HomeMy WebLinkAboutSBPF Submission 2013_11_1_201404011031364471
Attachment A
Epsilon Memorandum Regarding Retreat and Nourishment Calculations
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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
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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.
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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 –
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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
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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
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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