfield, Massachusetts 01103(413) 241-6920 Fax (413) 241-6911www.miloneandmacbroom.comLIST OF DRAWINGSSHEET NO.TITLE1TITLE SHEET2EXISTING CONDITIONS3GENERAL PLAN4TYPICAL CROSS SECTION5FLANKING DETAILDESIGNER:DATE:P.E. NO.:BY:MILONE & MACBROOM, INC.PROJECT SITEMilone & MacBroom, Inc. - 2013PROJECTSITEPROJECT SITEOctober 25, 2013REVISED: NOVEMBER 5, 2013James MacbroomNovember 5, 2013430526CONSTRUCTION STAGING AREA7-11CROSS SECTIONS
SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 20132967-111" = 200'EXISTING CONDITIONS - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'100'200'01/2"1"EXISTSHEET NO.2 OF 11
15+0016+0017+0018+0019+0020+0021+0022+0023+0024+0025+0026+0027+0028+0029+0030+0031+0032+0033+0034+0035+0036+0037+0038+0039+0040+0041+0042+0043+0044+0044+08.52SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 20132967-111" = 200'GENERAL PLAN - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATIONGENSHEET NO.3 OF 110'100'200'01/2"1"
SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 20132967-11HOR: 1"=20'VERT: 1"=20'TYPICAL CROSS SECTION - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'10'20'01/2"1"TYPSHEET NO.4 OF 11
SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 20132967-11HOR: 1"=20'VERT: 1"=20'TYPICAL FLANKING DETAIL - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'10' 20'0 1/2"1"TYPSHEET NO.5 OF 1111/19/2013REVISED DETAIL
30+0031+0032+0033+0034+0035+00SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 20132967-111" = 40'STAGING PLAN - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATIONSTAGINGSHEET NO.6 OF 110'20'40'01/2"1"
23+00-1001020304050607080-10010203040506070800.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0024+00-1001020304050607080-10010203040506070800.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0025+00-1001020304050607080-10010203040506070800.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.00SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 2013HOR: 1"=40'VERT: 1"=40'CROSS SECTIONS - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'20'40'01/2"1"PROJECT NO.2967-11X-SECTION7 OF 11SHEET NO.
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29+00-1001020304050607080-10010203040506070800.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0030+00-100102030405060708090-1001020304050607080900.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0031+00-100102030405060708090-1001020304050607080900.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.00SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 2013HOR: 1"=40'VERT: 1"=40'CROSS SECTIONS - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'20'40'01/2"1"PROJECT NO.2967-11X-SECTION9 OF 11SHEET NO.
32+00-100102030405060708090-1001020304050607080900.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0033+00-100102030405060708090-1001020304050607080900.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0034+00-100102030405060708090-1001020304050607080900.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.00SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 2013HOR: 1"=40'VERT: 1"=40'CROSS SECTIONS - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'20'40'01/2"1"PROJECT NO.2967-11X-SECTION10 OF 11SHEET NO.
35+00-100102030405060708090100-1001020304050607080901000.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0036+00-100102030405060708090100-1001020304050607080901000.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.0037+00-100102030405060708090100-1001020304050607080901000.0025.0050.0075.00100.00125.00150.00175.00200.00225.00250.00275.00300.00325.00350.00375.00400.00425.00450.000.00SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 2013HOR: 1"=40'VERT: 1"=40'CROSS SECTIONS - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'20'40'01/2"1"PROJECT NO.2967-11X-SECTION11 OF 11SHEET NO.
M E M O R A N D U M
Date: November 25, 2013
To: Joshua Posner, President, Sconset Beach Preservation Fund
From: Maria Hartnett and Les Smith, Epsilon Associates, Inc.
Subject: Emergency Status for Homes and Public Infrastructure Along Baxter Road,
Nantucket, MA
This memo defines those properties within the “Baxter Road Temporary Stabilization” project area
(DEP File No. 048-2610) from 85-107A Baxter Road that require protection under an Emergency
Certification. This analysis is based upon existing distances from the top of the coastal bank to
homes and Baxter Road, the long-term erosion rate, and the maximum anticipated winter erosion
rate (based on actual top of bank loss during the 2012-2013 winter season).
Existing Conditions
Existing Conditions at Sconset are presented on Figures 1 and 2. Figure 1 is an oblique aerial photo
taken in June 2013; the distances presented on Figure 1 are based on May 30, 2013 field
measurements of the minimum distances between the top of the coastal bank and existing homes
and Baxter Road. Figure 2 is an aerial photo taken in July 2013 with transects spaced every 20 feet
that list the distance between the edge of Baxter Road and the top of the coastal bank. To develop
this figure, GIS was utilized to digitize the eastern edge of pavement for Baxter Road and the 2013
top of coastal bank line, and then to generate the 20 foot transects with listed distances. These
figures demonstrate the following:
o The homes at 93 and 97 Baxter Road are between 8 and 24 feet from the edge of the bluff.
o The distance from the edge of Baxter Road to the top of the bluff for the vacant lots at 91,
99, 101, and 105 Baxter Road is as little as 29 feet and averages approximately 50.6 feet
(Table 1). This distance gradually starts to increase south of 91 Baxter Road.
Potential Threat from Erosion
We have previously determined the long-term erosion rate for the area from 85-107A Baxter Road
as 4.6 feet/year, in a memo from Epsilon Associates dated November 1, 2013. Given the
significant bank losses that occurred during the winter of 2012-2013, we also recommend the
consideration of potential single-season coastal bank loss at Sconset when determining those
properties that require immediate protection.
As previously mentioned, the winter of 2012-2013 resulted in significant coastal bank erosion at
Sconset. An analysis of the erosion that occurred between 2012-2013 was conducted by digitizing
top of bank lines from 2012 and 2013 aerial photographs (Figure 3) and then calculating retreat
distances along shore-perpendicular transects spaced every 20 feet. This analysis indicates that the
average erosion in the area from 85-107A was 20 feet and ranged up to 40 feet (Table 2).
Criteria for Defining an “Emergency”
It is our opinion that the situation at Sconset constitutes an emergency, and that all properties and
public infrastructure that may be lost due to erosion during the next few winter months require
immediate protection. The above analysis of 2012-2013 erosion demonstrates that up to 40 feet of
coastal bank can be lost during a single winter; this distance represents the basis of the below
criteria for those homes and sections of roadway that require immediate protection.
o Emergency Criteria for Homes. All homes within 40 feet of the top of the coastal bank
require immediate protection.
o Emergency Criteria for Public Infrastructure (Baxter Road). All sections of Baxter Road
within 65 feet of the top of the coastal bank require immediate protection. This distance is
based upon the sum of the potential single-season erosion (40 feet) plus the minimum
distance of 25 feet that needs to be maintained seaward of the Baxter Road pavement for
structural stability. This 25 foot distance is based upon information presented in the
November 8, 2013 letter from Milone & MacBroom. After conferring with the well-
respected geotechnical firm Haley & Aldrich, Milone & MacBroom reported that “[t]he
town can maintain travel on Baxter Road until such time as the top of the bluff is 25 feet or
less from the edge of pavement. When the top of the bluff is within 25 feet of the pavement
edge, the road should be closed to traffic until a detailed assessment can be completed by a
geotechnical engineer.”
Utilizing the above criteria to define which portions of 85-107A Baxter Road require immediate
protection yields the following conclusions:
o Homes Requiring Immediate Protection. The pre-1978 homes located at 93 and 97 Baxter
Road require immediate protection.
o Sections of Baxter Road Requiring Immediate Protection. The sections of Baxter Road along
the southern two-thirds of 105 Baxter Road, 101 Baxter Road, 99 Baxter Road, and 91
Baxter Road require immediate protection. (In limited parts of 91 Baxter Road, the distance
between the edge of pavement and top of the bluff is just over 65 feet. We recommend
providing protection across 91 Baxter its entirety to avoid discontinuous protection and/or
end effects.) We also note that the adjacent lot at 87 Baxter Road has a small section where
the distance between the top of the bluff and edge of pavement are less than 65-feet.
Protection for 87 Baxter Road could also be provided if this can be accomplished without
compromising the ability to protect the more threatened areas from 91-105 Baxter Road
during the limited time available before the winter storm season. Finally, we note that
Baxter Road provides access to pre-1978 homes both on its seaward and landward sides,
and that protecting Baxter Road is critical to maintain access to the pre-1978 homes on the
landward side of Baxter Road adjacent to 91, 99, 101, and 105 Baxter Road.
Figure 1
Existing Conditions - 91 to 109 Baxter Road
Setbacks based on 5/31/2013 on-the-ground measurements and July 2013 aerial photo
Baxter Road and Sconset Bluff Storm Damage Prevention Project Nantucket, MA
Lot 99 Lot 101 Lot 105
44’-60’
road
setback
37-45’
road
setback
29’-50’
road
setback
Lot 93 Lot 97
Lot 91
Lot 107 Lot
107A
Lot 109 17’
shed
setback
24’
house
setback
8’
house
setback
Note: “Lot” label refers to street numbers. Photo Date: June 2013
102
679995911049999997697969695949391898710686103851031028382.6182.5982100817876757575
747373737271
70
69
68
68
67
6767
66
66
65
65
64
63
6363
62
6160
6058
57
57
5754
43
54
54
41
52
5050504646
4544
43
43
43
42
4241
4037
66
99
97
87
85
101
105
109
93
91
117
115
113
107
119
107A
G:\Projects\Lighthouse\2013\Geotube\Revised\transects_117-87.mxd
Figure 2Geotube Analysis
Baxter Road and Sconset Bluff Storm Damage Prevention Project Nantucket, Massachusetts
LEGEND
Basemap: 2013 Aerial Imagery, Col-East, Inc.
Edge of Pavement
2013 Top of Bank
Transects (feet)
Parcels
0 60 12030Feet1 inch = 120 feetScale1:1,440
BA
X
T
ER
RO
AD
BAXTER ROADSANKATY ROAD
85
99
97
87
101
83
105
109
93
91
107
81
107A
113
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP,swisstopo, and the GIS User Community
G:\Projects\Lighthouse\2013\ConCom\Retreat\Revised\Detailed_Analysis_v3\2012-2013.mxd
Baxter Road and Sconset Bluff Storm Damage Prevention Project Nantucket, MA
Coastal Bank Retreat
LEGEND
Basemap: 2013 Aerial Imagery, Col-East, Inc.
2012 Top of Coastal Bank
2013 Top of Coastal Bank
Parcel Boundary
°0 60 12030Feet1 inch = 120 feetScale1:1,440
Transect Lot Distance from EOP to 2013 TOB (ft)
37 105 63
38 105 50
39 105 46
40 105 43
41 105 41
42 105 42
43 101 41
44 101 42
45 101 43
46 101 43
47 101 45
48 101 44
49 101 40
50 101 37
51 101 43
52 101 46
53 99 54
54 99 52
55 99 50
56 99 50
57 99 54
58 99 57
59 99 54
60 99 57
61 99 60
75 91 67
76 91 69
77 91 63
78 91 57
79 91 67
Average 50.6
Table 1. Distance from Edge of Pavement to Top of Coastal Bank,
91, 99, 101, and 105 Baxter Road, Nantucket, MA
Table 1. 2012-2013 Retreat Distances for 85-107A Baxter Road, Nantucket, MA
Transect
ID
Street Number 2012-2013
Retreat
(ft)
30 107A 22.1
31 107A 22.6
32 107A 25.9
33 107 24.4
34 107 27.2
35 107 18.6
36 107 20.6
37 107 22.0
38 105 16.9
39 105 10.5
40 105 17.5
41 105 33.2
42 105 27.3
43 105 25.2
44 105 25.0
45 105 24.5
46 105 21.8
47 101 25.2
48 101 17.1
49 101 21.5
50 101 18.6
51 101 13.9
52 101 18.9
53 101 25.2
54 101 23.6
55 101 17.0
56 Public Access 18.9
57 99 16.6
58 99 23.6
59 99 22.4
60 99 26.4
61 99 19.1
62 99 17.2
63 99 19.6
64 99 15.3
65 99 22.5
66 97 23.5
67 97 20.5
68 97 19.0
69 97 25.0
70 97 27.4
71 97 22.4
72 97 23.5
Transect
ID
Street Number 2012-2013
Retreat
(ft)
73 97 26.8
74 93 25.6
75 93 11.5
76 93 8.1
77 93 16.5
78 93 20.0
79 91 16.4
80 91 6.1
81 91 7.5
82 91 20.5
83 87 13.2
84 87 22.8
85 87 22.1
86 87 27.2
87 87 40.1
88 87 38.2
89 87 18.7
90 87 11.4
91 85 18.2
92 85 11.2
93 85 11.4
94 85 15.4
95 85 5.8
96 85 21.5
97 85 17.4
98 85 17.3
99 85 13.1
100 85 16.2
101 85 23.7
102 85 25.7
103 85 24.9
Average Retreat 20.3
Maximum Retreat 40.1
M E M O R A N D U M
Date: November 26, 2013
To: Joshua Posner, President, Sconset Beach Preservation Fund
From: Maria Hartnett, Epsilon Associates, Inc.
Subject: Review of Ratner Geotextile Application
This memo presents a discussion of information obtained about the erosion control installation at
the Ratner property on Sheep Pond Road, Nantucket involving geotextile materials and compares
that installation with the proposed emergency geotextile tube project. This memo summarizes
information conveyed to Epsilon Associates by Jamie Feeley (Cottage & Castle) and Bill Smallwood
(a professional engineer at Flint Industries), both of whom have knowledge of the site and/or of the
materials utilized.
Summary of Ratner Installation
The Ratner installation involved small-sized geotextile bags referred to as “coastal sand bags”. One
of these bags was recovered and retained by Peter Kaiser, and was measured by Jamie Feeley on
11/25/2013 to be 14 feet by 14 feet in size (see following photos). It is uncertain if the dimensions
of these bags were consistent with the permitted dimensions. Anecdotal evidence suggests that the
installation of these bags was not performed according to the permitted specifications. Bags were
observed being filled with sand by shovel and/or being filled with concrete. Bags did not appear to
be filled to their design capacity. Additionally, the seams of the bags do not appear to have been
mechanically sewn. The seams appear to have been hand-sewn (see following photo); such hand-
stitching would leave the seams vulnerable to failure.
The Ratner project did not appear to include any type of scour protection, such as installation of toe
protection or a scour apron and/or anchor tube to prevent undermining. Similarly, there appeared
to be no monitoring program of the bags for scour. No sand cover appears to have been
maintained on top of the geotextile tubes. The installation also did not extend high enough to
prevent overtopping during major storms which is another common failure mode for coastal bank
protection. In short, the installation at Ratner was clearly not done according to best engineering
design and specifications or by qualified personnel, and no rigorous monitoring or sand
maintenance program was maintained. The project failed as a result of these deficiencies.
Summary of Proposed Emergency Geotextile Tube Project and Comparison with Ratner Installation
The proposed emergency geotextile tube project differs from the Ratner installation in several
important ways.
1. Size of Geotextile Tubes/Coastal Sand Bags. The proposed geotextile tube project involves
massive, 45-foot circumference geotextile tubes, most with lengths of 200-feet. While
failure is not expected to occur due to the scour protection, monitoring, and sand
mitigation programs, the proposed geotextile tubes would be expected to fail as one
massive unit that could be readily recovered, rather than as widespread, small coastal sand
bags, such as were used at Ratner.
2. Seams within Geotextile Material. The seams in the Ratner coastal sand bags appear to be
hand-stitched and thereby vulnerable to failure (see below photos). For the proposed
geotextile tube project, the seams will be mechanically closed and are designed to be more
durable than the geotextile material itself.
3. Scour Protection. The proposed geotextile tubes will involve an anchor tube and scour
apron, which will greatly minimize the risk of failure due to tube collapse into a scour
hole. The Ratner project does not appear to have utilized any scour protection.
4. Sand Mitigation Program. The proposed geotextile tube project includes a rigorous sand
mitigation program and a requirement to provide an annual minimum volume of sediment
(equivalent to that which would have eroded by the coastal bank absent the project). For
the Ratner project, a sand cover was not maintained over the small geotextile coastal sand
bags.
5. Monitoring Program. The proposed geotextile project includes protocols for monitoring
and repairing the geotextile tubes. At the Ratner project, there appears to have been no
such regular monitoring or maintenance.
In conclusion, the Ratner geotextile project involved numerous deficiencies (in terms of design,
installation, monitoring, maintenance and substandard geotextile size), that have all been fully
addressed by the proposed project. The proposed contractor for the project, Fish Tec, is well-
qualified: Fish Tec specializes in geotextile tube installation and is one of the top installers for
the US Army Corps of Engineers. Finally, the proposed project design has been carefully
vetted by numerous professional engineers and is not anticipated to fail or generate marine
debris (see memo from Dr. Michael Bruno dated November 25, 2013).
Photo 1. Ratner Coastal Sand Bag
Photo 2. Seam in Ratner Coastal Sand Bag
M E M O R A N D U M
Date: November 26, 2013
To: Joshua Posner, President, Sconset Beach Preservation Fund
From: Maria Hartnett and Les Smith, Epsilon Associates, Inc.
Subject: Performance Standard Compliance for Emergency Certification Geotextile Tube
Project, Nantucket, MA
WETLAND RESOURCE AREAS
Work associated with the Project will be located in the following coastal wetland resource areas
subject to the Wetlands Protection Act (WPA), local Bylaw, and the respective state wetlands
regulations and local wetlands regulations:
Coastal Bank;
Coastal Beach; and
Land Subject to Coastal Storm Flowage (LSCSF).
The Natural Heritage and Endangered Species Program (NHESP) Estimated and Priority Habitat of
Rare and Endangered Species Map does not show priority habitats for state-protected species and
estimated habitats for rare wildlife in the project area. While other unlisted wildlife species may be
present in the Project area (likely on a transient basis); swallows are the only known (unlisted)
nesting species within the Project area. Properties north of this point may serve as swallow habitat,
including those properties from 91 to 99 Baxter Road where swallow nest cavities at the top of the
coastal bank were still observable at the time of the survey. To maintain areas where unlisted
swallow species can nest, the Project will not vegetate a 5 to 7 foot section at the top of the coastal
bank to reserve this section for swallows’ nests.
COMPLIANCE WITH STATE WETLANDS REGULATIONS
The installation of the Project will require authorization under the WPA for work occurring within
the following wetland resource areas: Coastal Bank, Coastal Beach, and Land Subject to Coastal
Storm Flowage. The protected interests within the wetland resource areas the project area include
storm damage prevention, flood control, and protection of wildlife habitat.
2
The following sections assess Project compliance with the state wetlands regulations for each
resource area.
Coastal Bank (310 CMR 10.30)
Coastal Bank is defined at 310 CMR 10.30(2) as “the seaward face or side of any elevated landform,
other than a coastal dune, which lies at the landward edge of a coastal beach, land subject to tidal
action, or other wetland.”
The state wetlands regulation at 310 CMR 10.30 includes the following conditions:
“WHEN A COASTAL BANK IS DETERMINED TO BE SIGNIFICANT TO STORM DAMAGE
PREVENTION OR FLOOD CONTROL BECAUSE IT SUPPLIES SEDIMENT TO COASTAL
BEACHES, COASTAL DUNES OR BARRIER BEACHES, 310 CRM 10.30(3) through (5) SHALL
APPLY.
“WHEN A COASTAL BANK IS DETERMINED TO BE SIGNIFICANT TO STORM DAMAGE
PREVENTION OR FLOOD CONTROL BECAUSE IT IS A VERTICAL BUFFER TO STORM WATERS,
310 CMR 10.30(6) through (8) SHALL APPLY.”
The coastal bank in the Project area supplies sand to nearby coastal landforms; therefore, it is
significant to storm damage prevention and flood control. The coastal bank in the Project area also
is significant to storm damage prevention and flood control because it is a vertical buffer to storm
waters. The following sections describe how the Project design complies with state regulations for
coastal banks (310 CMR 10.30(3) through (8)).
310 CMR 10.30(3) – Performance Standard
“No new bulkhead, geotube, seawall, groin or other coastal engineering structure shall be
permitted on such a coastal bank except that such a coastal engineering structure shall be
permitted when required to prevent storm damage to buildings constructed prior to the effective
date of 310 CMR 1.21 through 10.37 or constructed pursuant to a Notice of Intent filed prior to
the effective date of 310 CMR 10.21 through 10.37 (August 10, 1978), including
reconstructions of such buildings subsequent to the effective date of 310 CMR 10.21 through
10.37, provided that the following requirements are met:
(a) a coastal engineering structure or modification thereto shall be designed and
constructed so as to minimize, using best available measures, adverse effects on
adjacent or nearby coastal beaches due to changes in wave action, and
(b) the applicant demonstrates that no method of protecting the building other than the
proposed coastal engineering structure is feasible.
(c) Protective planting designed to reduce erosion may be permitted.”
3
Project Compliance: The Project is necessary to prevent storm damage to buildings constructed
prior to August 10, 1978, including reconstructions of such buildings. The status of the lots in
terms of which have pre-1978 structures or houses on them are included on Figure 11 appended to
the cover letter.
Lot 91 has a pre-1978 house on the lot landward of Baxter Road. Lot 93 and 97 have pre-1978
houses. The next three lots, 99, 101 and 105, all have pre-1978 houses on the landward side of
Baxter Road. Thus all lots meet the state requirement for pre-1978 protection. The Project
conforms to the other requirements of this regulation, as described below:
Coastal Engineering Structure: With respect to subsection (a) above, the project has been designed
and will be constructed using best available measures to minimize adverse effects on adjacent or
nearby coastal beaches caused by changes in wave action. Best Available Measures are defined at
310 CMR 10.04 as “the most up-to-date technology or the best designs, measures or engineering
practices that have been developed and that are commercially available.” The Applicant evaluated
alternative slopes and geotextile tube configurations an attempt to minimize changes in wave
action that might have adverse effects on beaches in the Project area. The project will be located
on the coastal bank with the toe of the geotextile tube extending beneath the beach at the toe of the
coastal bank, as shown on the Proposed Typical Section in the project plans. Waves will only
reach the geotextile tube toe during storms. The Project also incorporates design features that will
minimize the potential for adverse effects on adjacent beaches due to changes in wave action.
