HomeMy WebLinkAbout20130607-Cohen email re wshop info_201404071227111988SCONSET BLUFF
EROSION CONTOL ALTERNATIVES
and
RECOMMENDATIONS
21597/Sconset i Erosion Control Recommendations for Sconset
TABLE OF CONTENTS
EXECUTIVE SUMMARY 1
EROSION CONTROL ALTERNATIVES AND RECOMMENDATIONS FOR SCONSET 3
1.0 Current Status of Baxter Road and Adjacent Homes 3
2.0 Alternatives for Road and Bluff Protection 6
2.1 Geotextile Tubes 6
2.2 Beach Nourishment 6
2.3 Dewatering 7
2.4 Breakwater 7
2.5 Groin 8
2.6 Seawall 8
2.7 Drift Fence 9
2.8 Coastal Bank Terraces 9
2.9 Marine Mattress and Gabion System 10
2.10 Revetment 11
3.0 Recommended Action and Conclusions 12
3.1 Recommended Action - Revetment 12
3.1.1 Description of Revetment Design 12
3.1.2 Environmental Considerations 12
3.1.3 Regulatory Considerations 13
3.1.4 Cost 14
3.1.5 Construction Information 14
3.1.6 Assessment of Effectiveness 14
3.2 Public Benefits 14
3.3 Obtaining Environmental Approval for Recommended Action 15
3.3.1 Emergency Project Approval 15
3.3.2 Normal (Non-Emergency) Project Approval 16
3.4 Conclusions 17
ATTACHMENT A - FIGURES AND PLAN
Figure 1 Aerial Locus Map
Figure 2 Current Setback of Road and Houses to Top of Bank, 93 to 105 Baxter
Road
Figure 3 Current Setback of Houses to Top of Bank, 69 to 83 Baxter Road
Figure 4 Current Setback of Houses to Top of Bank, 109 to 115 Baxter Road
Typical Revetment Section
21597/Sconset ii Erosion Control Recommendations for Sconset
ATTACHMENT B REVETMENT EXAMPLES
Figure B-1 Hull, Point Atherton Revetment
Figure B-2 Hull, Green Hill Revetment
Figure B-3 Scituate, Fourth Cliff Revetment
Figure B-4 Plymouth, Gurnet Revetment
Figure B-5 Plymouth, Cedarville Landing Revetments
Figure B-6 Nantucket, Capaum Pond Road Revetment
Figure B-7 Nantucket, Westcliff Lane Revetment
21597/Sconset 1 Erosion Control Recommendations for Sconset
EXECUTIVE SUMMARY
Immediate protection of Baxter Road is critical for the 551-foot stretch from 99-105 Baxter Road,
where the distance between the road and the top of the coastal bank is less than 45 feet (this
distance ranges from 29 to 44 feet). The coastal bank in this area eroded 20 to 30 feet or more
from 2012 to 2013; such catastrophic erosion may well occur again during the coming winter
storm season. Therefore, some type of protection must be in place before the start of the 2013-
2014 winter storm season or else certain sections of Baxter Road may have to be closed or may
actually be breached by the erosion.
Similarly, immediate protection of Baxter Road is also necessary for all remaining homes along
Baxter Road from 75 Baxter to the Lighthouse, to protect both Baxter Road and threatened homes
that are as close as 11-feet to the top of the coastal bank. Significantly, most of the homes from 75
Baxter Road to the Lighthouse would be lost if another severe winter storm season occurs. The
opportunity for private funding of erosion control efforts will become more limited if additional
homes are lost, moved, or demolished.
Multiple erosion control alternatives have been considered. Sconset is a high-energy environment;
therefore, some alternatives are highly unlikely to be effective, such as geotextile tubes and
breakwaters. Other alternatives, such as beach dewatering and drift fences, have been tried and
proven ineffective over the long term at this location. Still other alternatives, such as seawalls and
beach nourishment, could be effective but would be difficult or impossible to get permitted under
the Massachusetts Wetlands Protection Act (WPA) and Nantucket Wetlands Bylaw regulations, and
have other negative aspects. The groin alternative would not be effective or environmentally
feasible at Sconset without an accompanying beach nourishment project or significant sand
mitigation program. The coastal bank terraces alternative has been implemented at Sconset for
multiple years as a short-term measure, but it has been repeatedly documented that this alternative
simply cannot withstand major storms and requires nearly constant post-storm maintenance, which
is not always feasible in successive storm situations. A marine mattress/gabion alternative or a
revetment alternative would both provide effective protection for Baxter Road and adjoining homes
in a high-energy environment like Sconset. Revetments offer an advantage over the marine
mattress system in that they do not require anchoring into the bank face, are composed of natural
rock material, would not require removal in the event of system failure, and have a comparable or
lower cost than the marine mattress/gabion alternative. Revetments are also easy to install and offer
a very long service life. Revetments are therefore the recommended alternative for erosion control
at Sconset.
Given the high erosion rates of the coastal bank, it is critical that the revetment is installed by late
fall of 2013. Meeting this schedule would likely require authorizing the revetment as an
“Emergency Project” under the WPA or obtaining Conservation Commission approval for the
project in a single public hearing or compressed public hearing process during the month of July.
Subsequent to this approval, 4-6 weeks would be required to select a contractor; followed by 8-10
weeks for construction of a 551-linear foot revetment, or 4 months for a 1,500-linear foot
21597/Sconset 2 Erosion Control Recommendations for Sconset
revetment. Provided regulatory approval is secured by July, the contractor could be selected in
August, and construction could occur from mid-August or September through early December,
depending on the length of revetment installed.
In addition to the areas requiring immediate protection, near term (2014) protection of Baxter Road
is also recommended for the remaining stretch of Baxter Road from 53 Baxter to the Lighthouse.
The significant erosion occurring along Baxter Road from 2012-2013 underscores the point that this
entire section is vulnerable to erosion and should be protected as soon as possible. Private support
for those areas from 75 Baxter Road to the Lighthouse requiring immediate protection cannot be
achieved without a Board of Selectmen commitment to support near term (2014) extension of the
revetment across the entire area from 53 Baxter to the Lighthouse.
21597/Sconset 3 Erosion Control Recommendations for Sconset
EROSION CONTROL ALTERNATIVES AND RECOMMENDATIONS FOR
SCONSET
1.0 Current Status of Baxter Road and Adjacent Homes
Baxter Road is a public way owned by the Town of Nantucket that 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 (Figure 1). Various utilities
(water, sewer, communication and electric services) are also included within Baxter Road.
At present, sections of Baxter Road are now within 29 to 44 feet of the edge of Sankaty Bluff
(referred to herein as the “coastal bank”) (Figure 2). Similarly, several homes along Baxter
Road are within 10 to 20 feet of the top of the coastal bank (Figures 3 and 4). Many of
these homes have significant historical value1 and they also represent a significant financial
contribution to the Town of Nantucket via taxes paid.
Erosion of the coastal bank along the northern portion of Baxter Road has been apparent
since 1994, and has been accelerating over the past decade. As detailed in Table 1 below
(which provides information on loss of costal bank for each lot from 55 Baxter Road to 119
Baxter Road), last winter (2012-2013) was particularly severe and resulted in coastal bank
retreat of 20 to 30 feet or more along most Baxter Road properties. While the 2012-2013
winter season yielded higher-than-average erosion rates, it underscores the point that
chronic erosion along this portion of the coastal bank can be catastrophic and that annual
coastal bank losses of 20 to 30 feet or more must be anticipated.
Table 1 also presents the following information:
1. Current (May 2013) minimum setback distance between Baxter Road and the top of
the bank,
2. March 2012 setback distance between Baxter Road and the top of the bank,
3. Amount of erosion of the bank last year (March 2012 to May 2013), and
4. Current (May 2013) setback distance between buildings on each lot and the top of
the bank.
This table makes it clear that sections of Baxter Road are within one major storm (or series
of successive storms) of being undermined or lost. Likewise, several homes are within one
major storm or series of successive storms of having to be moved or demolished, while
many others will be threatened in the near future (See Figures 3 and 4).