First, the sloped design of the geotextile tubes will minimize wave reflection or focusing of wave
energy onto adjacent, unprotected areas of the bank along the shoreline. In addition, voids
between the stones in the geotube will absorb and dissipate energy from breaking waves, thereby
minimizing the potential for wave reflection onto adjacent landforms.
No other feasible protection: With respect to subsection (b) above, there is no other feasible
method of protecting the existing buildings in the Project area in the long-term other than the
proposed project.
Sand Mitigation: Sand mitigation consists of the placement of compatible sand on the geotextile
tubes and downdrift beach areas. This sand mitigation protocol has been described in the
appended letters from Milone & MacBroom dated November 1, 2013, November 12, 2019, and
November 19, 2013.
Protective Plantings: With respect to subsection (c) above, the Project design includes native
vegetation plantings to protect the upper bank face and reduce runoff-related erosion.
310 CMR 10.30(4) - Performance Standard
“Any project on a coastal bank or within 100 feet landward of the top of a coastal bank, other than
a structure permitted by 310 CMR 10.30(3), shall not have an adverse effect due to wave action on
the movement of sediment from the coastal bank to coastal beaches or land subject to tidal action.”
4
Project Compliance: The Project is being permitted under 310 CMR 10.30(3), and thus this
standard does not apply.
310 CMR 10.30(5) - Performance Standard
“The Order of Conditions and the Certificate of Compliance for any new building within 100 feet
landward of the top of a coastal bank permitted by issuing authority under M.G.L. c. 131, § 40 shall
contain the specific condition: 310 CMR 10.30(3), promulgated under M.G.L. c. 131, § 40,
requires that no coastal engineering structure, such as a bulkhead, geotube, or seawall shall be
permitted on an eroding bank at any time in the future to protect the project allowed by this Order
of Conditions.”
Project Compliance: This standard does not apply to this NOI because the proposed Project does
not include the construction of any new buildings.
310 CMR 10.30(6) - Performance Standard
“Any project on such a coastal bank or within 100 feet landward of the top of such a coastal bank
shall have no adverse effects on the stability of the coastal bank.”
Project Compliance: The Project has been designed to maintain the stability of the coastal bank;
thus, it complies with this standard.
310 CMR 10.30(7) - Performance Standard
“Bulkheads, geotubes, seawalls, groins or other coastal engineering structures may be permitted on
such a coastal bank except when such bank is significant to storm damage prevention or flood
control because it supplies sediment to coastal beaches, coastal dunes, and barrier beaches.”
Project Compliance: As described above, the coastal bank in the Project area provides sand to the
coastal beach and adjacent coastal system. However, the Project is designed to stabilize the
existing bank face in accordance with 310 CMR 10.30(3) and mitigation will be provided as
described above.
310 CMR 10.30(8) - Performance Standard
“Notwithstanding the provisions of 310 CMR 10.30(3) through (7), no project may be permitted
which will have any adverse effect on specified habitat sites of rare vertebrate or invertebrate
species, as identified by procedures established under 310 CMR 10.37.”
Project Compliance: The Project area is not within estimated habitat indicated on the most recent
Estimated Habitat Map of State-Listed Rare Wetlands Wildlife published by the NHESP (NHESP
2008 Atlas, MassGIS).
5
Coastal Beaches (310 CMR 10.27)
Coastal Beach is defined at 310 CMR 10.27(2) as “unconsolidated sediment subject to wave, tidal
and coastal storm action which forms the gently sloping shore of a body of salt water and includes
tidal flats. Coastal beaches extend from the mean low water line landward to the dune line, coastal
bankline or the seaward edge of existing man-made structures when these structures replace one of
the above lines, whichever is closest to the ocean.”
The state wetlands regulation at 310 CMR 10.27(2) includes the following conditions:
“WHEN A COASTAL BEACH IS DETERMINED TO BE SIGNIFICANT TO STORM DAMAGE
PREVENTION, FLOOD CONTROL, OR PROTECTION OF WILDLIFE HABITAT, 310 CMR 10.27(3)
through (7) SHALL APPLY.”
The coastal beach in the Project area dissipates wave energy, reduces the height of approaching
waves, and acts as a sand source to nearby coastal landforms. It also provides habitat for non-listed
and listed foraging shorebirds. As such, the coastal beach is significant to storm damage
prevention, flood control, and the protection of wildlife habitat. As discussed below, the Project is
designed to comply with state regulations for coastal beaches 10.27(3) through (7).
310 CMR 10.27(3) – Performance Standard
“Any project on a coastal beach, except any project permitted under 310 CMR 10.30(3)(a), shall
not have an adverse effect by increasing erosion, decreasing the volume or changing the form of
any such coastal beach or an adjacent or downdrift coastal beach.”
Project Compliance: As explained above, the Project is being permitted under 310 CMR
10.30(3)(a). Therefore, this standard does not apply.
310 CMR 10.27(4) – Performance Standard
“Any groin, jetty, solid pier or other such solid fill structure which will interfere with littoral
drift, in addition to complying with 310 CMR 10.27(3), shall be constructed as follows:
(a) It shall be the minimum length and height demonstrated to be necessary to maintain
beach form and volume. In evaluating necessity, coastal engineering, physical
oceanographic and/or coastal geologic information shall be considered.
(b) Immediately after construction any groin shall be filled to entrapment capacity in height
and length with sediment of grain size compatible with that of the adjacent beach.
(c) To transfer sediments to the downdrift side of the inlet or shall be periodically
redredged to provide beach nourishment to ensure that downdrift or adjacent beaches
are not starved of sediments.”
6
Project Compliance: This standard does not apply. The Project is located above the zone of littoral
drift (i.e., at the landward edge of the coastal beach).
310 CMR 10.27(5) – Performance Standard
“Notwithstanding 310 CMR 10.27(3), beach nourishment with clean sediment of a grain size
compatible with that on the existing beach may be permitted.”
Project Compliance: The Project includes the initial placement of clean sand, of a grain size
compatible with that on the existing beach. Additional sand mitigation will be provided if the
shoreline monitoring demonstrates that adjacent or downdrift beaches are being impacted by the
geotextile tubes.
310 CMR 10.27(6) – Performance Standard
“In addition to complying with the requirements of 310 CMR 10.27(3) and 10.27(4), a project
on a tidal flat shall if water-dependent be designed and constructed, using best available
measures, so as to minimize adverse effects, and if non-water-dependent, have no adverse
effects, on marine fisheries and wildlife habitat cause by:
(a) alterations in water circulation,
(b) alterations in the distribution of sediment grain size, and
(c) changes in water quality, including, but not limited to, other than natural fluctuations in
the levels of dissolved oxygen, temperature or turbidity, or the addition of pollutants.”
Project Compliance: This standard does not apply. The Project does not include any work on a
tidal flat, which is defined at 310 CMR 10.27(2) as “any nearly level part of a coastal beach which
usually extends from the mean low water line landward to the more steeply sloping face of the
coastal beach, or which may be separated from the beach by land under the ocean.” All proposed
Project activities will occur on the coastal bank and at the toe of the coastal bank on a narrow strip
of the coastal beach well landward of and above the steeply sloping beach face.
310 CMR 10.27(7) – Performance Standard
“Notwithstanding the provisions of 310 CMR 10.27(3) through 10.27(6), no project may be
permitted which will have any adverse effect on specified habitat sites or rare vertebrate or
invertebrate species, as identified by procedures established under 310 CMR 10.37.”
Project Compliance: The Project area is not within estimated habitat shown on the most recent
Estimated Habitat Map of State-Listed Rare Wetlands Wildlife published by the NHESP (NHESP
2008 Atlas, MassGIS).
7
Land Subject to Coastal Storm Flowage
Land Subject to Coastal Storm Flowage is defined at 310 CMR 10.04 as “… land subject to any
inundation caused by coastal storms up to and including that caused by the 100-year storm, surge
of record or storm of record, whichever is greater”
The Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) for the
Project area, which defines the 100-year storm elevations, is provided in a separate attachment.
Project Compliance: There are no performance standards in the state wetlands regulations for Land
Subject to Coastal Storm Flowage (LSCSF). However, since LSCSF overlays the coastal beach and
the coastal bank up to the 100-year storm elevation as shown on the project plans, the relevant
performance standards have been reviewed and addressed above.
COMPLIANCE WITH LOCAL WETLANDS REGULATIONS
The Project will require authorization under the Nantucket Wetlands Bylaw for work occurring
within the following wetland resource areas: Coastal Bank, Coastal Beach, and Land Subject to
Coastal Storm Flowage. While a narrow coastal dune is present from Lot 59 south, no geotube
construction would occur on these lots unless and until future coastal erosion removes this narrow
coastal dune resource area. Since this dune is very thin, it could be removed by a single winter
storm.
The protected interests within these wetland resource areas include storm damage prevention,
flood control, protection of wildlife habitat and wetland scenic views.
Nantucket Coastal Banks (Section 2.05)
“Bank (coastal)” is defined in Section 1.02 of the local wetlands regulations as “the seaward face or
side of any elevated land form, other than coastal dune, which lies at the landward edge of coastal
beach, coastal dune, land subject to tidal action or coastal storm flowage, or other coastal wetland.
Any minor discontinuity of the slope notwithstanding, the top of the bank shall be the first
significant break in slope as defined by site specific topographic plan information, site inspection,
wetland habitat evaluation, geologic origin, and /or relationship to land subject to coastal storm
flowage. A bank may be partially or totally vegetated, or it may be comprised of exposed soil,
gravel, stone or sand. A bank may be created by man and/or made of man-made materials. A bank
may or may not contribute sediment to coastal dunes, beaches and /or to the littoral drift system. A
bank may be significant as a major source of sediment, as a vertical buffer, for wildlife habitat and
for wetland scenic views.”
8
Performance Standards for work on coastal banks are set forth in Section 2.05 B of the local
wetlands regulations, which provides that “Coastal Banks or Land within 100 feet of a Coastal Bank
shall be presumed significant to the Interests Protected by the Bylaw as referenced in Section A,
therefore the following regulations shall apply.”
The Project does not include structures subject to the performance standards listed below, and
hence they are excluded from the following discussion:
Section 2.05 B(2) (piers);
Section 2.05 B(4) (elevated walkways);
Section 2.05 B(6) (septic leach facility); and
Section 2.05 B(8) (buildings).
The remaining applicable sections of the local wetlands regulations pertaining to coastal bank are
addressed below.
Section 2.05 B(1) – Performance Standard
“No new bulkheads, coastal geotubes, groins, or other coastal engineering structures shall be
permitted to protect structures constructed, or substantially improved, after 8/78 except for public
infrastructures. Bulkheads and groins may be rebuilt only if the Commission determines there is no
environmentally better way to control an erosion problem, including in appropriate cases the
moving of the threatened buildings and/or public infrastructure. Other coastal engineering
structures may be permitted only upon a clear showing that no other alternative exists to protect a
structure that has not been substantially improved or public infrastructure built prior to 9/78, from
imminent danger.”
Project Compliance: The Project objectives are to protect Baxter Road and other infrastructure
and to preserve an entire historic largely pre-78 community, which necessarily includes a number
of post-78 homes as well as pre-78 structures to which various alterations have been made. In
addition, the project objectives include protecting and possibly extending the ‘Sconset Foot-path,
and proposed public access ways to the Foot-path and to the beach between properties. Note that
many houses have already been moved, others have little room to move, and the Town recently
voted that infrastructure is in imminent danger and should be protected in place.
The Project does not involve rebuilding any bulkhead or groin. No other alternative is feasible to
satisfy the project objectives.
Section 2.05 B(3) - Performance Standard
“All projects shall be restricted to activity determined by the Commission to have no adverse effect
on bank height, bank stability, wildlife habitat, vegetation, wetland scenic view, or the use of a
bank as a sediment source.”
9
Project Compliance: The Project will not have any adverse effect on bank height, bank stability,
wildlife habitat, vegetation, wetland scenic view, or the use of a bank as a sediment source as
described below.
Bank height: The Project will preserve rather than adversely affecting bank height, as it does
not involve any work which would lower the bank height.
Bank stability: As designed, the Project is intended to maintain bank stability by protecting
the lower bank from wave-induced erosion. Vegetation plantings on the upper bank face
will also prevent rain- and wind-induced scour.
Wildlife habitat: See discussion in discussion in the state review above.
Vegetation: Planting beach grass and other native vegetation on the bank face will enhance
vegetation in the Project area. Native vegetation will be planted from +28 feet MLW up to
5 to 7 feet below the top of the coastal bank.
Wetland scenic view: The project with sand cover will have wetland scenic views similar to
bank appearance before it was denuded by erosion. Subsequent sand mitigation will be
placed on the geotextile tubes and in downdrift. Otherwise the rock geotextile tubes will
have the appearance of a natural shoreline area. Planting of beach grass and other native
vegetation over the majority of the rest of the coastal bank will also enhance the wetland
scenic view of the overall coastal bank.
Sediment Source: With respect to the bank as a sediment source, although the Project is
designed to stabilize the existing bank face in accordance with local performance standard
Section 2.05 B(1) and state performance standard 310 CMR 10.30(3), the Project also
proposes to provide sand mitigation as described previously.
Section 2.05 B(5) - Performance Standard
“All projects which are not water dependent shall maintain at least a 25-foot natural undisturbed
area adjacent to a coastal bank. All structures which are not water dependent shall be at least 50
feet from a coastal bank.”
Project Compliance: “Water Dependent Projects or Uses” are defined in the local wetlands
regulations as “projects which require direct access for their intended use and therefore cannot be
located out of the Area Subject to Protection under the Bylaw. Examples include but are not
limited to: docks, piers, boat landings, boathouses, marinas, stairs to beaches, and boardwalks over
wetland vegetation. Projects which benefit from wetlands access but which do not require it are
not water dependent uses. Examples include: restaurants, dwellings, and commercial enterprises
servicing marine-related uses such as fish markets, repair facilities, ships’ chandleries, and general
use recreational trails.”
10
The Project is water dependent because direct access to the coastal bank and coastal beach is
required to achieve the intended purpose of the Project in stabilizing the coastal bank to provide
storm damage prevention and flood control. This Project cannot be located out of the coastal bank
and coastal beach resource areas. The project will also include preserving and enhancing
recreational trails and beach access.
Section 2.05 B(7) - Performance Standard
“In areas of an eroding coastal bank, the distance from all new structures to the coastal bank shall
be at least 20 times the average annual erosion rate or 100 feet, whichever is the lesser. The
average annual erosion rate shall be determined by averaging the annual erosion over a 150-year
period ending with the date the NOI was filed, or if no NOI was filed, the date construction began.
If erosion data is not available for the 150 year period, the Commission shall determine the average
annual erosion rate from such lesser time for which erosion data is available. In cases where
documentation can be provided to show that the use of the I50-year period is inappropriate to
existing coastal shoreline characteristics and trends, alternate shoreline change rates may be used
with the approval of the Commission.”
Project Compliance: This regulation is designed to ensure a substantial setback for new structures
built on the land above an eroding coastal bank. The Project does not involve construction of any
new buildings or other structures at the top of the coastal bank that might require future protection;
therefore, this standard does not apply.
Nantucket Coastal Beaches (Section 2.02)
“Beach” is defined in Section 1.02 of the local wetlands regulations as “unconsolidated sediment
subject to wave, tidal, or storm action which forms the gently sloping shore of a body of water,
including land which is separated from other land by a body of water or marsh system. Beaches
extend from the mean low water line landward to the dune line, bank line, or the edge of existing
man-made structures, when these structures replace one of the above lines, whichever is closest to
the defining water body.”
Performance Standards for coastal beaches are set forth in Section 2.02 B of the local wetlands
regulations, which provides that “[a] Coastal Beach, Tidal Flat or Land within 100 feet of a Coastal
Beach or Tidal Flat shall be presumed significant to the interests Protected by the Bylaw, as
referenced in Section A, therefore the following regulations shall apply:”
Project activities on the coastal beach consist of: (1) installing the toe of the geotextile tube beneath
the landward edge of the coastal beach at the toe of the coastal bank; (2) temporarily
accommodating construction equipment and personnel access to construct the proposed geotextile
tubes; and (3) periodic access for construction equipment and personnel to maintain the project.
11
The Project does not include activities or structures subject to the following performance standards,
and hence they are excluded from the subsequent discussion:
Section 2.02 B(3) (dredging);
Section 2.02 B(5) (septic systems);
Section 2.02 B(6) (non-water dependent activities);
Section 2.02 B(7) (new buildings); and
Section 2.02 B(8) (vehicular access for houses and recreational areas).
The remaining applicable sections of the local wetlands regulations pertaining to coastal beach are
addressed below.
Section 2.02 B(1) – Performance Standard
“The provisions of Section 2.01 B(1-8) (Land Under the Ocean) shall apply to coastal beaches and
tidal flats.”
Project Compliance: “Land Under the Ocean” is not defined in the local wetlands regulations.
Therefore, in accordance with Section 1.02, “To the extent not defined herein or in the Bylaw,
words used in the Bylaw or in the regulations shall have the definitions contained in the
Massachusetts Wetlands Protection Act (M.G.L. c. 131, sec. 40) and the rules and regulations
promulgated thereunder.”
“Land Under the Ocean” is defined in the state wetland regulations at 310 CMR 10.25(2) to mean
“land extending from the mean low water line seaward to the boundary of the municipality’s
jurisdiction and includes land under estuaries.”
The Project does not involve any work seaward of the mean low water line; therefore, none of the
performance standards at Section 2.01 B(1-8) of the local wetlands regulations apply. However,
even if these performance standards did apply, the Project does not include activities or structures
subject to the following performance standards:
Section 2.01 B(1) (improvement and maintenance dredging);
Section 2.01 B(2) (dredging);
Section 2.01 B(3) (residential piers);
Section 2.01 B(4) (commercial piers);
Section 2.01 B(5) (commercial or residential piers);
Section 2.01 B(6) (aquaculture); and
Section 2.01 B(9) (non-water dependent project).
If Section 2.01 B were applicable, the only standards in Section 2.01 B that would be relevant are
those provided in 2.01 B(7) (coastal engineering structure) and 2.01 B(8) (water dependent project),
which are both discussed below for informational purposes only:
12
Section 2.01 B(7) – Performance Standard
“No new bulkheads or coastal engineering structures shall be permitted to protect structures
constructed or substantially improved after 8/78. Bulkheads may be rebuilt only if the Commission
determines that there is no environmentally better way to control an erosion problem, including in
appropriate cases the moving of the threatened building. Other coastal engineering structures may
be permitted only upon a clear showing that no other alternative exists to protect a structure built
prior to 9/78, but not substantially improved, from imminent danger.”
Project Compliance: Project compliance with the language of this standard is described in our
project compliance for Section 2.05 B(1) above.
Section 2.01 B(8) – Performance Standard
“Water dependent projects shall be designed and performed so as to cause no adverse effects on
wildlife, erosion control, marine fisheries, shellfish beds, storm damage prevention, flood control,
and recreation.”
Project Compliance: As described in Section 2.05 B(5), the Project is water dependent because it
requires direct access to the coastal bank and landward portion of the coastal beach.
Wildlife: See previous discussion.
Erosion Control: The Project will have a positive effect on erosion control, as it will protect
the bank from wave-induced erosion and sand mitigation will be added to mimic the
natural sand supply from the coastal bank.
Marine Fisheries and Shellfish beds: No Project activities will occur seaward of Mean High
Water, and hence the Project will not have any adverse effects on marine fisheries or
shellfish beds.
Storm damage prevention and flood control: Storm damage prevention and flood control
will benefit from the Project since it will protect the coastal bank and sand mitigation will
be added to mimic the natural sand supply from the coastal bank.
Recreation: The Project will not adversely impact recreational uses along the beach or in
the water, as it will be located on the coastal bank and a portion of the coastal beach. The
Project will maintain the recreational use of the ’Sconset Foot-path at the top of the coastal
bank and the public access ways leading to the Foot-path and the beach. Also a walking
path will be available to the public, April through October, on top of the geotextile tube.
Also, the project will not have any structures perpendicular to the bank that would block
public access.
13
Section 2.02 B(2) – Performance Standard
“No new bulkhead or coastal engineering structure shall be permitted to protect structures
constructed, or substantially improved, after 8/78. Bulkheads may be rebuilt only if the
Commission determines there is no environmentally better way to control an erosion problem,
including in appropriate cases the moving of the threatened building. Other coastal
engineering structures may be permitted only upon a clear showing that no other alternative
exists to protect a structure built prior to 9/78, and not substantially improved, from imminent
danger.”
Project Compliance: Project compliance with this standard is discussed below.
Protect Buildings: The Project is designed to preserve an entire historic largely pre-78
community, which necessarily includes a number of post-78 homes as well as pre-78
structures to which various additions have been made.
Alternatives: Although numerous alternatives have been considered and evaluated (or even
implemented previously in the Project area), no suitable alternatives to the proposed Project
exist which would satisfy the objective of providing long-term protection for existing
structures that pre-date September 1978 and have not been substantially improved. The
criteria used to define those properties in imminent danger are described above.
Section 2.02 B(4) - Performance Standard
“Clean fill of a compatible grain size may be used on a Coastal Beach, but not on a Tidal Flat,
only if the Commission authorizes its use, and only if such fill is to be used for a beach or dune
nourishment project. All possible mitigation measures shall be taken, as determined by the
Commission, to limit the adverse effects of the fill.”
Project Compliance: Project compliance with this standard is discussed below.
Clean Fill: Sand mitigation will consist of placing clean, beach-compatible sand into the
littoral system.
Tidal Flats: The Project does not involve placement of sand on any tidal flats.
Section 2.02 B(9) – Performance Standard
“The Commission may impose such additional requirements as are necessary to protect the
Interests Protected by the Bylaw.”
Project Compliance: The Project has been designed using best available measures to stabilize the
coastal bank and protect existing landward structures and public infrastructure while
simultaneously avoiding, minimizing, and mitigating for potential impacts. All of the interests
protected by the Bylaw have been considered during the process of Project design.