1 Nantucket Preservation Trust. 2007. ‘Sconset Historic Site Survey.
21597/Sconset 4 Erosion Control Recommendations for Sconset
Table 1 Baxter Road and Building Setback Distances to Top of Coastal Bank & 2012-
2013 Coastal Bank Erosion Rates for 55 to 119 Baxter Road
Lot #
Property
Length (ft)
2013 Minimum
Baxter Road
Setback to Top
of Bank (ft)1
2012 Baxter
Road Setback to
Top of Bank (ft)2
2012-2013 Loss
of Coastal Bank
(ft)
2013 Minimum
Building Setback to
Top of Bank (ft)
119 67 84 94 10 n/a
117 113 95 100 5 n/a
115 104 91 103 12 18
113 93 84 98 14 13
Way 20 96 102 6
109 163 80 101 21 11
107 162 65 92 27 n/a
Way 18 65 90 25
105 166 29 60 31 n/a
101 188 37 60 23 n/a
24
99 173 44 74 30 17
97 171 61 93 32 24
Way 18 88
93 89 58 81 23 8
91 94 52 87 35 n/a
87 164 57 100 43 n/a
Way 26
85 281 73 100 27 n/a
83 134 91 115 24 32
81 106 115 138 23 18
79 99 122 144 22 13
77 88 120 144 24 75
75 80 127 148 21 11
73 149 122 148 26 56
Way 30
71 102 144 167 23 67
69 109 160 n/a n/a 46
67 79 181 n/a n/a 50
Way 30
65 69 179 n/a n/a 29
63 182 164 n/a n/a 30
Way 23
61 128 162 n/a n/a 69
59 147 153 n/a n/a 60
55 119 93 n/a n/a 32
Notes:
1. 2013 distances based on field measurements taken on 5/30/13.
2. 2012 distances based on March 2012 aerial photo; distances were measured in the same location on each lot
as the 2013 measurements for consistency.
n/a - Top of bank retreat not yet apparent on 2012 aerial photo; erosion of lower portion of bank apparent on
many properties.
21597/Sconset 5 Erosion Control Recommendations for Sconset
In light of these conditions, the following recommendations are made:
Immediate protection of Baxter Road is critical for the 551-linear foot stretch
from 99-105 Baxter Road, where the distance between Baxter Road and the top
of the coastal bank is less than 45 feet (this distance ranges from 29 to 44 feet;
see shaded area in Table 1 below and Figure 2). Bank protection must be in
place before the 2013 winter storm season in order to eliminate the risk of road
closure.
Immediate protection is also necessary for all remaining homes along Baxter
Road from 75 Baxter to the Lighthouse, to protect both Baxter Road and
threatened homes that are as close as 11-feet to the top of the coastal bank (see
Figures 2-4). Most of the homes from 75 Baxter Road to the Lighthouse would
be lost if another severe winter occurred. Protection of these homes likewise
protects Baxter Road and its associated utilities. Further, the opportunity for
private funding of erosion control efforts will become more limited if additional
homes are lost, moved, or demolished.
In addition to the areas requiring immediate protection, near term (2014) protection of
Baxter Road is also recommended for the remaining stretch of Baxter Road from 53 Baxter
to the Lighthouse. The significant erosion occurring along Baxter Road from 2012-2013
underscores the point that this entire section is vulnerable to erosion and should be
protected as soon as possible. Private support for those areas from 75 Baxter Road to the
Lighthouse requiring immediate protection cannot be achieved without a Board of
Selectmen commitment to support near term (2014) extension of the revetment across the
entire area from 53 Baxter to the Lighthouse.
21597/Sconset 6 Erosion Control Recommendations for Sconset
2.0 Alternatives for Road and Bluff Protection
This section provides a summary description of ten alternatives for preventing erosion of the
coastal bank at Sconset.
2.1 Geotextile Tubes
Geotextile tubes (geotubes) are fabricated from high strength, woven polyester or
polypropylene sewn together into a tube shape and filled with sand. A conceptual geotube
design for a 50-year storm would consist of at least four 30-foot-circumference geotextile
tubes installed in a terraced alignment and covered with clean sand fill. Construction
would require excavating the existing profile to +4.5 feet MLW and installing a 3-foot-
circumference anchor tube and scour apron. Geotubes would then be installed and filled
on the excavated terraces to approximately 5 feet tall and 11 feet wide. After the geotubes
were filled, a clean sand fill would be placed to a top elevation of approximately +23.5
feet MLW. The sand fill would be placed on a 1 vertical: 2.5 horizontal slope to meet
existing grade while maintaining a continuous one foot thick sand cover over the filled
tubes.