14
Nantucket Land Subject to Coastal Storm Flowage (Section 2.10)
“Land Subject to Coastal Storm Flowage” is defined in Section 1.02 of the local wetlands
regulations as “land subject to any inundation caused by coastal storms up to and including that
caused by the 100-year storm, surge of record, or storm of record, whichever is greater.”
Performance Standards for Land Subject to Coastal Storm Flowage (LSCSF) are defined in Section
2.10 of the local wetlands regulations, which provides that “Land Subject to Coastal Storm Flowage
or Land within 100 feet of Land Subject to Coastal Storm Flowage shall be presumed significant to
the interests Protected by the Bylaw, as referenced in Section A, therefore the following regulations
shall apply:”
The Project does not include activities or structures subject to the following performance standards,
and hence they are excluded from the subsequent discussion:
Section 2.10 B(2) (use of pollutants or septic systems);
Section 2.10 B(3) (underground fuel tanks); and
Section 2.10 B(4) (new buildings).
The applicable Sections 2.10 B(1) and 2.10 B(5) are discussed below.
Section 2.10 B(1) – Performance Standard
“The work shall not reduce the ability of the land to absorb and contain flood waters, or to buffer
inland areas from flooding and wave damage.”
Project Compliance: The Project will not reduce the ability of LSCSF to absorb and contain flood
waters. By stabilizing the bank face and providing protection at the toe of bank, the Project will
enhance the coastal bank’s function of buffering inland areas and buildings from storm damage.
Section 2.10 B(5)
“The Commission may impose such additional requirements as are necessary to protect the
Interests Protected by the Bylaw.”
Project Compliance: The Project has been designed using best available measures to stabilize the
coastal bank and protect existing landward structures while simultaneously avoiding, minimizing,
and mitigating for potential impacts. All of the interests protected by the Bylaw have been
considered during the process of Project design.
CONCLUSIONS
SBPF proposes to install a geotextile tubes on the lower coastal bank, along with planting
vegetation on the upper bank, to stabilize the eroding coastal bank in the Project area, thereby
protecting an historic residential community and public infrastructure at the top of the bank.
15
Geotextile tubes have proven successful on Nantucket and elsewhere in New England. The Project
offers a design with straightforward installation and maintenance, with limited construction and
environmental impacts. Sand mitigation will be provided as described above.
The Project is consistent with the protected interests and performance standards of both the WPA
and the Bylaw (and their respective wetlands regulations) and will further those interests by
providing storm damage prevention and flood control while restoring and maintaining wetland
scenic views. Many alternatives were considered for the Project, none of which was deemed
preferable due to a lack of proven effectiveness, regulatory prohibitions, or unacceptable impacts to
the sediment supply system. The Project proposal offers a natural-looking, effective means of
preventing storm damage to existing residences and public infrastructure while avoiding adverse
impacts to resource areas and littoral drift.
BAXTER
ROADSANKATY ROADISABE
L
LE
'S
WAY
ISOBELS WAYSANKATY HEAD ROADPOL
P
I
S
R
O
A
D
BAYBERRY LANE
53
52
86
63
97
51B
73
83
59
92
68
82
109
106
6970
100
81
79
61
55
104
51A
71
93
84
67
77
75
115
96
65
113
116
58
112
72
85
99
87
101
105
91
117
80
119
107
107A
64
60
54
3
54A
56
76
94
62
108
110
90
120
78
114
98
82A
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP,
swisstopo, and the GIS User Community
G:\Projects\Lighthouse\2013\NOI\year_built.mxd
Figure 11Pre-1978 House Status
Baxter Road and Sconset Bluff Storm Damage Prevention Project Nantucket, MA
LEGEND
Basemap: 2011 Aerial Imagery, ESRI
Pre-1978 House Status
Pre-1978
Post-1978
Vacant
Not Specified
°0 150 30075
Feet1 inch = 300 feet
Scale 1:3,600
Figure 1
Existing Jute Terraces
Existing Jute Terraces, Sconset
Existing Jute Terraces, Sconset
Emergency Project Nantucket, MA
Figure 2
Failure of Jute Terraces
Failure of Jute Terraces at 79 Baxter
Failure of Jute Terraces at 79 Baxter
Emergency Project Nantucket, MA
Attachment A
Epsilon Memorandum Regarding Retreat and Nourishment Calculations
Page 1
M E M O R A N D U M
Date: November 1, 2013
To: Kara Buzanoski, Nantucket DPW
From: Maria Hartnett, Epsilon Associates
Subject: Baxter Road Geotube Project – Coastal Bank Retreat Calculations
The following memo summarizes information about the ‘Sconset bluff volume contribution
calculation, including (1) a comparison of the current proposed sand mitigation volume
with past Sconset Beach Preservation Fund (SBPF) proposals; (2) details on how the bank
retreat rate and associated volume were calculated, including data tables; (3) comparison of
the calculated bank retreat rates with shoreline change rates; (4) comparison of the
calculated bank contribution volume with bank survey data; (5) a discussion of CZM’s sand
volume mitigation recommendations for the Project area; and (6) a discussion of Coastal
Planning & Engineering’s littoral budget prepared for the previously-proposed beach
nourishment project. The Town of Nantucket requested that I prepare this memo due to my
long history of calculating the bank retreat rates and associated volumes.
1.0 Comparison with Bank Retreat Rates and Volumes in Previous Submittals
The following table (Table 1) summarizes the bank retreat rates and volumes provided by
SBPF during project filings for the marine mattress and gabion projects, the revetment, and
the geotube project. There is significant spatial and temporal variation in coastal bank
retreat rates along the ‘Sconset bluff. Retreat rates are calculated along multiple transects
for each lot; therefore, different project areas will have different retreat rates and associated
volumes. The table below shows that each of the SBPF filings has involved a different
project area.
Variations in the sand mitigation volume proposed by SBPF are also a result of the varying
nature of bluff erosion over time. Erosion of the bluff is an ongoing process and SBPF has
periodically undertaken additional LIDAR surveys of the project site; therefore, more recent
data (2013 LIDAR survey) were available for use for the geotube and revetment project than
for the gabion project (2010 LIDAR survey). Similarly, the geotube and revetment project
areas include project areas farther to the north, where bank retreat was occurring as far back
as 1994, and therefore a more long-term bank retreat rate could be determined for the
geotube and revetment projects (bank retreat rates from 1994-2013 and 2003-2013 could
Page 2
be determined for the geotube and revetment projects vs. a 2003-2010 bank retreat rate for
the gabion project).
For the geotube project, the Town intends to follow the state standard of “Best Available
Measure,” which has been consistently required by DEP, CZM, and many local
Conservation Commissions. The state standard of “Best Available Measure1” for sand
mitigation is to provide to the littoral system, on an annual basis, the average amount of
sand that would have been provided by the eroding bank absent the project. For the
marine mattress and gabion project, SBPF offered an additional component of sand
mitigation (~7 cy/lf to replicate the amount of sand eroded from the nearshore); this extra
component was only associated with that pilot project (which was never implemented) and
is not relevant for the current project.
Table I. Summary of Sand Mitigation Volumes in SBPF Proposals
Project Project Area Years Used in
Calculation
Retreat Rate
(ft/yr)
Volume (cy/lf)
Geotube
(Current Town
Application)
85-107A Baxter 1994-2013
(91-107A Baxter)
2003-2013
(85-91 Baxter)
4.6 14.3
Revetment 63-119 Baxter 1994-2013
(91-119 Baxter)
2003-2013
(71-91 Baxter)
3.8 12.0
Gabion 77-85 Baxter (North)
63-67 Baxter (South)
2003-2010 (North)
2001-2011 (South)
4.96 (North)
3.62 (South)
North
11.6* (Bank)
6.8 (Nearshore)
20** TOTAL
South
7.5* (Bank)
7.2 (Nearshore)
16** TOTAL
*Excludes 13% fines
**Includes overfill allowance
2.0 Description of Methodology
The coastal bank retreat calculation was developed using the 2013 LIDAR data and high-
resolution georeferenced aerial photographs dating back to 1994 to establish a long-term
bank retreat average.
1 Best Available Measure(s) is defined in 310 CMR 10.04 as “… the most up‐to‐date technology or the best
designs, measures or engineering practices that have been developed and that are commercially
available.
Page 3
Bank Retreat Rate. The top of the coastal bank was digitized for 1994, 2003, and
2013 using ESRI ArcGIS software to produce the attached figure (see Figure 1). Top
of coastal bank retreat was analyzed along shore-perpendicular transects spaced
approximately every 20 feet.
o For the portions of the geotube project area from 91-107A Baxter Road, the
top of coastal bank was actively retreating as early as 1994. For these lots, a
long-term (1994-2013) coastal bank retreat rate of 4.0 feet/yr was calculated.
This was calculated by taking the average of the coastal bank retreat along
each transect within the area from 91-107A Baxter Road (see Table 1).
o For the portions of the project area from 85-91 Baxter Road, the top of
coastal bank was not actively retreating in 1994 (Figure 1 shows that the
1994 and 2003 top of bank lines are coincident south of the southern half of
91 Baxter Road). For these lots, a 10-year (2003-2013) bank retreat rate of
5.8 feet/yr was calculated. This was calculated by taking the average of the
coastal bank retreat along each transect within the area from 85-91 Baxter
Road (see Table 1).
o For the entire Project area, a single average coastal bank retreat rate was
calculated by averaging the above two rates. The average is distance-
weighted by transect, which reflects the fact that the majority of the geotube
project area has a long-term erosion rate of 4.0 feet/yr, with only the
southern 30% exhibiting the higher erosion rate of 5.8 feet/yr. The distance-
weighted average is 4.6 ft/yr (see Table 2).
Volume Calculation: Section views from each of the Project lots from 85-107A
Baxter Road were developed from the 2013 LIDAR survey. The volume associated
with a bank retreat of 4.6 ft/yr was then determined for each lot using AutoCAD (see
typical Figure 2, which shows how the cross-sectional area and associated volume
were calculated for each lot). A distance-weighted average volume for all the
project lots was then determined (see Table 3), yielding 14.3 cubic yards/linear
foot/year (cy/lf/yr).
3.0 Corroboration of Methodology by Survey Data
The bank retreat volume contribution methodology, based on LIDAR data and aerial
photography, was corroborated by independent calculations performed by Woods Hole
Group (WHG). WHG has top and toe of bank survey data available at profiles 90 (near
69/71 Baxter Road), 90.5 (near 79/81 Baxter Road), and 91 (near 91 Baxter Road), in years
2006, 2008, and 2013. While these data are too limited to use for the geotube project area
since they do not extend far enough northward, they provide a useful check of the above
methodology. WHG utilized the top and toe of bluff survey data to calculate a bank
contribution volume of 12.4 cy/lf for the area covered by the profiles (69/71 Baxter Road –
Page 4
91 Baxter Road); see Tables 4a and 4b. When the above methodology as described in
Section 2 was applied to the same project area (71-91 Baxter Road, for years 2003-2013),
the volume calculated was 13.2 cy/lf. The high degree of similarity between these two
numbers (they are within 10% of one another) suggests that the methodology used by
Epsilon provides an accurate representation of the bank contribution volume, and may even
slightly over-estimate the bank contribution volume.
4.0 Corroboration of Methodology by Shoreline Change Data
This calculation was also corroborated by shoreline change data. The WHG shoreline
change data for the area from 91-107A Baxter Road were compared to the calculated bank
retreat rate for 91-107A Baxter Road. The complete March 2013 WHG Shoreline
Monitoring Report is included as Attachment A.
Epsilon Methodology: the 1994-2013 bank retreat rate from 91-107A Baxter Road
was calculated as 4.0 ft/yr.
Shoreline Data: the 1994-2013 distance-weighted shoreline change rate for those
profiles located nearest to 91-107A Baxter Road (profiles 91, 91.5, and 92) is 3.9
ft/yr. (See Table 5.)
The high similarity between these two numbers again supports the accuracy of the
calculated bank retreat rate, and suggests that the above methodology may also be slightly
conservative.
Comparisons between 1994-2013 shoreline change rates and bank retreat rates were not
made for areas farther south of 91 Baxter Road, since the coastal bank was not actively
retreating throughout this time period.
5.0 Discussion of CZM Recommendations
Ms. Rebecca Haney of CZM provided a recommended sand volume to the Conservation
Commission in a letter dated August 26, 2013 for the revetment project. As noted in
SBPF’s submission to the Conservation Commission on September 6, 2013, Ms. Haney’s
suggestion to utilize short-term shoreline change rates from 1978-2009 to estimate the
volume of sediment eroded from the coastal bank fails to consider the coastal setting at
Sconset and, by doing so, recommends the use of irrelevant data. The Sconset shoreline
and beyond (from the Sewer Beds at the south to Wauwinet at the north) have been
carefully monitored on a quarterly or semi-annual basis for nearly twenty years, yielding an
impressive record of highly-accurate data. This monitoring has consistently shown that
shoreline erosion rates in areas where the coastal bank is fronted by dunes are significantly
higher than shoreline rates in areas with an eroding coastal bank. (This observation is as
expected, since an eroding dune contributes less to the littoral system than an eroding
bank.) In other words, survey data show that the shoreline change rates in areas fronted by
Page 5
dunes are not representative of the coastal bank retreat rate. Rather, the shoreline change
rate and coastal bank retreat rate may only begin to approximate one another after the
coastal dune and any vegetated portion of the coastal bank have completely eroded and
sufficient time has passed for an equilibrium to be reached. The coastal dune in the Project
area was still present during much of the 1978-2009 time period; therefore, Ms. Haney’s
suggestion to use a 1978-2009 shoreline change rate to approximate coastal bank retreat is
untenable.
Ms. Haney quotes a shoreline change rate of 6 to 10 feet/yr from 1978-2009 in the "project
area," but this analysis apparently overlooks the northern section of the revetment project
area. The CZM shoreline change data for the Project area (63-119 Baxter Road; CZM
transects 285 through 306) indicates somewhat lower shoreline change rates, in the range
of 4 to 9.7 feet/yr, and even these rates are in applicable given that they reflect dune
erosion, not bank erosion, in the earlier years. Additionally, the CZM data is subject to
uncertainty; such uncertainty is inherent to the methodology of identifying a shoreline from
aerial photographs used for the broad-reaching CZM shoreline change data project.
Although CZM quantifies this uncertainty for each transect; Ms. Haney fails to acknowledge
this uncertainty, even though the average uncertainty for the transects in the Project area is
almost 3 feet.
Ultimately, Ms. Haney’s analysis does not consider the coastal setting at Sconset and
therefore in our opinion does not provide an accurate representation for this project.
6.0 Discussion of the 2005 CP&E Sediment Budget
During the permitting effort for the beach nourishment project, Coastal Planning &
Engineering (CP&E) prepared a littoral budget based upon data from 1995-2005. (See FEIR,
Sconset Beach Nourishment Project, November 30. 2006. Attachment A, Coastal Planning
and Engineering (CPE) Engineering Design Report, Sconset Beach Nourishment Project,
Nantucket, Massachusetts. Section 8.0, “Littoral Budget” is included as Attachment B to this
memo.) This sediment budget relied upon several assumptions (such as locating the nodal
point at the area of greatest erosion, applying the shoreline change rate to entire coastal
profile [including eroding coastal bank], determining the volume associated with each
profile by multiplying the active profile height times the shoreline recession rate and
effective distance between profiles) that are appropriate for use in designing a beach
nourishment project, but that may not be as appropriate for quantifying the volume and
direction of sediment transport in the project area for the purposes of designing a sand
mitigation program. While we feel that the CP&E analysis for the beach nourishment
project has limitations when applied to the geotube or revetment project, we nonetheless
reviewed their analysis to serve as another check of the proposed sediment mitigation
volume.
Table 6 presents the CP&E sediment budget values for those profiles within the geotube
Project area (profiles 91, 92, and 92.5). The table has been updated from the original CP&E
Page 6
analysis in three places: (1) the shoreline change rates have been updated to reflect the most
current conditions, based on the results of the March 2013 shoreline survey; (2) the active
profile height has been changed to reflect the height of the eroding bank, rather than the
entire coastal profile out to the depth of closure, to reflect the geotube project’s
commitment to mitigate the amount of sand eroded from the coastal bank; (3) the discount
of the silt percentage applied by CP&E has been removed. This analysis yields an estimated
bank contribution volume of 11.4 cy/lf (see Table 6). This volume is lower than the
proposed volume of 14.3 cy/lf, again indicating that the sand mitigation volume proposed
for the geotube project is adequate and possibly conservative (i.e., it may slightly
overestimate the bank contribution volume).
BAXTER ROAD
SAN
K
A
T
Y
R
O
A
D
85
99
97
87
101
83
105
109
93
91
107
81
107A
113
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP,
swisstopo, and the GIS User Community
G:\Projects\Lighthouse\2013\ConCom\Retreat\Revised\Detailed_Analysis_v2\1994-2003-2013.mxd
Baxter Road Nantucket, MA Figure 1
1994-2013 Average Retreat for Lots 91-107A = . Feet/Year
2003-2013 Average Retreat for Lots 85-91 = 5. Feet/Year
Overall Average Retreat for Lots 85-107A = 4.6 Feet/Year
Average Coastal Bank Retreat Summary
Coastal Bank Retreat
LEGEND
Basemap: 2013 Aerial Imagery, Col-East, Inc.
1994 Top of Coastal Bank
2003 Top of Coastal Bank
2013 Top of Coastal Bank
Parcel Boundary
°0 60 12030
Feet1 inch = 120 feet
Scale 1:1,440
Figure 2
Coastal Bank Sediment Contribution – Representative Profile (85 Baxter Road)
Baxter Road Nantucket, Massachusetts
Retreat = 4.6 ft/yr
Average = 14.3 cy/lf
Table 2. Top of Coastal Bank Retreat Rate Data for 85-107A Baxter Road (1994-2013)
Retreat (ft) Rate (ft/yr) Retreat (ft) Rate (ft/yr)
30 107A 46.2 2.4
31 107A 43.9 2.3
32 107A 47.5 2.5
33 107 51.1 2.7
34 107 56.2 3.0
35 107 53.8 2.8
36 107 57.7 3.0
37 107 57.3 3.0
38 105 50.2 2.6
39 105 50.0 2.6
40 105 58.5 3.1
41 105 82.3 4.3
42 105 84.0 4.4
43 105 79.8 4.2
44 105 77.4 4.1
45 105 75.9 4.0
46 105 74.7 3.9
47 101 79.4 4.2
48 101 76.8 4.0
49 101 77.3 4.1
50 101 73.7 3.9
51 101 75.1 4.0
52 101 76.3 4.0
53 101 78.8 4.1
54 101 77.5 4.1
55 101 67.8 3.6
56 Public Access 74.5 3.9
57 99 70.2 3.7
58 99 68.1 3.6
59 99 75.7 4.0
60 99 80.4 4.2
61 99 75.1 4.0
62 99 77.3 4.1
63 99 84.0 4.4
64 99 85.5 4.5
65 99 85.9 4.5
66 97 81.0 4.3
67 97 77.2 4.1
68 97 84.7 4.5
69 97 91.4 4.8
70 97 99.2 5.2
71 97 99.0 5.2
72 97 100.4 5.3
73 97 98.1 5.2
74 93 85.6 4.5
75 93 95.4 5.0
76 93 98.8 5.2
77 93 104.5 5.5
78 93 108.2 5.7
79 91 97.7 5.1
80 91 71.1 3.7
1994-2013 2003-2013 (ft)Transect Lot
Page 1 of 2
Retreat (ft) Rate (ft/yr) Retreat (ft) Rate (ft/yr)
1994-2013 2003-2013 (ft)Transect Lot
81 91 31.9 3.2
82 91 20.5 2.1
83 87 13.2 1.3
84 87 22.8 2.3
85 87 55.1 5.5
86 87 76.8 7.7
87 87 84.5 8.5
88 87 81.1 8.1
89 87 61.6 6.2
90 87 48.3 4.8
91 85 67.7 6.8
92 85 67.4 6.7
93 85 61.0 6.1
94 85 60.6 6.1
95 85 54.9 5.5
96 85 59.1 5.9
97 85 66.8 6.7
98 85 72.3 7.2
99 85 67.3 6.7
100 85 67.2 6.7
101 85 67.9 6.8
102 85 64.3 6.4
103 85 64.5 6.5
Average Bank Retreat Rate by Section 4.0 5.8
Distance weight (#transects/total transects)0.7 0.3
Average Bank Retreat Rate 85-107A 4.6
Page 2 of 2
Table 3. Coastal Bank Contribution Volume for 85-107A Baxter Road
Lot Retreat Rate
ft/yr
Section Volume
cy Lot Length1
ft
Weight
(Lot Length/Total
Project Length)
Volume*Weight
cy
107A 4.6 17.2 71 0.05 0.8
107 4.6 16.9 100 0.06 1.1
105 4.6 16.0 175 0.11 1.8
101 4.6 14.7 200 0.13 1.9
99 4.6 13.9 185 0.12 1.6
97 4.6 13.6 180 0.11 1.6
93 4.6 13.3 98 0.06 0.8
91 4.6 13.3 94 0.06 0.8
87 4.6 13.5 177 0.11 1.5
85 4.6 13.3 294 0.19 2.5
Total Project Length1(ft)1574
14.3
1. Length measured along the +26 MLW contour.
Average Bank Contribution Volume (cy)
Table 4a. WHG Sconset Bluff and Shoreline Change Data for Profiles 90, 90.6, and 91 (2006, 2008, 2013)D (ft) Z (ft, MLW) D (ft) Z (ft, MLW) D (ft) Z (ft, MLW)2006 34.60 -144.19 73.1 -68.59 11.72008 43.30 -154.89 72.31 -76.8 12.232013 50.30 -161.5 74.04 -75.5 9.412006 14.10 -128.49 81.9 -33.59 9.32008 29.50 -135.04 84.4 -27.85 8.932013 36.10 -167.04 84.86 -71.68 9.442006 21.80 -174.24 76.3 -71.65 8.42008 21.70 -174.1 76.3 -77.34 10.62013 26.20 -197.52 76.72 -113.61 9.64D is distance along baseline relative to 0 at benchmarkZ is elevation relative to MLW 1992Table 4E. WHG Sconset Bluff Volume Change Data for Profiles 90, 90.6, and 91 (2006-2013)ProfileDistanceftDistance Weight2006-2013 Bank Contribution Volume1cy90 425 0.254.590.6 639 0.38 17.691 622 0.37 12.4Weighted Bluff Retreat Volume12.41. Determined by calculating that volume associated with the difference in bluff positions from 2006 to 2013.Profile 90.6Profile 9169/71 Baxter Road79/81 Baxter Road91 Baxter RoadTop of BluffToe of BluffShoreline (0-MLW ft)Approximate LocationYearProfile 90
Table 5. Shoreline Change Rates from November 1994 to March 20131Profile Approximate LocationEffective Distance2ftWeight(Effective Distance Total Distance)Shoreline Change Per Profile1 (Nov 1994-Mar2013)ftAverage Annual Shoreline Change ft(Shoreline Change/ 18.4 years)91 91 Baxter6220.43 -96.5-5.291.599/101 Baxter4310.30 -58.9-3.292105 Baxter4040.28-45.4-2.51457Weighted average 85-107A Baxter Road-3.91. From Southeast Nantucket Beach Monitoring, March 2013, 60th Survey Report, prepared by Woods Hole Group, August 2013.2. From FEIR, Sconset Beach Nourishment Project, November 30. 2006. Attachment A, Coastal Planning and Engineering (CP&E) Engineering Design Report, Sconset Beach Nourishment Project, Nantucket, Massachusetts.Total Distance (ft)
Table 6. Update of Coastal Planning & Engineering 1995-2005 Littoral Budget Analysis Profile Approximate LocationEffective Distance2ftShoreline Change Per Profile1 (Nov 1994-Mar2013)ftAverage Annual Shoreline Change ft(Shoreline Change/ 18.4 years)Top of Bank Height2 ft, MLWToe of Bankft, MLWActive Profile HeightftVolume3 (cy)91 91 Baxter622 -96.5-5.2 82 8 74 -894191.5 99/101 Baxter431 -58.9-3.2 90 8 82 -419092 105 Baxter404 -45.4-2.5 102 8 94 -3470Total Volume Eroded from Project Area (CY)-16601Total Volume Eroded from Project Area (CY/LF)-11.41. From Southeast Nantucket Beach Monitoring, March 2013, 60th Survey Report, prepared by Woods Hole Group, August 2013.2. From FEIR, Sconset Beach Nourishment Project, November 30. 2006. Attachment A, Coastal Planning and Engineering (CP&E) Engineering Design Report, Sconset Beach Nourishment Project, Nantucket, Massachusetts.3. Volume determined by multiplying the effective distance * active profile height * average annual shoreline change, then dividing by 27 to convert to cy (per Section 8.0 of CP&E report referenced above in #2).
SOUTHEAST NANTUCKET BEACH MONITORING
March 2013
60th SURVEY REPORT
81 Technology Park Drive
East Falmouth MA 02536
August 2013
Southeast Nantucket Beach Monitoring
March 2013
60th SURVEY REPORT
August 2013
Prepared for:
Siasconset Beach Preservation Fund
18 Sasapana Road
Nantucket, MA 02554
Prepared by:
Mitchell Buck and Robert P. Hamilton, Jr.
Woods Hole Group
81 Technology Park Drive
East Falmouth MA 02536
(508) 540-8080
Woods Hole Group
Siasconset 60th Survey 2000-162 i August 2013
TABLE OF CONTENTS
1.0 INTRODUCTION .................................................................................................. 1
2.0 MARCH 2013 SURVEY AND PROFILES .......................................................... 2
2.1 LAND-BASED SURVEY ............................................................................................. 2
3.0 RESULTS ................................................................................................................ 5
3.1 VOLUME CALCULATIONS ......................................................................................... 5
3.1.1 November 1994 to December 2001 ............................................................ 9
3.1.2 December 2001 to September 2012 ............................................................ 9
3.1.3 September 2012 to March 2013 .................................................................. 9
3.2 SHORELINE CHANGE ANALYSIS ............................................................................. 10
3.2.1 November 1994 to March 2013 ................................................................ 10
3.2.2 December 2001 to March 2013................................................................. 10
3.2.3 September 2012 to March 2013 ................................................................ 11
3.3 WAVE CONDITIONS ................................................................................................ 14
4.0 SUMMARY ........................................................................................................... 16
APPENDIX A ................................................................................................................ A-1
Woods Hole Group
Siasconset 60th Survey 2000-162 ii August 2013
LIST OF FIGURES
Figure 1. Project Location and Profile Map ................................................................... 3
Figure 2. Profile for 90.6 and 91 indicating how the volume calculation region
expanded for the March 2013 profiles. .......................................................... 6
Figure 3. Previous Lighthouse dewatering system sites and project area ...................... 7
Figure 4. MLW shoreline change from November 1994, December 2001, and
September 2012 to March 2013. .................................................................. 13
Figure 5. Time series of wave height for 60th survey period ........................................ 14
LIST OF TABLES
Table 1. Profiles Surveyed (Project area shaded) ......................................................... 4
Table 2. Volume change per profile from Nov. 1994 to Dec. 2001, Dec. 2001 to Sept.
2012, and Sept. 2012 to Mar. 2013 (+ Accretion, - Erosion) ........................ 8
Table 3. Shoreline changes from Nov. 1994, Dec. 2001, and Sept 2012 to March 2013
(Distances seaward from benchmark to 0 ft MLW92 contour) ................... 12
Woods Hole Group
Siasconset 60th Survey 2000-162 1 August 2013
1.0 INTRODUCTION
Woods Hole Group, Inc. was contracted by the Siasconset Beach Preservation Fund
(SBPF) to collect and analyze beach profile data related to the ongoing shoreline
monitoring efforts. This report summarizes the March 2013 topographic survey data,
which is the 60th survey conducted at Siasconset since 1994. WHG prepared similar data
reports beginning with the 23rd survey. Previously, Coastal Planning & Engineering, Inc.
(CP&E) completed more than five-years of monitoring at Siasconset, Nantucket Island,
including 22 reports, after the installation of the initial dewatering systems. Coastal
Stabilization, Inc. (original license holder in US) installed the original systems in August
1994 in an effort to mitigate beach erosion. One of these systems (Lighthouse South-
South) was upgraded during 2001, subject to new permit conditions, as summarized in
the SOC (SE 48-1248), U.S. Army Corps of Engineers (USACE) permit, local OOC,
Waterways license, and CZM Consistency Statement. SOC SE 48-1248 required
quarterly surveys with comparisons against the December 2001 baseline survey. The
dewatering systems were shut down in December 2004, and the 3 years of post-upgrade
surveys required by the SOC SE 48-1248 were completed. Subsequently, the systems
have since been removed. Since this time, the focus of the surveys and reports is not on
the performance of the dewatering system. Instead, surveys are intended to document
beach profile and shoreline change in the region, and to help plan for and monitor
ongoing and future shore protection initiatives.
This report provides comparisons of the recent March 2013 survey to previous data sets
back to 1994. This report summarizes the results of volume and shoreline change
calculations for three time periods:
November 1994 survey through December 2001 (pre-operational period prior to
the system upgrade);
December 2001 through September 2012 (post-upgrade); and
September 2012 through March 2013 (the last survey period).
The survey reports present new beach profile data and compare new beach profiles to
previous data. Volume calculations and shoreline change analysis are provided to reveal
erosion and accretion trends along the beach. This report does not discuss dewatering
system performance or mitigation issues, which are not relevant at this time.
This report is presented in three sections plus one appendix.
Specific information regarding the March 2013 topographic survey and beach
profiles is presented in Section 2.0;
Section 3.0 presents results of the volume and shoreline change calculations;
Profile data are plotted in Appendix A.
Woods Hole Group
Siasconset 60th Survey 2000-162 2 August 2013
2.0 MARCH 2013 SURVEY AND PROFILES
2.1 LAND-BASED SURVEY
Woods Hole Group conducted the 60th beach survey to a depth of -5 MLW from March
27-28th, 2013. Profile locations are shown in Figure 1. The horizontal datum for the
project is the Massachusetts State Plane Coordinate System, Island Zone (1927), and the
vertical datum is MLW, set in 1934 and corrected with 1992 NOAA adjustments by
Blackwell and Associates, Inc. (BAI). Profiles were constructed based on RTK GPS data
collected along the subaerial beach profile and traditional electronic total station survey
data collected in the surfzone. Three geodetic control points were utilized for this survey:
U.S. Coast and Geodetic Survey disk set in a large boulder located near the
intersection of Quidnet and Squam Roads and stamped with the date 1934 and
locally known as “Sugarloaf” (N 111,450.63, E 342,409.99, EL.=40.16 MLW92).
Beach profile Station 84.6, a capped rebar set in a 4” PVC pipe located in the
dune at the intersection of Beach Street and Codfish Park Road (N 96,006.53, E
347,614.23, EL.=12.31 MLW92).
U.S. Coast Guard Disk #1, a brass disk stamped with the date 1961 located across
the street from the entrance to the U.S.C.G. family housing near the Loran tower
at Low Beach (N 92,601.73, E 344,906.23, EL=13.50 MLW92).
Woods Hole Group conducted the March 2013 survey using a Trimble® R7 GPS, a real-
time kinematic global positioning system (RTK GPS). This GPS equipment provides
centimeter-level geodetic positioning. The surveyor navigates to previously established
(but unmarked) beach monitoring benchmarks, and collects topographic profile data
without having to recover and reoccupy beach monuments at each profile. The system
operates by establishing a GPS base station over a known geodetic control point. The
base station communicates via a radio link with a second GPS receiver in a backpack
worn while collecting the survey points on a hand-held data logger. The real-time
horizontal positioning data is used to "steer to" the coordinates of the benchmark for each
profile, and then walk perpendicular to the bank/bluff to collect the profile data. The
RTK GPS equipment limits the surveyor’s ability to wade to -5 MLW due to cabling, and
is incapable of collecting wading shots due to excess movement. To remedy this, a
Topcon GTS-3B electronic total station was utilized to collect the wading profile data.
Table 1 lists the profiles surveyed by BAI for the November 1994 and December 2001
surveys, and the profiles surveyed by Woods Hole Group for the March 2012 survey. All
profiles reached -5 MLW. As explained in Section 3, ongoing erosion in the area
afforded surveys of certain profiles extending landward of earlier 1994 and 2001 profiles,
providing data for more informative volume calculations farther landward compared to
most recent data sets. The “Distance” column in Table 1 represents the landward
distance from the original benchmarks for which volume calculations were made between
the two most recent surveys. Red numbers represent beach profiles for which volume
change was calculated farther landward than in previous reports.
Woods Hole Group
Siasconset 60th Survey 2000-162 3 August 2013
Figure 1. Project Location and Profile Map
Woods Hole Group
Siasconset 60th Survey 2000-162 4 August 2013
Table 1. Profiles Surveyed (Project area shaded)
PROFILE SURVEY DATE
NAME Distance (ft) Nov-94 Dec-01 Sep-12 Mar-13
81 -200
82 -70
82.6 -50 N/A
83 -20
83.5 -50
84 -20
84.3 0
84.6 0
85 0
86 -30
86.5 -223
87 -75
87.4 -146 N/A
87.5 -155
88 -130
88.3 -110
88.6 -110
89 -167
89.2 -98
89.5 -89
89.8 -72
90 -102
90.6 -59
91 -111
91.5 -72
92 -68
92.5 -53
93 -26
93.5 -50
94 -52
95 -54
95.5 -56
96 -33
96.5 -19
96.7 -18
96.9 -5
97 -11
97.3 -15
97.6 -12
98 0
99 0
Q -24
S 0
W -30
N/A Not Available
RED NUMBER = profile using updated volume calculation windows
Woods Hole Group
Siasconset 60th Survey 2000-162 5 August 2013
3.0 RESULTS
3.1 VOLUME CALCULATIONS
Volume calculations were performed using Matlab, and are presented in this report for
these time periods:
November 1994 to December 2001 (the dewatering system pre-operational
period);
December 2001 to September 2012 (the period from dewatering system activation
through the last survey);
September 2012 to March 2013 (the duration since last survey).
These surveys characterize volume change in the profile from the seaward position of the
–5 ft isobath, landward to the toe of the dune (Xon). Volume calculations were computed
from a landward limit (“baseline distance”), as specified in Table 1, to an offshore depth
of –5 ft MLW. This baseline distance location was determined based on the toe of the
bank locations for the December 2001 pre-operational survey (where applicable) or as far
back as data were available for comparison with other surveys. Specific profiles were
also translated horizontally to account for the movement of the benchmarks over time as
the beach eroded in certain places (i.e., the 0 point in the field is the stake location, which
had changed). Some of these translations are cumulative since December 2001, since
five benchmarks were relocated between December 2002 and March 2003 (profiles 81,
87.5, 88.3, 91, and 93) as documented in the 32nd report. A different set of baseline
distances was specified for comparisons with November 1994, since surveys at that time
did not extend landward of the benchmarks (original baseline). For profiles 91 and 91.5,
the baseline distance was modified from 0 ft to -20 ft because the ground survey in
December 2001 did not extend landward beyond the toe of dune.
More recently, progressive erosion of the profiles since 2001 has resulted in a scenario
where the active portion of certain profiles retreated landward of the baseline distance
within which prior volume calculations are made. Figure 2 shows an example for
profiles 90.6 and 91; the vertical dashed lines indicate the region within which volume
calculations were made in this and prior reports. Prior to 2001, the “Old” area shown in
Figure 2 represented the active profile; however, prevailing erosion produced a scenario
where recent volume calculations limited to within the Old baseline distance do not
represent overall profile change, since a significant portion of the active berm extends
landward of the Old baseline. For instance, volume change for several profiles known to
have eroded substantially would result in a positive volume change calculation indicating
accretion if limited to the Old baseline distance. This trend exists for other profiles, but is
not consistent across all profiles. Based upon discussions with SBPF, it was determined
that volume calculation will now be extended landward as needed to more accurately
represent beach volume change. The seaward limit of -5ft MLW isobath was maintained,
while the landward limit of the profile was extended as far landward as practical to
compare recent profiles (“New” distance shown by Figure 2). The adjusted profiles are
highlighted red in column two of Table 1. The New results are not directly comparable
Woods Hole Group
Siasconset 60th Survey 2000-162 6 August 2013
to calculations made for prior time periods in previous reports, but are more
representative of recent dynamic beach response.
Figure 2. Profile for 90.6 and 91 indicating how the volume calculation region
expanded for the March 2013 profiles.
Volume and shoreline change were calculated for the profiles in the entire monitoring
area (profiles 81 to W), and the narrower project area as defined in the modified SOC.
The project area is defined as the area extending from profile 89.2 through profile 92.5
(Figure 3). The mitigation areas, 1,000 ft to both sides of the previous Lighthouse South-
South dewatering system site, are included in the definition of the project area. Profiles
90, 90.6 and 91 are used to calculate the treated area changes, profiles 89.2, 89.5, 89.8, 90
and 90.6 are used to calculate the south mitigation area changes, and profiles 90.6, 91,
91.5, 92, and 92.5 are used to calculate the north mitigation area changes. Although the
dewatering system is no longer performing, these “project” and “mitigation” area
definitions are maintained for consistency and comparison to past reports.
Old
New
New
Old
Woods Hole Group
Siasconset 60th Survey 2000-162 7 August 2013
Table 2 lists the volume change for each profile station from November 1994 to March
December 2001, December 2001 to September 2012, and September 2012 to March
2013. Results are summarized below.
Figure 3. Previous Lighthouse dewatering system sites and project area
Woods Hole Group
Siasconset 60th Survey 2000-162 8 August 2013
Table 2. Volume change per profile from Nov. 1994 to Dec. 2001, Dec. 2001 to Sept.
2012, and Sept. 2012 to Mar. 2013 (+ Accretion, - Erosion)
VOLUME CHANGE PER PROFILE
PROFILE Nov-94 to Dec-01
cy/ft
Dec-01 to Sept-12
cy/ft
Sep-12 to Mar-13
cy/ft
81 -69 5.8 26.8
82 -31.7 12.9 6.2
82.6 N/A 13.3 3.9
83 47.7 22.4 -1.8
83.5 37.6 65.2 -16.5
84 11.8 75.1 -28.9
84.3 14.1 59.4 -31.9
84.6 36.4 0.6 -5.1
85 39.4 -8.1 -25.7
86 4 -16.9 -14.3
86.5 -27.1 -27.8 -20.0
87 -56 -14.5 -22.2
87.4 N/A -13.4 -25.7
87.5 -50.4 -18.8 -32.5
88 -41.5 -33.9 -29.5
88.3 -48.5 -30.8 -23.9
88.6 -48.8 -24 -23.6
89 -55.5 -18 -19.1
89.2 -60.7 -11.2 -21.1
89.5 -65.2 -14.5 -12.8
89.8 -67.9 -11.3 -11.2
90 -61.5 -9.9 -8.8
90.6 -51.6 -11.9 -6.5
91 -42 -30.1 5.6
91.5 -21.1 -36.6 6.9
92 -12.5 -21.9 6.5
92.5 -21.1 -4.4 -4.7
93 -30.9 1.3 -6.5
93.5 -35.7 2.6 -8.7
94 -25.9 -10 -0.5
95 -25.3 -11.2 -8.2
95.5 -33.2 -23.1 -4.2
96 -6.2 -13.9 -5.4
96.5 -1.9 2.8 -9.7
96.7 -2 3.8 -3.0
96.9 -2.1 14.8 -11.6
97 -7.2 20.9 -5.9
97.3 -3.1 15 -6.0
97.6 3.4 8.5 -2.8
98 -0.3 13.3 -6.6
99 -1.9 25.8 -7.7
Q 6.7 -0.3 -2.4
S 21.4 19.6 -7.9
W 16.5 16.9 1.7
(Project area shaded) (N/A: Not Available)
RED NUMBER = profile using updated volume calculation windows
Woods Hole Group
Siasconset 60th Survey 2000-162 9 August 2013
3.1.1 November 1994 to December 2001
This dewatering system preoperational period extends from the November 1994 (the
earliest pre-construction survey) to the December 2001 survey (Table 3).
Overall, 31 of the 42 profiles eroded since November 1994 (Note profiles 82.6
and 87.4 did not exist in November 1994).
The central portion of the monitoring area eroded (profile lines from 86.5 through
95.5), from just north of Codfish Park to Sesachacha Pond). Maximum erosion
was focused between profiles 87 and 91, where total erosion since 1994 exceeds
40 cy/ft; with a maximum of 68 cy/ft of erosion at profile 89.8.
The southernmost profiles, characterized by profiles 83 through 86, accreted with
the exception of profiles 81 and 82. Maximum accretion was more than 47 cy/ft
at profile 83.
The beach has been relatively stable and even accreting over the long-term from
profiles 96 through W.
In the project area, all profiles from 89.2 to 92.5 eroded between 12 and 67 cy/ft
in over 7 years since November 1994.
3.1.2 December 2001 to September 2012
This period extends from the activation of the dewatering system through the last survey
in September 2012. Table 3 presents volume change for the monitoring area.
The monitoring area performed as follows:
Overall 24 transects eroded during the reporting period.
The southern portion of the monitoring area, profile 81 through profile 84.6,
gained sediment over the past 11 years.
Maximum accretion occurred at profile 84, where more than 75 cy/ft of sediment
accumulated in the past 11 years.
The central portion of the study area, between profiles 85 through 92.5 eroded
Maximum erosion of more than 36 cy/ft occurred at profile 91.5.
In the northern reach beach volume changes from profile 96.5 to W were
generally positive (0 to 25 cy/ft of accretion).
In the project area, all profiles from 89.2 to 92.5 eroded between 4 and 36 cy/ft in
11 years since December 2001.
3.1.3 September 2012 to March 2013
This period spans the duration since the last survey in September 2012. Table 3 presents
the results. The volume change calculation was adjusted for a number of profiles
(highlighted in red in Table 3) as discussed in Section 3.1.
Woods Hole Group
Siasconset 60th Survey 2000-162 10 August 2013
The monitoring area performed as follows:
Of the 44 profiles surveyed in the monitoring area, 37 profiles eroded, and 7
profiles accreted since September 2012; erosion was the dominate trend for most
profiles since the last survey.
Maximum erosion occurred at profile 87.5, which eroded more than -31 cy/ft and
maximum accretion occurred at profile 81, which gained 26 cy/ft.
Erosion was concentrated between profiles 83.5 and 89.8, where erosion ranged
from 5 cy/ft and up to 31 cy/ft.
In the project area, three profiles accreted and six profiles eroded with a
maximum erosion of -21 cy/ft.
3.2 SHORELINE CHANGE ANALYSIS
Woods Hole Group evaluated shoreline change (retreat or advance of the mean low water
line) to provide qualitative insight regarding beach response in the project vicinity. This
section provides a comparison of shoreline changes since November 1994 for the
monitoring area for the three periods under investigation.
Shoreline distances were measured from the baseline horizontally to the 0 ft MLW92
contour level. This elevation was selected for comparison with prior reports. These
surveys included comparisons between the earliest survey of November 1994, the pre-
operation survey of December 2001, the last survey in September 2012, and the latest
March 2013 survey. Table 3 lists shoreline change by profile for the surveys under
investigation. Figure 4 illustrates the change in the shoreline positions.
Results can be summarized as follows:
3.2.1 November 1994 to March 2013
In general, the shoreline advanced in the southern portion of the monitoring area
(profiles 82 to 85), retreated substantially in the middle (profiles 86 to 96.7), and
was relatively stable or accreting at the northern portion (profiles 96.9 to W).
Maximum shoreline advance occurred between profiles 83 and 84.6, where the
shoreline advanced more than 65 ft, and as much as 150 ft at profile 83.5.
Maximum shoreline retreat occurred between profiles 86.5 and 91, where the
shoreline retreated more than 87 ft and as much as 134 ft at profile 88.3.
3.2.2 December 2001 to March 2013
Although there has been more variability, the shoreline change trend since
December 2001 is similar to the trend since 1994. The southern and northern
limits accreted while the middle of the monitoring area eroded.