Geotextile tubes are not well-suited to a high energy environment like Sconset. Too much
scour at the toe could potentially lead to structural failure (even when a scour apron is
included in the design). Geotubes are susceptible to damage from vandalism, debris, and
storm waves; storm-driven debris may puncture and tear the tube. For this reason,
maintenance costs for geotubes tend to be higher than for other alternatives. When ripped
open by storm waves, geotextile tubes may fail in place, emptying sand onto the beach and
possibly releasing geotextile material to the coastal environment. The release of sacrificial
sand would not have any adverse environmental effects since clean, beach-compatible sand
would be used to fill the tubes. However, replacement of the geotube would be expected
to be required on a frequent basis (one or more times annually). Such replacement often
cannot be accomplished between successive storms, potentially leaving the bank
vulnerable to wave-induced scarping at the toe (and subsequent slumping of the upper
bank, which undermines vegetative stabilization that otherwise works) at the time when
protection is most needed. For these reasons, geotubes are not considered a viable long-
term erosion control solution.
2.2 Beach Nourishment
Beach nourishment would involve the placement of approximately 2.6 million cubic yards
of sand on Sconset Beach. The nourished beach would be approximately 200 feet wide
with a berm height of 12-16 feet above MLW. Sand would be obtained from an offshore
borrow site; a likely candidate would be the offshore shoal system known as Bass Rip,
though other potential sites could also be evaluated. The wider beach would absorb and
dissipate wave energy, thereby increasing protection to infrastructure and property
threatened by erosion and storm damage. Additionally, the wider beach would potentially
21597/Sconset 7 Erosion Control Recommendations for Sconset
offer increased public recreation opportunities. Renourishment would be required
approximately every 5 years.
While the beach nourishment alternative offers significant benefits, there are potential
adverse impacts that must be carefully minimized and/or mitigated. The nourishment
envelope would cover more than 125 acres of beach, inter-tidal, and sub-tidal habitats.
Direct mortality to the marine organisms living in or on the seafloor would be unavoidable
in those areas covered by the beach nourishment sand, although these resources would be
expected to recover in 1-3 years. Areas of nearshore cobble habitat important to the local
fishing community would be temporarily or permanently impacted by the placement of the
beach fill and would require replication of such habitat nearby. While beach nourishment
could be planned and executed in a manner that protects threatened public infrastructure
and homes while resulting in a minimal level of environmental impact, beach nourishment
is not desirable at Sconset due to potential impacts to locally important fishery resources.
Therefore, beach nourishment is not a recommended erosion control alternative.
2.3 Dewatering
Beach dewatering systems are based on the supposition that draining water from the beach
face can increase the percolation of swash zone water into the beach and promote the
deposition of new sediment that is actively moving through the swash zone. The Sconset
Beach Preservation Fund (SBPF) installed four different systems that met with mixed levels
of performance success. Three of the systems, Lighthouse North (LHN), Lighthouse South
(LHS), and Lighthouse South-South (LHS-S) were severely damaged and ultimately removed.
The fourth system at Codfish Park is covered by the existing beach, due to beach accretion
after its installation and subsequent system upgrade. Once the beach accreted to a point
where the system could no longer pump effectively, operations were terminated and the
beach responded by eroding.
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 long-term, effective protection of
threatened public infrastructure and homes.
2.4 Breakwater
Manufactured breakwaters typically consist of structures placed in water depths of 7 to 9
feet that are intended to break wave action and artificially perch the landward beach.
Breakwaters could consist of reef balls, narrow-crested artificial reefs, sunken barges, or
rubble mounds. Artificial reef balls and narrow-crested artificial reefs would have limited
effectiveness in a high-energy environment like Sconset. Sunken barges would offer a
21597/Sconset 8 Erosion Control Recommendations for Sconset
broader profile to potentially dissipate wave energy but would be subject to corrosion,
settlement, and movement along the seafloor due to hydrodynamic forces. Environmental
permits for sunken barges may be difficult to obtain given their likely impacts and uncertain
effectiveness.
An effective offshore breakwater design at Sconset would likely require a large emergent
rubble-mound breakwater system, which would occupy a large area of the bottom,
permanently impacting nearshore habitat. 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 long-term efficacy of such a structure is also
questionable. Future consideration of a breakwater on an experimental basis, particularly if
combined with other erosion control measures, may be warranted. At present, breakwaters
are not a preferred alternative for erosion control at Sconset due to limited or questionable
long-term effectiveness, potential impacts to nearshore habitat, and potential impacts to
downdrift beaches.
2.5 Groin
Groins are concrete, rock, and/or timber structures constructed perpendicular to the
shoreline that are designed to catch and trap sand being transported downdrift. Groins
compartmentalize the shoreline, minimizing sediment losses from a given section of beach
which can have adverse impacts to adjacent downdrift beaches by reducing the sediment
supply available to these areas. When used in conjunction with a beach nourishment
project, low-profile or semi-permeable groins can reduce the rate of sediment loss from
erosion hotspots within a nourished design beach profile and enhance the effectiveness of
nourishment. When used on their own without sand mitigation; however, traditional
coastal groins are not preferred due to downdrift impacts and consequently environmental
regulatory constraints. Groins would not be effective or environmentally feasible at Sconset
without an accompanying beach nourishment project or significant sand mitigation
program.
2.6 Seawall
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. 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.
21597/Sconset 9 Erosion Control Recommendations for Sconset
Seawalls are not a preferred erosion control method for Sconset primarily due to
environmental regulatory constraints stemming from likely environmental effects. Since
they are vertical, smooth structures, seawalls in high energy beach areas like Sconset can
cause additional wave reflection and turbulence and thus maintaining a beach in front of
them can be a challenge. Depending on local conditions, seawalls can decrease the supply
of sediment in the littoral system. Additionally, removing a seawall of the size and depth
below the beach required at Sconset in the event of negative impacts would be extremely
expensive and difficult.
2.7 Drift Fence
Drift fence is designed to trap wind-blown sand and is typically placed a few feet seaward
of an eroding dune or coastal bank. While drift fence can cause some minor accumulation
of sand, it has been shown ineffective over the long term in a high energy environment at
Sconset. The SBPF installed “Duneguard” drift fence along with the beach dewatering
systems over a decade ago. Sand nourishment was placed landward of the drift fence to
add a layer of protection to the toe of the coastal bank. Even with this provision, however,
the drift fence did not make a measureable impact on the erosion of the coastal bank over
the long term. Drift fence can also be a significant source of marine debris when it fails
after coastal storms. Drift fence is therefore not a suitable alternative for preventing erosion
at Sconset.
2.8 Coastal Bank Terraces
Coastal bank terraces of differing designs (utilizing mats or bags fashioned from coir or jute)
have been utilized seaward of portions of Baxter Road since the early 2000’s. The current
design has been in use since 2006, most consistently at 79 Baxter Road, and consists of jute
fabric terraces 3-feet high by 5-feet deep by 50-feet long, constructed to an elevation of 12
to 13 above the back of the beach (approximately +20 to +23 MLW). The terraces are
secured using duckbill ground anchors buried into the coastal bank. The current design
eliminates the anchor stake and coir debris that occurred when earlier terrace installations
were damaged during storms. These terraces are designed to be sacrificial. During storm
events, they rip open and release sand to the beach; therefore, the terraces have to be
completely replaced once or twice a year. Sand for the terraces is obtained from on-island
pits and is trucked to Baxter Road, where it is dumped over the top of the bank at
designated access points and then placed in the terraces using bobcat or skid steer
equipment. Approximately 9-10 cubic yards of sand per linear foot of terraces have been
added to the littoral system each year through the maintenance of the terraces. Nearby
properties have benefited from the diffusion of sand onto their adjacent beaches.
The terraces have helped to prevent erosion of the toe of the coastal bank at 79 Baxter
Road; however, these terraces do not provide sufficient protection during major or
successive storm events and cannot be considered a viable long-term option. The strongest
storms the terraces have been subjected to have been 10-year storms, and the terraces have
21597/Sconset 10 Erosion Control Recommendations for Sconset
not withstood those events. Rather, the sacrificial terraces have ripped open and released
their sand as designed, subsequently requiring complete replacement multiple times a year.