An exception to the trend is at the very southern end (profiles 81 and 82) where
the overall trend of erosion since 1994 has been accreting since 2001.
Shoreline advance since December 2001 occurred between profiles 81 and 84.6,
with a maximum shoreline advanced of 113 ft at profile 81.
Woods Hole Group
Siasconset 60th Survey 2000-162 11 August 2013
Shoreline retreat since December 2001 occurred between profiles 85 and 96.5,
with a maximum shoreline loss of 63 ft along profile 88.
3.2.3 September 2012 to March 2013
The shoreline advanced (29 of 44 profiles) between profiles 81 and 83.5 and 89.5
and W since the last survey.
Maximum shoreline advance in the past seven months occurred at profile 81,
accreting 114 ft.
Erosion was focused between profiles 84 and 89.2 with maximum retreat of 24 ft
and 25 ft at profiles 87.5 and 84.3.
In the project area the shoreline along all profiles, except 89.2, advanced likely
due to a portion of sediment eroded from the bluffs remaining on the beach.
Woods Hole Group
Siasconset 60th Survey 2000-162 12 August 2013
Table 3. Shoreline changes from Nov. 1994, Dec. 2001, and Sept 2012 to March
2013 (Distances seaward from benchmark to 0 ft MLW92 contour)
PROFILE
SHORELINE
CHANGE
PER PROFILE
Nov-94 to Mar-13
ft
SHORELINE
CHANGE
PER PROFILE
Dec-01 to Mar-13
ft
SHORELINE
CHANGE
PER PROFILE
Sept-12 to Mar-13
ft
81 -15.7 113.0 114.1
82 8.9 52.7 46.9
82.6 N/A 42.9 37.8
83 129.0 43.9 22.7
83.5 150.2 84.7 0.2
84 119.5 100.5 -19.4
84.3 88.4 64.6 -25.6
84.6 65.8 13.6 17.3
85 32.0 -30.3 -14.8
86 -33.3 -38.6 -4.1
86.5 -87.2 -40.8 -1.1
87 -120.8 -26.5 -0.8
87.4 N/A -32.7 -16.1
87.5 -133.3 -53.3 -24.3
88 -131.0 -63.0 -13.1
88.3 -134.8 -53.7 -9.1
88.6 -130.6 -44.4 -7.7
89 -129.7 -33.4 -1.9
89.2 -125.9 -27.6 -6.8
89.5 -115.0 -15.6 18.2
89.8 -117.6 -10.5 14.9
90 -121.9 -14.1 8.6
90.6 -103.5 -21.6 5.0
91 -96.5 -6.6 30.2
91.5 -58.9 8.4 37.9
92 -45.4 -27.1 26.2
92.5 -41.3 -0.7 11.8
93 -47.3 -2.8 2.5
93.5 -66.8 -2.0 1.4
94 -50.8 -10.2 13.4
95 -64.7 -22.7 3.2
95.5 -76.1 -40.6 3.1
96 -42.5 -10.5 15.0
96.5 -6.2 -1.1 0.9
96.7 -0.9 6.2 8.1
96.9 4.6 7.9 -3.5
97 13.2 22.9 -0.5
97.3 13.6 18.9 7.2
97.6 14.3 10.2 8.0
98 4.5 5.7 2.5
99 23.7 24.3 1.2
Q 3.7 4.3 11.8
S 33.2 12.9 1.3
W 25.8 22.3 12.0
(N/A : Not Available)
Woods Hole Group Siasconset 60th Survey 2000-162 13 August 2013 Figure 4. MLW shoreline change from November 1994, December 2001, and September 2012 to March 2013.
Woods Hole Group
Siasconset 60th Survey 2000-162 14 August 2013
3.3 WAVE CONDITIONS
The 60th survey is defined by the time period of September 12, 2012 through March 30,
2013. Wave data for this time period was obtained from the Woods Hole Oceanographic
Institution’s Martha’s Vineyard Coastal Observatory (MVCO), located approximately 1.5
kilometers south of Edgartown Great Pond in 12 meters of water. The MVCO collects
wave data every 20 minutes and functioned with a 98% data return for the period.
Although the MVCO data is not entirely representative of nearshore conditions at
Siasconset (due to partial sheltering of the MVCO from waves arriving from the East to
Northeast) the MVCO is the only source for measurements of the directional distribution
of waves in the region. At the location of the MVCO, waves arrive primarily from West-
Southwest to East-Southeast, with the majority arriving from the South. This is expected
since the waves are becoming more shore-normal as they approach the southern-facing
shoreline of Martha’s Vineyard. Wave data were also obtained from the National
Oceanic and Atmospheric Administration’s National Data Buoy Center (NDBC) Station
44008. This station recorded data for a 20-minute sampling period every hour and was
located 54 nautical miles southeast of Nantucket Island in 62.5 meters of water. NDBC
Station 44008 achieved a data return of 99.5% for this sampling period; however, the
station went adrift on 2/9/13 and data recorded after this date are not included in Figure 5,
nor incorporated elsewhere in this report. Both data sets were processed to evaluate wave
characteristics and storm events for the period of interest.
Figure 5. Time series of wave height for 60th survey period
Woods Hole Group
Siasconset 60th Survey 2000-162 15 August 2013
Time series of wave height data for the period show a variety of storms during the 60th
survey period. Both the NDBC Station 44008 and MVCO data are shown in Figure 5,
indicating which storms observed in the offshore data had an impact on the islands of
Nantucket and Martha’s Vineyard. There were approximately thirty (30) events when
wave heights exceeded 1 meter at the MVCO location for an extended duration. The
most significant storm was Hurricane Sandy from October 22-31st that generated waves
over 4 m at the MVCO station and 10 m at NDBC Station 44008. The overall energy-
weighted average wave height for the time period was 1.3 meters at the MVCO location
and 2.67 meters at the offshore NDBC buoy. These heights are indicative of energetic
wave conditions for the winter season.
Woods Hole Group
Siasconset 60th Survey 2000-162 16 August 2013
4.0 SUMMARY
From the analysis of the data collected for the 60th survey (March 2013), the following
summary can be made
Significant erosion of the beach, dunes, and bluff was visually observed during
the March 2013 survey.
An analysis of the wave data between the September 2012 and March 2013
indicate this time period was energetic with the overall energy-weighted average
wave heights of 1.3 meters at the MVCO location and 2.67 at Station 44008.
Beach volume change calculations were made between November 1994 and
December 2001, December 2001 and September 2012, and September 2012 to
March 2013. This is a departure from previous volume change calculations that
were made comparing the historical results to the most recent survey. In addition,
the region for the volume calculations for the September 2012 to March 2013 was
adjusted for a number of profiles. As a result, these volume calculations are not
directly comparable to previous reports.
Between these three monitoring periods, the general trend for volume and
shoreline change demonstrated the northern and southern portions of the
monitoring area accreted, while the middle portions of the monitoring area
eroded.
The most recent survey shows erosion of more than 10 cy/ft between profiles 83.5
and 88.6 since September 2013 with a maximum of 32 cy/ft at profile 89.8.
Since September 2012, only six of the nine profiles lost beach volume, and eight
profiles exhibited shoreline advance up to 37 ft. This may seem counterintuitive
based on the long-term erosional trends for the project area, and given the
significant amount of regional erosion observed this winter. However, it appears
that a significant portion of sand eroded from the bluffs was deposited in the surf
zone between MLW and the -5 ft MLW contour. The profile comparison figures
in Appendix A illustrate this trend, and show the amount of material deposited
below MLW resulted in shoreline advance, and even an overall gain in beach
volume since September 2012 for certain profiles.
8.0 LITTORAL BUDGET
Waves and currents are the forces that transport beach sediment along the coastline, and are the
forces of beach erosion in some instances. A "littoral budget" is an assessment of the magnitude
and direction of sediment transport in the project area. The Sconset Beach littoral budget is
based on annual cumulative sand volume change rates at each profile line. The littoral budget
was developed for the time period of December 1995 through December 2005, which also
coincides with the period of highest shoreline recession rates.
The average shoreline recession rate was multiplied by the active profile height and effective
distance between profiles to develop a volume change rate at each profile line (Table 10). (It
would have been preferable to use the measured volumetric change but the lack of profile closure
necessitated using shoreline changes). The top of the active profile height varies along the
project length, peaking at a height of 110 feet at profile 92.5. (The active profile height (APH) is
the distance from the top of the active profile to the depth of closure). Light Detection and
Ranging (LIDAR) topographic mapping data was used to determine the top of the active profile
height in the highly eroding bluff area (profile 90.6 to 95). The bluff elevation was later
confirmed when the August 2006 survey data became available. In the areas where there is a
highly vegetated dune landward of the beach, the top of the active profile height was taken at the
base of the vegetation. A vegetated dune indicates that it has been stable Jong enough to vegetate
and is therefore not considered part of the 'active' profile height. The depth of closure (Section
5.7) was assumed to be -26 ft MLW along the entire project length.
Once the volume change was established, the percentage of silt at each profile line was removed
from the total volume change, in order to account exclusively for the longshore movement of
coarse grained material (sand and gravel). It was assumed that the silts were put into suspension·
by wave activity and swept out of the project area. The percentage of silt was determined by an
elevation weighted average of the silt content from 298 cross-shore samples. The cumulative
sand volume changes represent the littoral transport rates, as summarized in Table 10.
The total Jongshore transport could then be estimated by summing the volumetric changes at
each line using the sand conservation equation. A starting point for this summation had to be
assumed. A typical method of determining this starting point is to identify the point where there
is no net sediment transport, which is termed the nodal point. The nodal point is typically
located at the location experiencing the highest shoreline recession rates, as there is no sediment
being supplied from an updrift source. The largest shoreline recession rate occurs at profile 88.6,
which suggests that this is the location of the nodal point. Once the nodal point is located the
volumes are then summed going away from this point to provide the littoral transport rate. The
sign indicates the direction of transport so positive transport is to the south and west while
negative transport is to the north.
25
COASTAL PLANNING & ENGINEERING, INC
Table 10. Littoral Budget between 1995 and 2005
Profile Effective Top of DOC APH Sho reline Volume Percent Sand Vol Littoral
Distance APH Change Change Silt Change Transport
(ft) (ft) (ft) (ft) (ft/yr) (cv/vr) (%) (cy/yr) (cv/vr)
81 536 11 -26 37 -4.5 _-3!~00 0.3 -3,300 :20,500 -· ---··--·-
82 865 15 -26 41 7.9 10,300 0.3 10,300 -23,800
82.6 .. 546 13 -26 39 13.2 10,400 0.2 10,400 -13,500
83 522 15 -26 41 18.6 14,700 0.3 14_.?00 -3, 100 -----
83.5 529 15 -26 41 22.4 18,00Q_ 0.3 17,900 11,600
84 444 15 -26 41 18.4 12,400 0.3 12,400 I 29,500
84.3 396 15 -26 41 12.3 1.~90 0.2 7.409 41,900
84.6 351 13 -26 39 8.0 4,000 0.2 4,000 49,300
85 398 13 -26 39 3.9 2,200 0.2 2,200 53,300
86 441 13 -26 39 -0.7 -500 0.2 -500 55,500
86.5 435 14 -26 40 -6.5 -4J_QO 0.2 '·· -4,200 55,000 --. -
87 436 14 -26 40 -13.0 :8.400 0.3 -8,400 50,800 . . ·-· -· - -87.4 .. 351 15 -26 41 -12.0 -6,4QO 0.2 -6,400 42,400
87.5 533 16 -26 42 -11.0 -9!.~QQ 0.2 -9,200 36,000
88 514 18 -26 44 -18.2 -!_~.JOO 0.1 -15,200 26,800
88.3 223 18 -26 44 -20.6 -7,500 0.2 -7,500 11,600
88.6 223 18 -26 44 -22.6 -8,209 0.2 -8,200 0
89 248 15 -26 41 -20.7 . _-7t?.QO 0.2 -7,8.00 --~ 1,900 -· .,, --· 89.2 273 15 -26 41 -15.4 -6__.400 0.2 -6AOO -18,300 --
89.5 I 273 15 -26 41 -16.4 -6._800 _ 0.2 -6,800 -25,100
89.8 276 15 -26 41 -14.3 -61.QOQ_ 0.6 -6,000 _-31 ,100 -----·--
90 425 14 -26 40 -12.6 -7,90Q_ '-0.6 -7,900 :39,000
90.6 639 85 -26 111 -8.0 -21,100 7.6 -19,500 -58,500
91 622 82 -26 108 -7.2 -17,900 15.1 -15,200 -73,700 ---
91.5 431 90 -26 116 -3.8 -?.000 1.7 -6,9Q_O -80,600 ··---
92 404 102 -26 128 -4.3 -8,200 1.7 -8, 100 -88,700
92.5 483 110 -26 136 -2.0 -4!_?._Q9_ 6.2 -4,500 -93,200 --
93 393 108 -26 134 -8.0 -15!609 3.6 -1?.0.00 -108,200 . -----.
93.5 500 96 -26 122 -6.1 -13..._~Q_O 4.6 -13,109 ... -121,300 --. ·-.. --· -94 740 72 -26 98 -6.0 -16,200 3.1 -15,700 -137,000
95 802 57 -26 83 -4.9 -~2.200 9.4 -11,100 -148,100
95.5 888 42 -26 68 -4.2 -:9.500 4.2 -9,10Q -_157.!200 --.
96 979 26 -26 52 0.9 1,700 1.7 1,700 -155,500
96.5 606 26 -26 52 -1.0 -1,200 1.3 -1,200 -156,700
96.7 208 28 -26 54 0.9 400 0.6 400 -1_56,300 -· ----· . --·-96.9 210 24 -26 50 1.2 500 0.6 500 -155,800 --··-·-... . -·-. -· . -
97 272 25 -26 51 1.0 500 0.6 500 -155,300
97.3 350 24 -26 50 0.3 200 0.4 200 -155,100 -. ---·--. ,. ---~----· ------.
97.6 408 24 -26 50 0.1 100 0.4 100 -155,000
98 722 20 -26 46 -0.5 -600 0.4 -600 -155,600
99 1,225 21 -26 47 -0.4 -800 0.4 -800 -156!400 -----a 2,98_5 -23 -26 49 -0.3 -1.~00 0.4 -1,800 -158,200 -----s 5,159 18 -26 44 2.5 20,700 0.4 20,600 -137,600
w 2,900 21 -26 47 1.7 8,300 0.4 8,300 -129,300
26
COASTAL PLANNING & ENGINEERING, INC
A plot of the littoral transport curve is shown in Figure 8. This figure shows that from profile 86
through to profile 95.5 that the littoral transport rate is increasing, which is indicative of an
erosional area The graph al so shows that north of profile 95.5, the littoral transport rate curve is
relatively flat so the volume of material entering the section is similar to the volume leaving this
section and the beach is stable. North of profile Q, the sediment transport rate is decreasing,
which is indicative of an accretional area.
eo.ooo
«l.000
f 20.000
~'
86
63,300 cy/yr 85 ...
I .
~ s 0 tl j ~ ! -20.000 j 81 82
~ j -«l.000
z. ~ § -eo.ooo
'.::;' ~ s -80.000
~ }-100.000
...J
l -120.000
@:.
-1«l.OOO
. w
: .... 166,800 cy/yr
• .
96 97 98 99 a
-1eo.ooo L-------------~~~~~====2::'.::::_ ______ J
PROFILE LINE
Figure 8. Annual Littoral Transport (Dec 1995 to June 2005)
The southern end of the project is located at profile 85. At this location the sediment transport
rate is approximately 53,300 cy/yr being transported to the southwest. The northern end of the
project has a sediment transport rate of 155,600 cy/yr being transported to the north. Therefore,
the total net loss due to longshore transport is approximately 208,900 cy/yr.
27
COASTAL PLANNING & ENGINEERING, INC
Attachment B
Comparison of Retreat Rates at 79 Baxter Road and Nearby Properties
BAXTER ROADSANKATY ROADSANKATY HEAD ROADBAYBERRY LANE
85
99
97
87
73
101
83
105
109
81
79
71
93
91
77
117
75
115
113
119
107
107A
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP,
swisstopo, and the GIS User Community
G:\Projects\Lighthouse\2013\ConCom\Retreat\Revised\retreat_lots_71-119v2.mxd
Figure 1
Baxter Road Nantucket, MA
LEGEND
Basemap: 2013 Aerial Imagery, Col-East, Inc.
1994 Top of Coastal Bank
2003 Top of Coastal Bank
2013 Top of Coastal Bank
Parcel Boundary
°0 100 20050
Feet1 inch = 200 feet
Scale 1:2,400
1994-2013 Average Retreat for Lots 91-107A = 4.0 Feet/Year
2003-2013 Average Retreat for Lots 85-91 = 5.8 Feet/Year
Overall Average Retreat for Lots 85-107A = 4.6 Feet/Year
Average Coastal Bank Retreat Summary
Coastal Bank Retreat
Table 1. Top of Coastal Bank Retreat Rate Data for 75-83 Baxter Road
Transect Lot 2003-2012 (ft) 2012-2013 (ft)
83 Baxter Road
104 83 48.41 18.60
105 83 46.68 6.14
106 83 39.90 4.50
107 83 38.48 5.42
108 83 40.25 4.83
109 83 43.52 5.54
110 83 38.99 5.91
Avg Rate 83 4.70 7.28
81 Baxter Road
111 81 45.01 8.99
112 81 39.41 12.23
113 81 33.91 15.30
114 81 27.02 14.71
115 81 23.69 21.08
Avg Rate 81 3.76 14.46
79 Baxter Road
116 79 5.22 30.65
117 79 NA 20.87
118 79 NA 14.36
119 79 NA 12.22
120 79 1.68 6.29
Avg Rate 79 0.38 16.88
77 Baxter Road
121 77 10.64 6.31
122 77 3.68 8.93
123 77 6.04 15.44
124 77 11.35 19.33
Avg Rate 77 0.88 12.50
75 Baxter Road
125 75 21.25 11.07
126 75 25.40 12.19
127 75 18.98 11.57
128 75 22.43 12.20
Avg Rate 75 2.45 11.76
Q1
Q2
Q3
S
Q
81
83
84
85
82
86
87
88
89
90
91
92
93
94
95
96
9798
99
W
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP,
swisstopo, and the GIS User Community
G:\Projects\Lighthouse\2013\WHG_Profiles\fig1_coast_area.mxd
Figure 1WHG Survey Lines - Wauwinet to Sewer Beds
Baxter Road Nantucket, MA
LEGEND
Basemap Imagery: 2011 ESRI
Project Area Properties
WHG Survey Line
New WHG Survey Line
°0 1,100 2,200550
Feet1 inch = 2,200 feet
Scale 1:26,400
WAUWINET
SEWER BEDS
SQUAM
QUIDNET
SIASCONSET
SesachachaPond
AtlanticOcean
LIGHTHOUSE
see Figure 2for zoom-in toProject Area
BAXTER
ROADSANKATY ROADSANKATY HEAD
ROADISABELLE'S WAYPOL
P
I
S
R
O
A
D
ISOBELS WAY
ANNES LANE
BAYBERRY LANE
ELDRIDGE LANE 90.5
92.4
92.3
92.2
90.8
90.70
0
700
600
500
400
300
200
100
700
600
500
400
300
200
100
800 feet
800 feet
88
89
9091
92
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94
88.3
88.6
89.2
89.8
89.5
91.5
93.5
63
69
71
67
65
85
99
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87
73
83
101
105
109
81
79
93
91
77
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75
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107A
90.6
92.5
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP,
swisstopo, and the GIS User Community
G:\Projects\Lighthouse\2013\WHG_Profiles\fig2_project_area.mxd
Figure 2WHG Survey Lines - Project Area
Baxter Road Nantucket, MA
LEGEND
Basemap Imagery: 2013 Col-East/2011 ESRI
Participating Properties
WHG Survey Line
New WHG Survey Line
°0 200 400100
Feet1 inch = 400 feet
Scale 1:4,800
SHEET NAMEREVISIONSDATEPROJECT NO.DESIGNEDRSDSCALEDRAWNSMWCHECKED--OCT. 25, 20132967-11HOR: 1"=20'VERT: 1"=20'TYPICAL FLANKING DETAIL - TEMPORARY SLOPE STABILIZATIONNANTUCKET, MASSACHUSETTSBAXTER ROADSLOPE STABILIZATION0'10'20'01/2"1"TYPSHEET NO.5 OF 1111/19/2013REVISED DETAIL
Submitted To:COTTAGE & CASTLE, INC.
Address:37 OLD SOUTH RD. UNIT 6
City, State, Zip:NANTUCKET, MA 02554
Date:Tuesday, November 19, 2013
Job Name:SBPF - GEOTUBE REMOVAL
Location:BAXTER RD
Job Description:JOBNO:3435
Proposal
THE COST TO:
$218,000.00REMOVAL AND DISPOSAL OF 15OO LF OF GEOTUBE FROM SANKATY BEACH. REMOVAL
WOULD TAKE PLACE FROM THE TOP DOWN, ANY SMALL SECTIONS OF THE TUBE REMAINING
WOULD BE EXCAVATED AND RUN THRU A TROMMEL SCREEN TO ASSURE THAT ALL OF THE
TUBE MATERIAL IS REMOVED FROM THE SITE. ACCESS WOULD BE NEEDED AT HOICKS
HOLLOW.
1-
Payment is to be made as follows:
$218,000.00We Propose hereby to furnish material and labor in accordance with the above specifications for the sum of:
BILLING IS WEEKLY. PAYMENT IS DUE WITHIN TEN DAYS.
All material is guaranteed to be as specified. All work is to be completed in a professional manner according to standard practices. Any alteration or deviation from the
above specifications involving extra costs will be executed only upon written orders and will become an extra charge over and above the estimate. All agreements
contingent upon strikes, accidents or delays beyond our control. Owner shall carry all risk property insurance, to include but not limited to, fire, windstorm and flood coverage. Our workers are fully covered by Workman’s Compensation Insurance. We carry $500,000 of care, custody and control legal liability insurance for of any
building being moved. Coverage in excess of this amount is the sole responsibility of the Owner.