During major or successive storm events, wave-induced scarping can still occur at the toe of
the coastal bank, followed by slumping of the upper bank, which undermines vegetative
stabilization of the upper bank that otherwise works. Such slumping tears out vegetation on
the upper bank and emphasizes that vegetation on the upper bank face cannot have a
meaningful impact on slowing upper bank erosion without effective toe protection in place.
Terraces are not a viable long-term option because they cannot protect the toe of the coastal
bank during major or successive storms and require nearly constant post-storm
maintenance.
2.9 Marine Mattress and Gabion System
This alternative would consist of placing flexible marine mattresses and gabion baskets on
the lower portion of the coastal bank face and along the toe of the coastal bank. Three rows
of gabion baskets would be buried in the beach; each basket would be 4-foot high x 5-foot
wide x 10-foot-long and filled with 12- to 22-inch diameter stones. Up to three rows of
marine mattresses would be placed on top of the gabions along the bank face; each
mattress would be 6 feet wide x 18 inches high x 16-30 feet long and filled with 3- to 6-
inch diameter stones. The mattresses would extend up the bank face to an elevation of
approximately +26 feet Mean Low Water (MLW), in order to provide protection from a
100-year storm. Both the mattresses and the gabions would be formed from HDPE geogrid
material. Returns would be placed at the ends of the system to prevent end scour. The
marine mattresses and returns would be anchored to the existing bank slope using Platipus
ground anchors or helical anchors driven about 12 feet deep into the coastal bank under
the mattresses. Finally, a sand cover would be placed over the entire installation and the
upper portion of the bank would be planted with native vegetation to provide habitat and
decrease erosion of the upper bank.
Marine mattresses and gabions have been used effectively in multiple locations throughout
Massachusetts (Boston, Plymouth, Martha’s Vineyard, and at Hinckley Lane on Nantucket)
and also at Cape May, New Jersey. Experience at these locations has shown that properly-
designed marine mattress and gabion systems can be effective in high-energy environments
like Sconset. Including a sediment cover in the design may prevent potential impacts to
adjacent beaches. The sloped and porous nature of the system would minimize wave
reflection and encourage energy dissipation. Disadvantages associated with the system are
that the geogrid material must be removed if the system fails, the relatively high cost of this
alternative, and the (inaccurate) perception that the required anchors may serve to
destabilize the coastal bank. This perception is part of why a marine mattress and gabion
project at Sconset was not permitted by the Conservation Commission in 2012. For these
reasons, marine mattresses and gabions are not a preferred alternative.
21597/Sconset 11 Erosion Control Recommendations for Sconset
2.10 Revetment
A revetment is a coastal engineering structure consisting of rock facing to protect a sloping
embankment against erosion. Revetments generally consist of an outer protective layer of
large rocks (approximately 3-6 feet in diameter), an inner core layer of smaller rocks, and
toe protection. The outer protective layer provides the basic protection against wave
action, while the inner core layer acts as a filter layer to assure drainage and retention of the
underlying soil. The toe protection is needed to provide stability against undermining at the
bottom of the structure. The primary advantage of a revetment is its flexibility, which
allows it to settle into the underlying soil or experience minor damage and still retain its
ability to protect the bank from erosion. Revetments also have rough surfaces, which reduce
wave runup and overtopping.
Like other coastal engineering structures placed in front of or on the face of a coastal bank
to provide storm damage prevention, revetments can potentially interrupt the supply of
sediment to the littoral system; therefore, monitoring of adjacent beaches and/or some type
of initial or annual sand mitigation would be required. Additionally, like other coastal
engineering structures, scour may potentially occur at the either end of the structure from
wave reflection. The most suitable end treatment when there are adjacent eroding banks is
to taper the ends of the structure using smaller stone to help blend the structure into the
adjacent, unprotected bank.
Revetments are a common coastal structure used for erosion control and have been proven
effective in environments similar to Sconset (see Section 3.1.6 below and Attachment B).
Revetments are relatively easy to install and maintain, with very long service lives.