After ten (10) days, unpaid invoices accrue interest at a rate of 1 1/2% per month (18% per year). If Owner fails to make payment(s) as required under this Proposal or
commits any other breach or default under this Proposal, Toscana shall have the right to immediately cease all work to be performed and to recover from Owner all
moneys owed to Toscana for any work performed as well as any costs and expenses incurred by Toscana including reasonable attorney's fees, as a result of any
default or non-payment.
Note: This proposal may be withdrawn by us if not accepted with 30 days.
Acceptance of Proposal - The above prices, specifications and conditions are satisfactory and are hereby accepted. You are authorized to do the work
as specified. Payment will be made as outlined above.
Authorized Signature:
Signature: Date of Acceptance:
Page 1 of 1Toscana Corporation Proposal Printed: 11/19/2013
Emergency Management & Marine Safety
Town of Nantucket D. F. Fronzuto
4 Fairgrounds Road Coordinator Nantucket, MA 02554 508-325-4100 X 7007 dfronzuto@nantucket-ma.gov 508-228-7246 FAX
From: Emergency Management Coordinator, DPW Director
To: Town Manager
Date: November 19, 2013
RE: Emergency Monitoring of Baxter Road
The intent of this plan is to protect the health and safety of the public, including residents travelling on Baxter Road,
north of Bayberry Lane.
1. Monitor developing weather prior to any winter storm.
2. If extended periods of wind in excess of 50 MPH and rain/snow are forecast, erect barricades to block traffic in
the north bound lane of Baxter Road
3. Monitor distance from stakes installed by SBPF representative. Stakes to be placed 25’ from the eastern edge of
the bluff.
4. It is recommended that when the distance from the edge of the road to the edge of the bluff
is less than 25’ the road be closed to everyone except residents
5. Some widening of the road may be necessary on the west edge of the road within the road layout if additional
width is needed to maintain the 25’ separation.
6. Milone & MacBroom continue to recommend development of an alternative access plan for Baxter Road.
7. Install orange construction fencing with appropriate signs reading “Danger” “Stay Back from Bluff Edge”
8. Visual observations of distances and conditions will be made prior to during and post storm events
Notification ( Town Council to prepare) should be made to property owners north of Bayberry Lane that the closure of
Baxter Road may occur prior to and during storm events and that additional failure of the slope may cause there to be a
permanent closure.
Comments Regarding the Sconset Emergency Project
Dr. Michael S. Bruno, Dean, School of Engineering and Science, Stevens Institute of
Technology
November 25, 2013
The proposed project, comprised of 4 tiers of overlapping geotubes, will in my view
provide protection against bluff toe erosion during expected average and extreme
events.
The fact that the lowest, seaward geotube will be based at Mean Low‐Water (MLW)
will help assure the stability of the structure. The elevation of the crest of the
structure at +26' MLW, including scour apron and sand covering, will in my opinion
significantly reduce the potential bank loss from wave run‐up.
The characteristics of the geotextile material, coupled with the size of the geotubes
(~200' long, 45' in circumference) are such that there is minimal risk that the
structure will fail in a fashion that hazardous debris will be introduced to the marine
environment.
The proposed commitment to retain a volume of "sacrificial" sand at the geotube
structure, combined with ongoing monitoring of and adaptive response to the near
and far‐field impacts of the structure on the existing beach cross‐sections, is in my
view an effective strategy to avoid the potential negative impacts to the shoreline
associated with eliminating the bluff as a source of sand for the fronting and down
drift beaches. It is my understanding that the volume of sacrificial sand to be placed
is based on the approximately 18‐year average bluff recession rate. I believe that
this is a sound strategy to prevent adverse impacts to the adjacent and downdrift
shoreline areas associated with the loss of this sand source. It is also my
understanding that the planned beach monitoring program will be employed to
assess any possible sand loss adjacent to the structure, and that sand will be placed
in these areas at the earliest possible time following an erosion event. Taken
together, the commitment to replace the source of sand represented by bluff
erosion, and to ensure continued monitoring and maintenance of the beach areas
fronting and adjacent to the geotube structure are in my view effective strategies to
avoid any adverse impacts due to the proposed geotube structure. I support the
very simple approach of replacing the full volume of sand lost due to the elimination
of the bluff as a sand source, because any attempt to tie the volume of placed sand
to measured or modeled net sediment transport is in my view prone to error,
because of the complex and highly variable sediment transport system in this region.
MICHAEL S. BRUNO
DEAN, SCHOOL OF ENGINEERING AND SCIENCE
STEVENS INSTITUTE OF TECHNOLOGY
HOBOKEN, NJ 07030
TEL: 201.216.5338
www.stevens.edu
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Table of Contents
ALTERNATIVES ANALYSIS 1
1.0 Overview 1
2.0 No Action Alternative 2
2.1 Description of No Action Alternative 2
2.2 Review of No Action Alternative 3
3.0 Retreat Alternative 3
3.1 Description of Retreat Alternative 3
3.2 Review of Retreat 4
4.0 Bank Stabilization Alternative – Fabric Coastal Bank Terraces 4
4.1 Description of Fabric Coastal Bank Terraces Alternative 4
4.2 Review of Fabric Coastal Bank Terraces 5
5.0 Dewatering Alternative 6
5.1 Description of Dewatering Alternative 6
5.1.1 Beach Dewatering 6
5.1.2 Passive Bank Drains 8
5.2 Review of Dewatering 9
5.2.1 Beach Dewatering 9
5.2.2 Passive Bank Drains 9
6.0 Beach Nourishment Alternative 9
6.1 Description of Beach Nourishment Alternative 9
6.1.1 Upland Sand Sources 10
6.1.2 Offshore Sand Sources 10
6.2 Review of Beach Nourishment 11
7.0 Breakwater Alternative 12
7.1 Description of Narrow-Crested Breakwaters 13
7.2 Description of Broad-Crested Breakwaters 14
7.3 Description of Reef Balls 14
7.4 Review of Breakwaters 15
8.0 Groin Alternative 16
8.1 Description of Groin Alternative 16
8.2 Review of Groin Alternative 18
9.0 Seawall Alternative 18
9.1 Description of Seawall Alternative 19
9.2 Review of Seawalls for the Proposed Project 19
10.0 Mattress and Gabion System Alternative 20
10.1 Description Mattress and Gabion System 20
10.2 Review of Marine Mattress and Gabions Alternative 21
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Table of Contents (Continued)
11.0 Preferred Alternative: Emergency Project - Geotextile Tube Alternative 22
11.1 Description of Geotextile Tube Alternative 23
11.2 Review of Geotextile Tubes 23
12.0 Comparison of Alternatives and Conclusions 23
LIST OF TABLES
Table 12-1 Comparison of Alternatives Considered for the Project
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ALTERNATIVES ANALYSIS
1.0 Overview
Ongoing erosion on the eastern shore of Nantucket Island has substantially decreased the
setback between the top of the coastal bank and the existing public infrastructure and
residences along sections of Baxter Road. Without the implementation of measures to
protect the coastal bank and preserve the coastal beach, these existing homes and public
infrastructure are in imminent danger of storm damage. The Siasconset Beach Preservation
Fund (SBPF, or the Applicant) has undertaken efforts to prevent this storm damage, but none
has achieved the degree of stability and reliability deemed necessary. Accordingly, after
conducting a comprehensive alternatives analysis to evaluate these previous efforts and
other options, SBPF developed a preferred alternative for a bluff protection Emergency
project (the Project) that provides a robust and environmentally sensitive method of
protecting the Baxter Road public infrastructure and homes from storm damage by
preventing coastal bank erosion and preserving the coastal beach and allowing construction
to go forward to provide protection for this winter season.
As part of the permitting process for the preferred and selected alternative (the Project), the
Applicant has prepared this Alternatives Analysis to aid the Nantucket Conservation
Commission in its review of this Emergency Certification. The purpose of this Alternatives
Analysis is to assess the practicability of various strategies and methods that may have the
potential to achieve SBPF’s objective, which is to protect existing residential buildings and
public infrastructure (including Baxter Road and associated utilities therein) from storm
damage associated with coastal bank and coastal beach erosion.
To ensure a thorough vetting of each alternative, this analysis has considered various design
parameters, potential environmental impacts, likely effectiveness in achieving project
objectives, regulatory considerations, and cost. This has led to the selection of a preferred
Emergency Project alternative, which is required to provide coastal bank protection for this
winter season.
For nearly the past 20 years, a variety of shore protection alternatives have been evaluated
in concept and some have been put into practice in Sconset. An assortment of alternatives
has been implemented with varying degrees of success, including the No Action alternative,
managed retreat from the coastal bank, beach dewatering, Duneguard, coastal bank toe
terracing, coastal bank terracing and vegetation planting, and drainage wells to drain a
perched aquifer that was exacerbating erosion on the bank face. Despite these efforts, the
beach and bank have continued to erode, a process intensified by recent stormy winters. In
addition, many of these alternatives are no longer preferable or particularly practical due to
logistical or technical constraints. For example, larger areas of available space landward of
the threatened homes would be needed to continue a strategy of managed retreat, but
existing areas are nearly exhausted; thus, strategic retreat is no longer a viable option.
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Of the alternatives evaluated, the No Action strategy is unacceptable given the resources at
risk, and the retreat alternative is unacceptable due to the limited opportunities to relocate
these resources. The purely structural alternatives (e.g., seawall or groins) are not preferred
due to potential environmental impacts and strict regulatory obstacles. Bank stabilization
alone would not satisfy SBPF’s objectives and would require structural stabilization to
prevent the coastal bank from becoming undermined. A large-scale beach nourishment
proposal proposed by the Applicant several years ago met stiff local resistance that rendered
it infeasible.
Based on this alternatives analysis, a combination of bluff protection and sand nourishment
emerged as the preferred alternative. As described below this Emergency Project alternative
will be attained with this geotextile tube project. To maximize performance and aesthetics
while minimizing potential impacts, the design incorporates an element of replenishing a
supply of sacrificial sand over the proposed installation.
The Project developed from the preferred alternative is the product of site-specific
experience, sound scientific understanding of coastal processes, responsible engineering
design, and stakeholder input. The following sections provide a thorough review of Project
alternatives, associated environmental impacts, and subsequent conclusions.
2.0 No Action Alternative
The No Action alternative would allow natural processes to occur without any form of
human intervention to prevent coastal bank erosion and storm damage to existing
residences and public infrastructure.
2.1 Description of No Action Alternative
The existing homes in the Project area from are currently threatened due to coastal erosion
of the toe of the coastal bank and subsequent slumping of the upper bank areas due to
slope instability. Although the average coastal bank erosion rate is approximately 5 feet per
year, winter storms can lead to top of bank slumping in some areas of up to 25 feet within a
single storm season. Thus, there is an imminent threat that requires action to avoid serious
losses to the existing homes. The No Action alternative would ultimately require
abandonment or removal of these homes.
The Project area also encompasses public infrastructure and services that would be
threatened by the No Action alternative, including Baxter Road and associated sewer/water
services. Baxter Road is a Town road which provides the only access to the recently-
relocated Sankaty Head Lighthouse, as well as access for the public, residents, and
emergency vehicles to more than 100 homes.
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Land use controls are not practicable because the Project is intended to protect existing
resources, including public infrastructure and developed residential properties. All of these
existing resources are increasingly threatened by erosion and storm damage. Furthermore,
Baxter Road is a public road that provides access to residences on its western and eastern
sides, and loss of the road would cut off access to these residences.
2.2 Review of No Action Alternative
The No Action alternative is not preferable because by allowing erosion to continue, this
alternative would place the residential properties and public infrastructure at increasing risk.
3.0 Retreat Alternative
The Retreat alternative allows erosion to continue but seeks to mitigate its impacts through
land use restrictions. A retreat strategy may be an appropriate option where rapid shoreline
erosion threatens existing development or in areas where property values and other
economic considerations are not high (Dean, Davis and Erickson, 2005). If space is
available, threatened structures may be moved landward in response to a retreating
shoreline; perhaps the best-known example of this occurred when the Cape Hatteras
Lighthouse was relocated 1,600 feet inland in the year 2000 (Greene, 2002). In Sconset,
the Sankaty Head Lighthouse was moved 400 feet landward in 2007 in response to the
eroding bluff north of the Project.
In some cases, land use controls and managed retreat policies can be effective long-term
solutions to erosion problems. When practicable, such strategies are encouraged by the
Office of Ocean and Coastal Resource Management (OCRM) (NOAA2, 2005). In areas free
of dense coastal development, land use controls may maintain enough of a buffer between
the shoreline and upland structures to withstand periods of erosion until the trend shifts. By
avoiding development in high-risk coastal areas, the costs, maintenance requirements, and
potential impacts from shoreline management activities can also be avoided.
3.1 Description of Retreat Alternative
For this Sconset location, relocating the threatened structures and infrastructure is not a
practicable long-term solution, because of the temporary nature of a retreat solution
(especially when implemented within the same lot), the high cost associated with acquiring
a new lot and moving a home, and the constraints on landward retreat caused by existing
conditions. Most of the nearby landward lots in Sconset are now occupied or are
constrained by the presence of freshwater wetlands, thus reducing the feasibility of moving
additional homes away from the bank as erosion continues.
While the retreat alternative has been implemented in some circumstances, it does not offer
the most feasible alternative for long-term protection. As evidence of the limits of the
retreat option, the property owner at 83 Baxter Road has already moved his home landward
on the existing lot and is threatened again. In addition, a review of the history of property
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owners who have moved their homes landward on their lots at Sconset makes the
temporary nature of this alternative even clearer. On approximately 9 lots in Sconset near
the Project area, owners first moved their homes landward on their lots, only to be forced
within a few years of continued erosion to move their homes off of their lot entirely.
Additionally, public infrastructure (such as Baxter Road) cannot be moved. In the event
Baxter Road were compromised by erosion, this would eliminate access not only to homes
within the Project area, but also to homes located on the western side of Baxter Road and
the recently re-located Sankaty Lighthouse – further illustrating why the retreat option alone
is not feasible. Concern for the imminent danger of Baxter Road has been expressed by
vote and letter from the NP&EDC, the leading planning agency of Town and County
government charged with responsibility for this issue.
While relocating threatened homes to lots elsewhere on the island is theoretically possible,
this action would be extremely difficult to implement due to the large size of the structures
in question, the various physical limitations and impediments to their relocation (e.g.,
power lines, etc.) and the substantial cost. The retreat alternative is so expensive as to be
infeasible for many owners. Retreat requires purchasing another lot in Sconset (which is
$1-2 million at a minimum), plus the cost of moving a home (which is estimated at
$200,000 at a minimum).
Finally, there is a fundamental value to preserving the entire Sconset neighborhood and
infrastructure in its current form where the Project will be implemented without adverse
impacts and in compliance with applicable performance standards.
3.2 Review of Retreat
This alternative is not preferable due to the temporary nature of this alternative when
implemented within a single lot, the high cost associated with moving a home and/or
purchasing a new lot, and the limited nature of available nearby lots due to existing land
uses and the occupation of landward lots.
4.0 Bank Stabilization Alternative – Fabric Coastal Bank Terraces
The Bank Stabilization alternative involves the construction of sand-filled terraces
composed of natural fabric materials at the toe of the coastal bank. In many locations,
erosion at the toe of bank has caused an over-steepening of the coastal bank face. To
reduce the likelihood of bank failure, terraces have been constructed at the toe of the bank
along some properties in the Project area.
4.1 Description of Fabric Coastal Bank Terraces Alternative
Bank stabilization through a combination of bank terracing and vegetation plantings
involves constructing a series of terraces on the lower portion of the bank face using
biodegradable coconut fiber mats or jute fiber bags. The mats or bags are layered and filled
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with sand, after which they are covered with a layer of sand and stabilized with plantings of
American beach grass (Ammophila breviligulata) or other indigenous vegetation to stabilize
the terraces. By reducing the slope at the toe of the bank face and providing a buffer
against storm wave activity, these terraces can reduce or prevent bank retreat. However,
this alternative requires nearly constant post-storm maintenance since medium to large
storms cause the bags to rip open, releasing sand to the beach; thus, it is a temporary
solution. The cost of constructing jute terraces to a finished height of +26 feet Mean Low
Water (MLW) from the base elevation of the back of the beach is approximately $1,140 per
linear foot.
4.2 Review of Fabric Coastal Bank Terraces
On its own, a strategy of bank stabilization through temporary terracing and/or vegetation
plantings would not provide adequate storm damage protection for upland resources in the
Project area, and hence is not a preferred alternative. While vegetation helps prevent wind,
rain, and runoff-induced erosion from destabilizing the coastal bank, the primary cause of
bank retreat is wave-induced erosion or scarping of the toe of bank. This toe erosion
destabilizes the bank, resulting in bank slumping and upper slope failure. Fiber terraces
can prevent runoff-induced erosion while also providing a measure of protection near the
toe of the coastal bank under normal and moderate storm conditions; however, these
terraces do not provide sufficient protection during major storm events, when wave-induced
scarping occurs at the toe. A review of storms in the Project area indicates that the strongest
storms the terraces have been subjected to have been 10-year storms, and the terraces have
not withstood those events.1 Given this experience, terraces cannot provide protection
during a 100-year storm, which is one of the Project objectives. The preferred marine
mattress and gabion system alternative provides substantial protection for the toe of bank
and is also robust enough to withstand up to 100-year storm conditions should it become
uncovered.
Furthermore, the terraces are designed to release sand during storm events, and maintaining
the fiber toe terraces requires frequent (once or twice a year) replacement of all or most of
the terraces. This high level of required maintenance is not financially sustainable, and is in
contrast to the relatively low required maintenance of the robust marine mattress and
gabion system included in the preferred alternative. The fiber toe terraces are also
biodegradable and do not offer an effective long-term solution by themselves, but could be
effective as temporary impediments to erosion until the preferred alternative is completed.
1 Two 10-year storm events have occurred since January 2004: a December 2004 storm and a January 2005 storm.
These are the biggest storms since 2004.
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5.0 Dewatering Alternative
5.1 Description of Dewatering Alternative
Dewatering, the process of removing water by extracting it or promoting its drainage, can
be accomplished directly within the beach or within the buffer to the top of the coastal
bank.
5.1.1 Beach Dewatering
Beach dewatering systems are part of an experimental erosion-control strategy by which
water is removed from the beach face to promote accretion of actively-moving sediment in
the swash zone. The theory behind the concept is based on the supposition that draining
water from the beach face can reduce the local groundwater table, thereby stabilizing the
existing beach by reducing the buoyancy forces and lubrication between individual grains
of sand on the existing beach. In addition, beach dewatering promotes the percolation of
swash zone water into the beach, thus reducing seaward drainage across the beach face.
Beach dewatering systems are intended to halt the erosion of existing sediment as well as
promote deposition of new sediment that is actively moving through the swash zone,
thereby encouraging shoreline accretion.
The concept of beach dewatering was first employed in Denmark in 1981 at Hirtshals West,
when the North Sea Research Center constructed a pump drainage system designed to filter
water through the beach face for use in a saltwater aquarium. After providing high-quality
water to the aquarium for approximately six months, flow rates began to decline
substantially and local citizens began to observe accumulations of sand in the landward
roadways. It was also discovered that the beach had accreted 20-30 meters in the vicinity
of the pumping system. Beach accretion had decreased the efficiency of the pumps,
thereby compromising the effectiveness of the filtration system for the aquarium.
Furthermore, the accreted beach provided a greater source of dry beach sand for aeolian
transport onto local roadways. The pump system was lengthened to accommodate the
pumping of more water, and a plan was eventually adopted to remove sand from the
accreted beach on a regular basis so that sufficient pumping volumes could be achieved for
the research center. This prototype provided the template upon which later beach
dewatering systems were based. Including three systems that were planned or in progress
for construction in 2005, 41 systems are known to exist worldwide.
The performance of beach dewatering systems is the subject of much debate in coastal,
scientific, and engineering communities. Success stories have occurred at Sailfish Point,
Florida, and at Thorsminde, Denmark, where installations operated for several years and
monitoring data demonstrated meaningful beach accretion that corresponded to system
operation. The Danish Coastal Authority observed direct accretion of the beach
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corresponding with more than six years of system operation, and the authority observed a
return to beach erosion when the system was intentionally shut down for experimentation
purposes.
There have also been a number of disappointments with beach dewatering systems. Efforts
to adopt the systems in Malaysia have met with difficulties, and achieving accurate pipeline
installations, effective well configurations, and preventing clogging of shore-parallel beach
face filtration pipes have all presented challenges. Many systems have been damaged
during storms, and flooding has damaged electrical components. A recent article in the
Coastal Engineering journal reported on an installation in Alassio Beach in northern Italy2.
This study found that the drained beach and the control beach showed little difference in
volume. Another recent article in the Journal of Coastal Research reviewed a beach
dewatering system that was installed in 2002 at Chiaiolella Beach, Procida Island, Italy3.
No beach accretion was observed in this study and a storm subsequently damaged the
structure. Local authorities deactivated the system and instead pursued cliff stabilization
alternatives.
Although the volume of published research and experience is not sufficient to conclusively
demonstrate the effectiveness of beach dewatering systems, it is generally accepted that the
technology can be effective along sandy tidal beaches that are exposed to moderate wave
climates, have high groundwater tables, and where seasonal beach fluctuations are evident.
Typically, beach dewatering systems are expected to be less effective along chronically-
eroding beaches exposed to severe wave and current regimes.
There is a significant history with beach dewatering systems at Sconset Beach: the Applicant
installed four different systems that met with mixed levels of performance success, as
demonstrated by quarterly data reports published since 1994. At the time of installation,
the cost of the Lighthouse South (LHS) system, to protect approximately 800 linear feet of
shoreline, was approximately $1.1 million (see Table 13-1). Three of the systems,
Lighthouse North (LHN), Lighthouse South (LHS), and Lighthouse South-South (LHS-S) were
severely damaged and ultimately removed. All of these systems were installed along the
section of Sconset Beach seaward of the Project area that has experienced the most severe
erosion over the past decade, and have not remained operational due to problems with
construction, maintenance, and repeated storm damage.
One remaining system at Codfish Park is covered by the existing beach. Prior to 1999,
Codfish Park was severely eroding and had little active beach fronting the public roadway.
Prior to a system upgrade, the baseline erosion rate in the 600-foot area subject to the
2 “Efficacy of beach dewatering – Alassio, Italy” by D. Bowman, S. Ferri, and E. Pranzini (volume 54, issue 11,
November 2007, pages 791 -800).
3 “Performance of a Beach Dewatering System – Chiaiolella Beach, Procida Island, Italy” by D. Vicinanza, A. Guida, V
Ferrante, and P. Ciavola (volume 26, issue 4, pages 753-761, July 2010).
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beach dewatering system’s sphere of influence was a net loss of 3,219 cubic yards per year.
Following a system upgrade, the Codfish Park beach accreted at an annual rate of 15,418
cubic yards for the period from December 1999 to July 2002. Once the beach accreted to
a point where the system could no longer pump effectively, operations were terminated and
the beach responded by eroding at a rate of 5,102 cubic yards per year. This pattern is
consistent with the Thorsminde experience, absent a scenario to evaluate whether the
beach will recover upon reactivation of system operations. The Codfish Park dewatering
system has not been reactivated since the beach is sufficiently wide in this area. Any future
use of this system would require repairs to the electrical system and would require updating
permits.
5.1.2 Passive Bank Drains
Several impermeable near-surface clay layers are present in the subsurface of the buffer
zone along portions of the Sconset bank area. After rain and snow events, water percolates
from the ground surface into the underlying sediments and much of the water gets trapped
by these layers, resulting in areas of perched water table. During heavy precipitation
events, water contained within these perched water tables tends to flow by gravity towards
the coastal bank where it breaks out along upper portions of the bank face. This water
flowing out of the bank face exacerbates surface runoff erosion and causes gullies to form.
During particularly heavy precipitation events, the flowing water can act as a lubricant to
reduce the internal friction of overlying layers of sediment. This internal friction reduction
together with the additional weight of the sediment as a result of the water trapped by the
clay layer destabilizes the upper bank, causing failure to large sections of the upper bank by
slumping. Since the clay layers causing the perched water tables are relatively thin, shallow
passive drainage wells can penetrate into underlying pervious sediments, allowing the
perched water to drain.
In 2003, the Applicant performed a geophysical survey to determine the depth and lateral
extent of the confining clay layers to assist in the planning for installation of passive
drainage wells. The Applicant proposed to install up to 192 eight-inch-diameter passive
drainage wells extending to a maximum depth of 20-24 feet below surface. After receiving
an Order of Conditions from the Nantucket Conservation Commission in 2004, the
Applicant installed 130 passive drainage wells in an area east of Baxter Road and seaward
of a number of the existing homes; the wells installed immediately east of Baxter Road
remain operational, but a number of the passive drainage wells in more seaward locations
have been lost to bank erosion or were disturbed during excavation activities associated
with relocating some of the homes. The cost of installation to a depth of 15-20 feet below
grade is approximately $1,200-1,500 per well (see Table 13-1).
In terms of the current Project area, the clay layers are largely north of the areas where the
marine mattress system will be immediately installed. The clay layers are present in the
coastal bank at lots north of and including 49-34 (#83 Baxter Road).
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5.2 Review of Dewatering
5.2.1 Beach Dewatering
Although the patterns of shoreline erosion and accretion at Codfish Park have not been
linked conclusively with operations of the dewatering system, they are evidence of the
system’s potential effectiveness. However, based on the extensive storm damage
documented to the LHN, LHS and LHS-S systems, and the difficulties involved in
performing associated repair and maintenance, beach dewatering as a stand-alone
alternative is not preferred or considered viable for satisfying Project objectives.
5.2.2 Passive Bank Drains
After the installation of passive drainage wells, there was a significant reduction in the
amount of water breaking out onto the coastal bank where the confining clay layers
intersect the bank face, and there was a corresponding reduction in the amount of slumping
in the upper bank. While these measures were effective at reducing bank surface runoff
erosion and associated upper bank slumping, erosion at the toe of bank continued to result
in bank retreat. Bank erosion is ultimately a wave-driven process that cannot be resolved
without protecting the bank from direct wave action. Thus passive drainage wells alone are
not a viable alternative for satisfying Project objectives.
6.0 Beach Nourishment Alternative
Beach nourishment adds additional sand to the eroding shoreline to create a wider beach
across which wave energy is absorbed and dissipated, thereby increasing protection to
infrastructure and property threatened by erosion and storm damage. Once nourishment
sand is in place, natural coastal processes rework the nourishment material to create an
equilibrated beach profile, and since ongoing sediment transport will likely erode
nourishment material and transport it to adjacent shorelines as well as offshore, a
maintenance plan for renourishment is necessary for this alternative to be effective as a
long-term management strategy. Beach nourishment can be performed alone or in concert
with other shore protection measures. For the purposes of evaluating this alternative, a
typical design life of 5 years was selected.
6.1 Description of Beach Nourishment Alternative
To meet a target design life of approximately five years, Sconset Beach would be nourished
with approximately 2.6 million cubic yards of sand. The nourished beach would be
approximately 200 feet wide with a berm height of 12-16 feet above MLW. The seaward
portion of the nourished beach (the “advance nourishment”) would be considered sacrificial
and the landward portion (the “design beach”) would be maintained at all times to protect
against storm damage. Renourishment would occur once the advance nourishment eroded
due to natural longshore transport and in response to coastal storms.
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Beach nourishment could occur on its own or in combination with the other shoreline
protection strategies to increase the nourishment’s longevity and/or performance. A
vegetated dune or geotextile tube could be placed along the back of the beach near the toe
of coastal bank to provide additional protection. Alternatively, groins could be constructed
in select locations of accelerated erosion where longshore sediment transport occurs in both
directions to reduce sand loss from the nourishment template. These groins would start at
the toe of the coastal bank with an elevation of approximately two feet below the nourished
berm and would extend approximately 270 feet seaward, tapering down to the MLW
elevation at the seaward ends. Another option would be to supplement beach nourishment
with both groins and geotextile tubes to mitigate the rate of sand loss from erosion hotspots
along the nourished profile while protecting the toe of bank.
For all of the beach nourishment alternatives, beach nourishment would likely occur
between the end of May and November based on a number of considerations including
meteorological conditions, wave and current climates, safety, and time-of-year restrictions
related to biological resources. Although costs are dependent upon the construction
season, demand for equipment, and location of the borrow site (among other factors), the
approximate cost for a beach nourishment project to protect the Project area, with
construction between the end of May and November, is estimated at $8-10 per cubic yard.
6.1.1 Upland Sand Sources
Beach nourishment projects require a large volume of sand that is compatible in terms of
grain size and color with the native beach. Options for borrow sites include upland sources
(on-island and off-island) and offshore sources. Upland sand sources were evaluated and
determined to be infeasible. On-island upland sand sources have been depleted to the
point where the local sand supply would not meet nourishment requirements. Off-island
sand sources, from which sand would be transported via barge to Nantucket Harbor and
trucked to the site, would not meet production rate requirements for beach nourishment
without nearly nonstop truck delivery and associated local disturbance. Further, associated
costs would be prohibitive.
6.1.2 Offshore Sand Sources
Given the infeasibility of upland sand sources, offshore sand deposits were evaluated. In
general, a preferred offshore borrow site is one that has the necessary physical sediment
characteristics (i.e., volume and grain size) and is located where mining activities would
produce minimal impacts on existing offshore geomorphology, wave focusing, longshore
sediment transport patterns, and biological communities and habitat. A borrow site must
also satisfy operational parameters for dredging equipment, which include considerations of
proximity to the nourishment area, wave and current conditions, and water depth.
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Using these criteria, a potential offshore borrow site was identified through geophysical
surveys, sediment coring, and benthic/fisheries surveys. The borrow site is approximately
2.7 miles east-northeast of the Project shoreline near the offshore shoal system known as
Bass Rip, where water depths are -20 to -50 feet MLW. Less than half of the 345-acre site
would need to be excavated for the initial nourishment. Either a hopper or a hydraulic
dredge would be used to excavate the borrow site.
Based on a summer dredging schedule, estimates for the cost of offshore dredging and
beach nourishment are on the order of $5-8 per cubic yard.
6.2 Review of Beach Nourishment
Large-scale beach nourishment would restore the depleted sediment supply on the beach
without interfering with longshore sand transport, enhance storm protection, and provide a
wider beach (potentially providing protection and/or enhanced habitat for plants, listed
shorebirds, and other marine organisms). The beach nourishment alternative considered
herein would have a design life of approximately 5 years, at which point renourishment
would likely be necessary to replenish the advanced nourishment profile; including
additional shoreline management strategies would potentially enhance longevity of the
design beach and decrease renourishment impacts.
In spite of the tremendous benefits associated with beach nourishment, there are potential
adverse impacts that must be carefully minimized and/or mitigated. For example, the
nourishment envelope considered in this alternative would cover more than 125 acres of
beach, inter-tidal, and sub-tidal habitats. Direct mortality to the benthic community,
including invertebrates and shellfish, would be unavoidable. However, nearshore benthic
organisms would be expected to begin recolonizing the area within months of the
nourishment, and full recovery would be expected within one to three years. In addition,
areas of nearshore cobble habitat important to the local fishing community would be
temporarily or permanently impacted by the placement of fill. The Applicant has
thoroughly investigated various strategies for replicating nearshore cobble habitat, and it is
likely that these impacted areas of cobble could be successfully replicated nearby.
Nonetheless, it is worth noting that the preferred marine mattress and gabion system with
sand cover will contribute the same amount of sediment to the system as would occur
under natural conditions, and thus will not result in this nearshore habitat coverage.
While the Applicant is confident that beach nourishment could be planned and executed in
a manner that achieves Project objectives while resulting in a minimal level of
environmental impact, the local regulatory climate currently prohibits the use of beach
nourishment at the project location.
In addition to impacts along the nourished shoreline, dredging the nourishment material
from an offshore borrow site would also have potential adverse impacts. Based on the
borrow site considered in this analysis, no significant changes to offshore currents or
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sediment transport would be expected; the borrow site would be expected to shoal
relatively rapidly, with no detrimental impacts to adjacent morphological features such as
Bass Rip. Furthermore, no nearshore impacts from wave focusing would be expected, and
modeling results suggest that excavating an even thicker dredge cut than considered in this
analysis would not induce any significant changes to nearshore waves during average wave
conditions or during a representative storm.
No long-term impacts to marine life at the borrow site would be anticipated from the
dredging considered in this analysis. Anticipated impacts to offshore benthic habitat would
be minimal, as the post-dredging seafloor would resemble the existing sandy seafloor
conditions. Due to its adaptive nature, the benthic community would be expected to
recover rapidly, with early recolonization within months of dredging and species diversity
returning to pre-dredging conditions within one to three years. In addition, offshore
dredging would not be expected to significantly impact demersal or pelagic fish species,
which would avoid dredging activities and temporarily relocate to adjacent areas of suitable
habitat until dredging is complete and/or food sources recover. Likewise, marine birds at
the dredging site may be temporarily displaced during dredging; however, similar habitat
exists in ample supply nearby, and avian species would return once suitable food sources
were to recolonize the seafloor.
Fishing activities at the borrow site (which consist of limited commercial fishing, frequent
charter fishing, and an active recreational fishery) may be temporarily impacted during
dredging; however, more productive fishing grounds exist nearby (e.g., Old Man Shoal,
Great Point, and on Bass Rip itself) and would not be impacted by dredging. Finally, while
some marine mammals are present seasonally, the potential borrow site encompasses only
a small portion of their overall habitat and suitable habitat is available nearby; therefore, no
adverse impacts would be expected.
As stated previously, while the Applicant believes that an appropriate borrow site has been
identified and that all environmental impacts could be minimized and/or mitigated
adequately, the local regulatory climate prohibits the use of beach nourishment with an
offshore borrow site at the Project location.
7.0 Breakwater Alternative
In 1992-1993, the Applicant funded studies prepared on behalf of the Town of Nantucket
by Coastal Planning & Engineering, Inc. (CP&E) of Boca Raton, Florida to investigate
shoreline protection alternatives for the Project area. The first report, entitled An evaluation
of alternative technologies for beach erosion control at Sconset, Massachusetts and
prepared in May 1993, evaluated technologies for shore protection including reef-type
breakwaters and submerged barge breakwaters for construction in shallow water close to
shore. The Applicant has continued to investigate the practicability of breakwaters
subsequent to the CP&E report.
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7.1 Description of Narrow-Crested Breakwaters
Manufactured artificial reef breakwaters consist of concrete structural units typically placed
in water depths of 7 to 9 feet that are intended to break wave action and artificially perch
the landward beach. Two types of artificial reef breakwaters, PEP and Beachsaver, were
considered during the studies; these structures are very similar in design and placement,
and have similar effects on the beach. Both types of units are low-profile and narrow-
crested relative to wave length, features which inhibit effective dissipation of wave energy.
This is particularly true during storms, when storm surge and wave setup increase the
freeboard from the crest of the artificial reef to the water surface, thereby allowing more
wave energy to pass just when protection is most needed. Scaled physical modeling of
similar narrow-crested submerged breakwaters has shown that only a small fraction of wave
energy dissipates over these structures, even under simulated non-storm conditions. The
ability of these structures to dissipate wave energy also decreases as they settle into
underlying sediments, a problem that has been documented with artificial reefs. Examining
case studies where these structures have been employed provides valuable insight into their
potential performance benefits and limitations.
The first example is a PEP artificial reef that was constructed offshore from an eroding beach
in Palm Beach, Florida. The structure was not able to halt or reduce beach erosion, and
normal coastal conditions as well as storm-induced wave action significantly impacted the
artificial reef and caused problems with settlement. Monitoring studies also documented
that the structure’s interaction with local wave and current regimes strengthened the
longshore current along the landward shoreline, which contributed to beach erosion by
exacerbating sediment loss from the beach in the lee of the reef. Sections of the PEP reef
were later removed and used to form a shore-perpendicular groin system.
Other examples of PEP reefs have included a segmented systems, such as one constructed
at Vero Beach, Florida in September 1996. Segmenting this reef reduced the strength of the
longshore current and moderated the exacerbated rate of sediment loss, but ultimately the
beach response was not found to be significant. In fact, beaches landward of the
segmented system at Vero Beach measured less accretion than control areas (Stauble and
Tabar, 2003).
Beachsaver reefs intended to reduce beach erosion have been installed along the southern
coast of Long Island, New York and in New Jersey. Experience in New Jersey has shown
that Beachsaver reefs installed across the seaward ends of groins can be moderately
effective at maintaining the beach between the groins; however, the structures did not stop
beach erosion in the absence of groins. According to the CP&E report, Beachsaver
installations would not change the natural pattern of sand deposition in the Project area
sufficiently to maintain the beach.
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Artificial submerged breakwaters are not appropriate for this Project primarily because of
the steep shore face and dynamic coastal zone. Due to the steep nearshore slope,
submerged breakwaters would need to be located at water depths exceeding -12 feet MLW,
and at such depths the low-profile structures would be ineffective, particularly during
storms. Significant settlement would also be expected due to the energetic wave climate
and strong tidal currents, both of which could induce scour.
7.2 Description of Broad-Crested Breakwaters
Sunken barges, used as broad-crested breakwaters, are another type of artificial reef that can
be employed in erosion control efforts. With a broad profile, sunken barges have a greater
effect on waves than the narrow-crested breakwaters. CP&E evaluated this alternative
assuming the sunken barges would be approximately 195 feet long, 35 feet wide, 12-14 feet
deep, and located in approximately 15 feet of water. Several problems were associated
with using sunken barges as artificial reefs, including structural deterioration due to
corrosion, settlement, and movement along the seafloor due to hydrodynamic forces acting
on the bulky structures. Each of these factors adds significantly to cost of this type of
structure, which in 1993 would have cost approximately $730 per foot and would likely be
more expensive today.
7.3 Description of Reef Balls
An additional type of artificial reef involves reef balls placed offshore from an eroding
shoreline. Reef balls are relatively small, dome-shaped concrete structures with holes
intended to create structure and habitat for fish. Placed in nearshore areas, these structures
are intended to absorb wave energy, reducing the amount of energy impacting the shoreline
and thereby moderating erosion. By containing holes, however, the effectiveness of these
hollow structures at absorbing wave energy is drastically compromised.
Due to the intense and dynamic waves and currents in the Project area, installations of reef
balls may in fact be counterproductive because they would likely be scattered and crushed
in the active coastal zone. Stability, scour, and settlement are all design problems
associated with reef balls. When placed in a sandy substrate, it is generally recommended
that reef balls be attached to an articulating concrete mat foundation; when installed on
hard substrate, pilings should be driven through the reef ball units and into the seafloor to
enable the structures to resist horizontal and vertical movement (Harris, 2004).
A successful reef ball breakwater was constructed for a project in the Dominican Republic
where the tidal range was 1.2 feet and water depth was approximately 2.5 feet. In contrast,
the environment in the Project area exhibits tidal ranges from two to more than four feet,
contributing to strong currents, and water depths well in excess of those at the Dominican
site. It would not be possible to place reef balls in similarly shallow water in the Project
area due to vertical and horizontal changes observed in the beach profile. Furthermore,
reef balls employed in the Dominican project and elsewhere have weighed approximately
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2.2 tons; in contrast, armor stones weighing a minimum of five to seven tons would be
required to withstand the high-energy, dynamic conditions present in the nearshore
environment of this Project. Placing these structures further offshore, where wave energy
may not be as intense, would not be a practicable alternative since that would substantially
reduce the potential effectiveness at dissipating wave energy. The effectiveness of reef balls
is questionable even in a moderate wave climate, and they are not a practicable concept in
the environment offshore of Nantucket.
7.4 Review of Breakwaters
An effective offshore breakwater design at Sconset would likely require a large emergent
rubble-mound breakwater system, which would be extremely costly and occupy a large
area of the bottom. In addition, until the beach behind such a structure accreted to the
point where it formed a tombolo (i.e., it accreted seaward to the structure itself), a large
emergent breakwater would act as a barrier to longshore sediment transport, starving
downdrift beaches. The possibility of such an impact runs contrary to the Project objective.
In fact, the preferred marine mattress and gabion system with sand cover will not impede
longshore sediment transport and will not impact marine habitat.
Furthermore, manufactured artificial reef breakwaters are not preferred due to serious
questions about their effectiveness. Artificial reefs are generally submerged and have
exhibited limited effectiveness at causing waves to break before they impact the shoreline.
As described above, these structures are generally narrow in width compared to wave
length, which compromises effective dissipation of wave energy. Artificial reef breakwaters
are also susceptible to scour and settling, which can result in significant passing wave
energy, ultimately impacting the beach. In addition, as water depth increases over the
structure during storm surge, breakwaters offer the least protection when it is most needed.
More minor artificial submerged breakwater structures such as reef balls would be
completely ineffective in an environment as dynamic and active as the Project area. In
contrast, the preferred marine mattress and gabion system with sand cover is a robust
barrier to absorb wave energy and protect the toe of bank and bank face while effectively
minimizing wave reflection.
From an engineering standpoint, emergent breakwaters could be more effective than
submerged breakwaters, but only when constructed in massive rock configurations that are
not preferred because of likely environmental impacts and permitting constraints.
Employing a breakwater as a stand-alone measure could potentially shunt erosion problems
to downdrift beaches by interrupting natural sand supply and transport. Since the Project
beach slopes steeply offshore, a breakwater would have to be located in relatively deep
water with a large base foundation which would permanently impact nearshore marine
habitat; no such impact is associated with the preferred alternative. Breakwaters
constructed of stone would cost approximately $3,600 per linear foot.
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Finally, the feasibility of permitting sunken barge breakwaters to protect the high-energy
Project area is questionable.
8.0 Groin Alternative
Groins are concrete, rock, and/or timber structures constructed perpendicular to the
shoreline and extending seaward of the beach. These structures are designed to retain
beach sand along a shoreline or to trap sand being transported in an active littoral system in
order to stabilize and build out the beach directly up-drift of the groins (Douglass, 2002).
However, groin effectiveness is often achieved at the expense of inhibiting longshore
transport of sand, which can exacerbate downdrift shoreline erosion.
Because they impede longshore sand transport, if traditional coastal groins are installed
absent a corresponding nourishment program, they can transplant an erosion problem from
one area to another. Once a popular response to beach erosion, traditional high-profile and
impermeable groins are now considered problematic and even damaging given their
consequences for downdrift shorelines (Douglass, 2002).
8.1 Description of Groin Alternative
Design modifications have made it possible for groins to allow more sand to pass to
downdrift beaches while still benefiting the up-drift shoreline; this effect can mitigate sand
volume loss and erosion historically associated with these structures. For example, low-
profile and semi-permeable groins are designed to allow some longshore sediment transport
to pass over and through the structures. Novel designs also include removable segments,
which allows for structural adjustments to optimize performance and minimize adverse
impacts. These structures can have tapered offshore profiles designed to act as templates
for a desired beach profile, and they can be notched to allow sediment transport in the
swash zone. Variations of the semi-permeable groin concept have been successfully
employed in Florida (Naples Beach), northern New Jersey, and South Africa (Port Elizabeth),
among other locations.
T-head or headland-style groins have also been constructed in conjunction with offshore,
connected, or shore-parallel breakwater elements (typically rock or rubble-mound) designed
to minimize offshore sand losses around the end of the groin. The T-head also affords
added protection to adjacent shorelines by reducing direct wave impacts on the beach.
Groins are usually designed in conjunction with beach nourishment activities and are
intended to lengthen the design life of a nourishment project by maintaining the beach
design profile in areas of accelerated erosion (i.e., erosion hotspots). The strategy employed
in these cases is to maintain the minimum beach profile in these hotspots while allowing
some sediment transport to continue over and around the structures toward downdrift
beaches. To achieve this objective, properly-designed groins generally extend just into the
advance nourishment (i.e., sacrificial) portion of the nourished beach. As the sacrificial
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beach erodes away, the groins become exposed and begin to moderate the rate of
additional sand loss. At that time, renourishment would again bury the groins and rebuild
the advanced nourishment profile.