Additionally, the cost of revetment installation is comparable to or less than other
alternatives considered and maintenance costs tend to be lower. Revetments also consist of
natural rock material and do not require removal in the event of system failure. For these
reasons, a revetment is considered the preferred alternative for preventing further erosion of
the coastal bank, thereby protecting Baxter Road and the homes along it.
21597/Sconset 12 Erosion Control Recommendations for Sconset
3.0 Recommended Action and Conclusions
3.1 Recommended Action - Revetment
The recommended action to protect Baxter Road, its associated infrastructure, and private
homes along Baxter Road is a revetment. Revetments are proven effective in high-energy
environments such as Sconset, are easy to install and maintain, are composed of natural
rock material, are not anticipated to result in adverse environmental impacts, and offer a
cost of installation and maintenance that is comparable to or less than other alternatives.
Given the high-energy wave environment at Sconset, “softer” solutions simply cannot be
effective long-term against major storms.
Details of the revetment installation are provided below.
3.1.1 Description of Revetment Design
In Sconset, a rock revetment could be placed on the lower slope of the coastal bank from
Elevation +0.0 MLW to Elevation +26 MLW to provide protection against wave action
from a 100-year design storm. The revetment would consist of a geotextile fabric lining the
coastal bank, a filter layer of 4-inch to 8-inch diameter crushed stone, and two layers of
larger “armor” stone (see accompanying sheet for size gradation). The armor layer would
provide the basic protection against wave action, while the filter layer assures drainage and
retention of the underlying soil. The revetment would likely be graded to a maximum slope
of 1 foot vertical to 1.5 feet horizontal (see accompanying sheet in Attachment A).
During installation of such a revetment, a layer of geotextile filter fabric would first be
placed on the bank face. Next, a minimum 18-inch thick filter layer consisting of 4-inch to
8-inch diameter crushed stone would placed. Two layers of armor stone would be placed
on top of the filter layer to form a thickness of 6-feet (when measured perpendicular to the
face of the coastal bank). The revetment toe (consisting of armor stone, filter layer, and
geotextile filter fabric) would be buried a minimum of 5-feet deep into the existing beach.
The toe protection is needed to provide stability against undermining at the bottom of the
structure. Lastly, a sand cover or other form of sand mitigation could be added on the face
of the revetment or seaward of the revetment; the volume and frequency of this sand
placement would likely be determined during the Conservation Commission review
process. Sand covers are not typically placed on the face of revetments (see photographs of
typical revetments in Attachment B) but may be included for aesthetic reasons or to supply
sand to the littoral system. Native vegetation could be planted above the revetment in the
appropriate places to stabilize the upper bank.
3.1.2 Environmental Considerations
The revetment will be sloped, and the stones placed to form a rough surface to maximize
wave energy dissipation and minimize wave reflection to adjacent, unprotected areas. No
construction or placement of sand fill would occur seaward of the Mean High Water line.
21597/Sconset 13 Erosion Control Recommendations for Sconset
Thus, the revetment would not have any adverse effects on marine fisheries, shellfish beds,
or other biological communities such as mole crabs which reside the swash zone. A sand
cover could be included for aesthetic reasons and to provide sand to the littoral system to
avoid impacts to adjacent beaches; however, a sand cover is not necessary for the
revetment to effectively protect the toe of the coastal bank.
3.1.3 Regulatory Considerations
The Massachusetts Wetlands Protection Act (WPA) and Nantucket Wetlands Bylaw and
regulations (Bylaw) have specific provisions governing the installation of a coastal
engineering structure such as a revetment. The revetment will be placed on the face of the
lower coastal bank, and the toe portion of the revetment will be buried at the interface
between the coastal bank and coastal beach.
Under the WPA, coastal engineering structures are allowed to prevent storm
damage to pre-1978 buildings. The protected structures can be on the water or
land side of the road. Additionally, protection across “gap lots” between pre-
1978 homes may also be permitted.
Under the Nantucket Wetlands Bylaw regulations, coastal engineering structures
are allowed on a coastal bank to protect public infrastructure, or pre-1978
structures that have not been substantially improved, from imminent danger.2 A
revetment on a coastal bank is thus permissible. Coastal engineering structures
are also allowed on a coastal beach to protect pre-1978 structures that have not
been substantially improved from imminent danger.3 A revetment on a coastal
bank and coastal beach is thus permissible, but may require determinations or
waivers by the Town and Conservation Commission.