When design expectations are satisfied, groins can lengthen the design life of nourishment
in erosion hotspots to a temporal scale more consistent with the design life of the
comprehensive nourishment project. This not only improves a project’s performance and
cost-effectiveness, but can also lessen potential environmental impacts by reducing the
frequency and/or volumetric requirements of renourishment events. Increasing the number
of groins decreases the beach fill volume required to maintain a project’s design profile,
thus limiting the acreage of submerged area covered by nourishment material. Groin fields
installed in a closely-spaced configuration are effective at retaining sediment along a beach
in the presence of strong longshore currents and energetic wave activity. Spacing groins at
a distance less than or equal to twice the groin length (as measured from berm crest to
seaward end) would hold more sand along a given reach than if the structures were more
broadly spaced. Reducing the spacing and increasing the number of groins would result in
a steeper beach profile and more uniform shoreline.
There are numerous groin designs that can be adopted to achieve various objectives. One
such design is for a structure consisting of a marine mattress foundation filled with stone
which is then topped with armor stones. Gaps between the large armor stones can be left
empty, increasing the structure’s porosity. These groins can also be constructed with a
tapered profile to template the beach nourishment, extend nourishment’s longevity in
erosion hotspots, and minimize the nourishment footprint and downdrift impacts.
An alternative to a rubble-mound groin is a pile groin, which is constructed by driving a
series of pre-stressed reinforced concrete or wooden piles into the beach. Concrete piles
are generally preferred to wood, since the latter deteriorates over time. Permeability can be
controlled by altering the number of piles, pile spacing, and the number of rows of piles,
but performance of pile groins is difficult to predict. Also, as the beach erodes, more and
more of the piles would become exposed, increasing wave loading while simultaneously
reducing the embedment depth. In the proposed Project’s active and dynamic coastal
environment, this could result in structural damage.
A second alternative groin design involves king piles and panels. This type of groin is
constructed by inserting concrete or wooden panels into grooves cut into the piles; these
panels can be added or removed to alter the structure’s height, although such alteration is
not easy. King pile and panel groins can also be made semi-permeable by adding spacers
between the panels to control porosity. As with pile groins, performance of semi-
permeable king pile and panel groins is difficult to predict. Based on current conditions, a
groin alternative would cost approximately $2,700 per linear foot.
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8.2 Review of Groin Alternative
Designed to catch and trap sand being transported downdrift, groins can reduce the
sediment supply available to the littoral system and potentially exacerbate erosion problems
on adjacent, downdrift beaches. Traditional coastal groins are not preferred as a stand-
alone alternative for the proposed Project primarily due to environmental regulatory
constraints stemming from likely environmental effects. Furthermore, it is not likely that a
groin field absent an accompanying beach nourishment program would be effective
because it would not address the existing sediment deficit in the Project area. Employing
groins as a stand-alone measure could potentially shunt erosion problems to downdrift
beaches by interrupting natural sand supply and transport. In addition, groins are not
preferred as an alternative in conjunction with beach nourishment due to the challenges
associated with beach nourishment as described above.
Low-profile or semi-permeable groins, after filled to entrapment, can help maintain a more
natural rate of sand transport from erosion hotspots. These structures reduce the rate of
sediment loss from a nourished design beach profile and enhance the effectiveness of
nourishment. Implementing a beach nourishment program in conjunction with the
placement of adjustable structural alternatives enhances the sand supply in the littoral
system, moderates sediment losses from erosion hotspots, and increases the longevity of the
design beach profile. The National Research Council recommends that regulatory agencies
consider the use of well-designed structures, when proposed in conjunction with beach
nourishment activities, in cases where they would significantly improve the performance or
longevity of shore protection projects without promoting unacceptable adverse effects
(NRC, 1995).
Impacts from shoreline armoring absent nourishment, however, are increasingly recognized
by coastal engineers and regulatory authorities, and as a result many state and local
regulatory programs now discourage hard structures or require nourishment as a condition
of approval to maintain the sand supply in the littoral system. Since this Project does not
involve large-scale beach nourishment an alternative involving groins is not preferred.
9.0 Seawall Alternative
The Applicant has investigated the potential effectiveness of various shore protection
alternatives for the Project area. This investigation has encompassed several typical types of
structural shore protection that have been permitted elsewhere along the Massachusetts
coastline, including seawalls.
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9.1 Description of Seawall Alternative
Seawalls are generally vertical structures composed of concrete, steel, or other impervious
material that are installed immediately adjacent and parallel to a threatened shoreline.
These structures may be constructed at the water’s edge or in landward positions,
depending on the characteristics of the shoreline and the resources being protected.
A properly-designed seawall for the Project shoreline would be a massive vertical structure
composed of hard, impervious material such as concrete or steel that would be placed on
the coastal beach at the toe of the coastal bank. Composite seawalls could also be
constructed, which might include a seawall fronted by a rubble-mound structure designed
to protect the toe of the structure by dissipating wave energy during storms.
If constructed, a seawall would likely extend from below MLW to above the level attained
by waves during major storm events. A preliminary analysis of seawall designs for the
Project area suggests that in order to withstand long-term erosion pressures, the structure
would need to extend from 10 feet below the existing beach face to a height equal to at
least 20 feet up the face of the coastal bank.
9.2 Review of Seawalls for the Proposed Project
Without accompanying beach nourishment, seawalls are not preferred alternatives for the
proposed Project primarily due to environmental regulatory constraints stemming from
likely environmental effects. Although seawalls would effectively protect the bank and
upland property from erosion, potential issues associated with the interruption of natural
sand supply and transport could result in unacceptable environmental impacts and make
timely permitting problematic. Seawalls are not able to absorb and dissipate wave energy
as effectively as marine mattresses and gabions, which have voids between the enclosed
stones to better dissipate wave energy and minimize wave reflection onto adjacent
landforms. In addition, seawalls cannot conform to an existing bank shape as well as more
flexible structures such as marine mattresses. Finally, removing a seawall in the event of
negative impacts would be extremely expensive and difficult, whereas the mattresses and
gabions can be removed readily using a crane.
Furthermore, seawalls are specifically prohibited by the Nantucket Wetlands Bylaw
administered by the Nantucket Conservation Commission. These structures would also be
costly, with a steel sheet pile seawall costing approximately $5,000 per linear foot.
Although confident that an effective structural plan, with appropriate mitigation, could be
designed from an engineering perspective, these structures are not preferred at this time
given the regulatory climate and the potential detrimental environmental effects from
armoring the shoreline.
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10.0 Mattress and Gabion System Alternative
10.1 Description Mattress and Gabion System
This alternative consists of placing marine mattresses (i.e., rock-filled containers constructed
of strong, high-density polyethylene4 [HDPE] geogrid) on the lower portion of the coastal
bank face with a scour apron consisting of three rows of buried HDPE gabion baskets along
the toe of the bank; a layer of sand will then be placed over the entire installation. The
upper bank will be planted with native vegetation. This option is designed to protect
existing homes from a 100-year storm event by protecting the lower portion of the coastal
bank from erosion due to direct wave attack. This alternative uses HDPE gabion baskets at
the toe of the coastal bank to prevent scour, while the marine mattress installed on the bank
face will stabilize the slope by preventing storm damage erosion due to breaking waves and
run up.
This alternative design was detailed in a Notice of Intent filed previously by SBPF in April
2011. First, a scour apron consisting of three adjacent rows of buried 4-foot high x 5-foot
wide x 10-foot-long HDPE gabion baskets will be placed along the toe of the prepared
coastal bank. Each gabion basket will be constructed of a strong manufactured grid of high-
density polyethylene (HDPE) (geogrid) and filled with 12- to 22-inch-diameter stones.
These toe gabions will provide 15 feet of scour protection seaward of the marine mattresses.
As shown on Sheet 7 in Attachment B, the base of the seaward row of gabions will lie at
MLW, the base of the middle row will lie at +2.0 feet MLW, and the base of the landward
row will lie at +4.0 feet MLW. This stepped design will provide the intended level of scour
protection from a 100-year storm event. Excavation into the beach will be required to place
the gabions at their specified elevations; however, all excavated material will be re-used
onsite.
Next, the coastal bank will be prepared by grading the lower slope as required within the
Project area to create a maximum baseline slope of 1 vertical (V):1.5 horizontal (H), as
shown on Attachment B, Sheets 4 through 7 (section views).
Marine mattresses consisting of geogrid filled with angular crushed stone 3-6 inches in
diameter will be installed. The mattresses will extend up the bank face to an elevation of
approximately +26 feet MLW. The mattresses are 5 feet wide and 18 inches high, and will
be placed next to one another on the bank face to conform to the existing bank slope.
Mattress lengths will not exceed 16 feet up to 30 feet. The mattresses will be anchored to
the existing bank slope using approximately 4 Platipus ground anchors or helical anchors
4 Gabions or mattresses composed of wire would be so vulnerable to deterioration as to be infeasible.
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for each approximately 5 feet wide by 9 feet long mattress unit. Each anchor has a capacity
of one ton5, and would be driven about 12 feet deep into the coastal bank with percussion
tools. The anchor components are made from corrosion resistant materials.
The alternative also incorporates the use of returns to prevent the terminal ends (i.e., sides)
of the system from being flanked and undermined by continuing long-term erosion adjacent
to the Project. As with the marine mattresses proposed for the face of the coastal bank, the
returns will be covered with a portion of the sacrificial clean sand being supplied for
nourishment.
The final construction element will consist of placing a layer of clean, beach-compatible
sand over and in front of the marine mattresses and gabions, as shown on Sheets 4-7.
Native vegetation will be planted from elevation +28 feet MLW up to 5 to 7 feet below the
top of the coastal bank, as shown on Sheets 4 to 6.
Construction associated with installation of a marine mattress array and accompanying
gabions is less complicated than other shore protection methods (e.g., groins, geotextile
tubes). The marine mattresses and gabions will be fabricated off-island and shipped to the
island empty; they will be trucked to the site for filling and placement.
10.2 Review of Marine Mattress and Gabions Alternative
The marine mattress and gabion system offers several advantages:
1. This alternative utilizes an extremely durable yet flexible geogrid to form the marine
mattresses and gabions. The geogrid material used for the mattresses and gabions is
constructed of high tensile strength, ultraviolet-resistant HDPE material which does
not corrode or degrade from exposure to the marine environment. Existing
installations using older, less robust geogrids have been in place for more than 20
years and are still performing successfully (see Attachment I).
2. This alternative does not interfere with littoral drift. The stone-filled mattresses are
flexible and are laid directly on the prepared bank slope (1V:1.5H maximum for
stability). The gabions at the toe of the slope will be installed below the existing
beach elevation for scour protection. Therefore, the proposed system does not
extend perpendicular to the shoreline and will not interfere with the natural coastal
processes that drive the longshore transport of sediment along the beach.
5 The capacity refers to the amount of force required to pull the anchor out of the ground once installed. The capacity
can be verified in the field by pull testing a representative sample of installed anchors to verify the force required to
move it. If the actual capacity is less (or greater) than advertised due to differences in subsurface soil conditions, the
anchor spacing can be adjusted to fit more or fewer anchors per mattress.
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3. This alternative will maintain the supply of beach-compatible sediment to the littoral
system. The sacrificial sand berm covering the system will provide sufficient
sediment to the coastal system to mimic the natural, long-term erosion losses from
the coastal profile. Maintenance of the sacrificial sand cover will maintain a source
of sediment to the littoral system while preserving visual aesthetics.
4. This alternative does not cause adverse wave action onto adjacent, unprotected
areas. The voids in the stone fill in the mattresses and gabions provide a porous
surface which is better able to absorb and dissipate wave energy than "harder"
structures such as vertical seawalls. Unlike vertical barriers such as seawalls or
bulkheads, the sloped nature of the system also prevents wave reflection or focusing
of wave energy onto adjacent, unprotected areas of the bank along the shoreline.
5. This alternative utilizes several design features to minimize storm damage and
potential debris generation. The mattresses and gabions are fabricated with interior
diaphragm walls that form separate compartments within each unit. The interior
compartments prevent stone fill from settling all to one side of the unit, and also
prevent widespread loss of fill in the event the unit is damaged. The side-to-side
and end-to-end seams between mattress and gabion units will be joined together
with HDPE braided line to prevent large-scale damage or displacement of the units
during significant storms. These extremely durable connectors create a monolithic
system that further increases global stability of the installation. Total system failures
due to storm damage are not likely.
6. This alternative offers straightforward construction, installation, and, if necessary,
removal. Project removal can be easily accomplished.
7. This alternative has minimal aesthetic impacts. A sand-colored geogrid will be
used, and the Project will be covered with sand. The sand cover will be maintained
year-round.
The estimated cost for this Project’s design of mattresses and gabions with sacrificial sand
cover is approximately $3,900 per linear foot.
A Notice of Intent for the Marine Mattress and Gabion Alternative was submitted to the
Nantucket Conservation Commission and unfortunately is was denied, so this alternative is
not being pursued.
11.0 Preferred Alternative: Emergency Project - Geotextile Tube Alternative
Geotextile tubes (geotubes) are fabricated from high strength, woven polyester or
polypropylene sewn together into a tube shape that is typically 15 to 45 feet in
circumference. Once placed into position (generally parallel to the coastal resource
targeted for protection), these tubes are filled with sand pumped from the nearshore beach
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or deposited from an overhead gravity hopper into ports located on the top of the tube.
Tubes are most stable when the height to width ratio is 0.5 or less. This also corresponds to
the natural shape the tube assumes when properly filled. The tube will be filled to its
optimum shape when the width to height ratio is 2:1.
The geotextile fabrics are susceptible to UV degradation and debris damage. For this reason
it is important to geotextile tubes covered as much as possible with a sand cover.
Geotextile tubes, like any coastal engineering structure, need toe protection. This is often
provided with a polypropylene scour apron, which is typically connected to the seaward
edge of the geotextile tube to minimize undercutting by wave scour. As flexible, gravity
structures, geotextile tubes have some inherent flexibility to accommodate foundation
changes, however if the subsurface changes too much due to scour, the tube can become
unstable and can fail. Scour can occur due to wave impact and currents, so proper toe
protection is important.
11.1 Description of Geotextile Tube Alternative
The Emergency Project – Geotextile Tube design was prepared to protect the project area
from major storms. This design previously described consists of four geotextile tubes
installed in a terraced alignment, with clean sand fill cover. Toe protection would be
accomplished by use of a scour apron and anchor tube. See project description in
Emergency Certification submission
11.2 Review of Geotextile Tubes
Geotextile tubes can provide emergency protection to the coastal bank during storm events
and they can be constructed relatively quickly.
12.0 Comparison of Alternatives and Conclusions
The initial screening of alternatives has led to the emergence of the Emergency Project
option as the preferred alternative (see Table 12-1).
The No Action and retreat alternatives would not achieve Project objectives and
would lead to the loss of private homes and public infrastructure.
Coastal terraces are a temporary, financially unsustainable measure that do not offer
an effective long-term solution by themselves and cannot protect properties from
major storm events.
Beach dewatering was not effective in the Project area due to extensive storm
damage and operational difficulties.
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Passive bank drains were effective at reducing bank surface runoff erosion and
associated upper bank slumping; however, bank drains cannot prevent wave-
induced scarping at the toe of the bank, which in turn causes slumping and failure.
Beach nourishment is not feasible because the local regulatory climate currently
prohibits its use at the Project location.
The efficacy of breakwaters is highly questionable in the Project’s high-energy
environment where the beach face is quite steep.
Groins can reduce the sediment supply available to the littoral system and
potentially exacerbate erosion problems on adjacent, downdrift beaches.
Seawalls may interrupt natural sediment transport patterns and have detrimental
impacts to adjacent shorelines.
For these reasons, all of the above alternatives were determined to be not practicable.
By contrast, the preferred alternative of geotextile tubes offers an effective method of bank
protection while avoiding or minimizing impacts. The preferred Project features to
maximize bank stability and minimize the potential for debris generation, maintains the
supply of beach-compatible sand to the littoral system, does not cause adverse wave action
onto adjacent (unprotected) properties, maintains aesthetics, and can be easily constructed
and removed. The Project proposal alone offers a natural-looking, effective means of
combating erosion while avoiding adverse impacts to resources and dynamic processes
such as littoral drift.
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Table 12-1 Comparison of Alternatives Considered for the Project
Alternative Advantages Disadvantages Cost
($/linear foot)
No Action Abandonment or removal of
existing residences
Eventual destruction of existing
public services and
infrastructure, including: Baxter
Road and sewer/water services
Loss of tax revenues
Reduced public access through
loss of the Sconset Foot-Path and
public access ways
0
Retreat May avoid physical loss of
structures
Retreat within existing lot is a
temporary solution
Lack of space exists on adjacent
landward lots to move existing
homes
Infrastructure cannot be moved
Existing infrastructure (e.g.,
power lines, narrow roads) limits
the feasibility of moving large
homes longer distances
20,000-50,000
(abandonment
of lot)
1,000-3,000
(moving/re-
establishing
structure)
Bank
Stabilization –
Fabric Coastal
Bank Terraces
Temporary abatement of toe
of bank retreat
Sacrificial storm damage
prevention
Sacrificial, requiring near-
constant post-storm maintenance
Insufficient protection during
major storm events, resulting in
wave-induced scarping at the toe
of bank
1,140 initial
2,850 annual
maintenance
(finished height
of +26 feet
MLW)
Dewatering May foster beach accretion
by promoting drainage
Experimental, unproven
Susceptible to storm damage
1,375
Passive Bank
Drains
Drain perched aquifer to
prevent groundwater break-
out on bank face
Reduce upper-bank
slumping
Offers no protection to the toe of
bank
No robust stabilization of bank
face
Offers no protection from direct
wave action
1,200-1,500
(per well)
Beach
Nourishment
Uses natural shoreline
processes to achieve an
equilibrated beach profile
Protects the toe of bank by
increasing beach width
Creates a wider beach that
enhances recreational
opportunities
Allows natural longshore
transport to continue
Requires maintenance via
renourishment
Necessitates a large volume of
beach-compatible sand, likely
from offshore sources
Dredging offshore sand would
have temporary impacts on
benthic habitat and organisms
Nourishment envelope requires
covering some existing beach,
intertidal, and subtidal habitats
8-10 per cubic
yard
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Table 12-1 Comparison of Alternatives Considered for the Project (Continued)
Alternative Advantages Disadvantages Cost
($/linear foot)
Breakwater Break wave action,
artificially perching the
landward beach
Reduced effectiveness during
storm surge and storm wave
setup
Settling can reduce the
effectiveness at dissipating wave
energy
Interfere with longshore
sediment transport
Limited viability along steep
shore faces and dynamic coastal
zones, like the Project area
Occupy existing subtidal habitat
3,600
(stone
construction)
Groins Cause beach accretion,
thereby establishing a wider
beach that buffers the bank
from wave action
Interfere with longshore
sediment transport
Can be made permeable to allow
some longshore sediment
transport
Can lengthen the design life of
beach nourishment by reducing
erosion hotspots
2,700
Seawall Establishes a vertical rigid
barrier to protect the toe of
bank from wave action
Does not preserve the beach
Susceptible to scour, which can
lead to collapse
Interferes with longshore
sediment transport
Prohibited by the Nantucket
Wetlands Bylaw
The hard structure can reflect
wave energy onto adjacent,
unprotected areas
5,000
(steel sheet pile)
Mattresses and
Gabions with
Sand Cover
Effectively protect the toe of
bank by providing
protection against wave
action
Compartmentalized design
with robust connectors
creates a monolithic system
out of durable yet flexible
material, making total
system failure unlikely
Sand cover will provide the
same amount of sand to the
littoral system as would be
contributed by natural
erosion absent the Project
Does not interfere with
littoral drift
The sacrificial sand cover will
need to be replaced on a regular
basis
The NOI for this alternative was
denied by the NCC.
3,900
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Table 12-1 Comparison of Alternatives Considered for the Project (Continued)
Alternative Advantages Disadvantages Cost
($/linear foot)
The Project will absorb, not
reflect, wave energy
Removal, if necessary, is a
relatively simple process
Sand-colored geogrid and
sand cover mean minimal
aesthetic impacts
Geotextile Tubes When properly designed,
can provide effective toe of
bank protection
Susceptible to UV damage and
debris damage if left exposed
Susceptible to vandalism
Repair consists of replacing the
tube
Tubes must be filled on site
Tubes can be constructed to
provide the necessary coastal
bank protection for this winter
storm season.
912
(four 30-foot
geotubes; cost
differs for the
proposed four
45-foot
geotubes)
14.0 References Cited
Dean, Robert G.; Davis, Richard A.; and Erickson, Karyn M. 2005. Beach Nourishment with
Emphasis on Geological Characteristics Affecting Project Performance.
http://www3.csc.noaa.gov/beachnourishment/html/geo/scitech.htm. Accessed August 5, 2005.
Douglass, Scott L. 2002. Saving America’s Beaches: The Causes of and Solutions to Beach
Erosion. Advanced Series on Ocean Engineering – Volume 19. World Scientific. Pp. 48-49.
Greene, Karen. 2002. Beach Nourishment: A Review of the Biological and Physical Impacts.
Atlantic States Marine Fisheries Commission Habitat Management Series #7. November.
Harris, Rachel. 2004. Hurricanes ripped apart Florida’s shipwrecks, artificial reefs. CDNN.
October 18. www.cdnn.info/industry/i041018/i041018.html.
National Research Council (NRC). 1995. Beach Nourishment and Protection. Committee on
Beach Nourishment and Protection Marine Board; Commission on Engineering and Technical
Systems. National Academy of Sciences, National Academy Press, Washington, D.C. 352 pp.
2NOAA. 2005. Beach Nourishment: Law and Policy. NOAA Coastal Services Center.
http://www3.csc.noaa.gov/beachnourishment/html/human/law/law.htm. Accessed August 4,
2005.
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Stauble, D.K. and Tabar, J.R. 2003. The Use of Submerged Narrow-Crested Breakwaters for
Shoreline Erosion Control. Journal of Coastal Research. Volume 19, No. 3. West Palm Beach,
Florida, ISSN 0749-0208. Pp. 684-722.