Additionally, §67-1 of the Code of the Town of Nantucket provides for a temporary
moratorium (ending December 31, 2013) on coastal engineering structures on Town-owned
land from Great Point south to and including the Sconset sewer beds. Subsection D
provides for an exception from the moratorium for emergency actions to protect public
2 NAN2.05(B)1. “No new bulkheads, coastal revetments, 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.”
3 NAN 2.02(B)2. “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 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.”
21597/Sconset 14 Erosion Control Recommendations for Sconset
infrastructure: “This moratorium shall not prohibit emergency armoring measures necessary
to protect public roads, public buildings, or other public assets from imminent destruction.”
In the case of Sconset, Baxter Road is clearly threatened as well as public property assets.
3.1.4 Cost
The cost of installation and maintenance is comparable or lower than a marine
mattress/gabion system, and is substantially lower than terraces.
3.1.5 Construction Information
Approximately 8-10 weeks would be required to install the 551-foot section of revetment.
Approximately 1500-feet of revetment could be installed to protect the most severely
threatened homes from 75 Baxter to the Lighthouse in approximately 4 months.
Construction could occur from August or September into early December.
It is highly likely that the construction would be staged from the beach, with barges
bringing in rocks and other supplies and offloading them to a barge anchored at the beach
with a gangway to the beach. This approach would avoid truck traffic on local roads during
the late summer.
3.1.6 Assessment of Effectiveness
Revetments are a common coastal structure used for erosion control along shorelines in the
northeast due to their proven effectiveness and the availability of suitable rock from local
quarries. Numerous examples exist of successful revetment projects that have been
constructed along Cape Cod and Southeastern Massachusetts. Examples of several such
revetment projects are provided in Attachment B, which includes photographs of
revetments in Plymouth, Scituate, Hull, Marshfield, and Nantucket. Like Sconset, the
revetments included in Attachment B were installed on a coastal bank in a location facing
open water. This information shows that revetments can be used for long-term, effective
protection of public infrastructure and homes.
3.2 Public Benefits
Erosion protection at Sconset would provide the following public benefits:
Protection of Baxter Road and associated utilities.
Preservation of historic homes and historic settings, as well as the relocated but
still ultimately endangered Sankaty Head Lighthouse.
Preservation and possible restoration/expansion of the ‘Sconset Bluff Walk.
Preservation of tax revenue for Town of Nantucket and restoration of significant
future tax revenues from deeply depreciated currently threatened properties.
21597/Sconset 15 Erosion Control Recommendations for Sconset
Preservation of private funding sources for erosion control efforts that protect
Baxter Road through retention of private homes on Baxter Road, both now and
in the future.
Possible inclusion of additional points of beach access to the general public.
3.3 Obtaining Environmental Approval for Recommended Action
As indicated above, it is critical that the most vulnerable section of Baxter Road (from 99-
105 Baxter Road) is protected prior to the 2013-2014 winter storm season, or it is highly
likely that part of Baxter Road will have to be closed or will actually be lost by slumping
associated with ongoing bank erosion. Likewise, it is necessary to provide imminent
protection for most of the homes from 75 Baxter Road north to the Lighthouse, since most
of these homes could be lost during one severe storm season. A revetment would require
both the support of the Board of Selectmen and approval from the Nantucket Conservation
Commission. Such approval would be required by July in order to meet the following
scheduling requirements:
Installation of the system must be completed by late fall to provide protection
from winter storms.
Approximately 6 weeks will be required to prepare a request for proposals,
solicit bids, and select a contractor.
Installation of the system will require 8-10 weeks for a 551-foot long stretch; or
4 months for an up to 1500-foot long stretch.
Under these timeframes, approval from the Conservation Commission by July would be
necessary, to allow selection of the Contractor by August, with construction to commence
in mid-August or September and finish no later than early December.
There are two potential means through which a revetment project could receive
authorization from the Conservation Commission: as an “Emergency Project” or through the
typical Notice of Intent (NOI) review process under the WPA and Bylaw. Each of these
options is discussed below.
3.3.1 Emergency Project Approval
The procedure for obtaining approval for an emergency project is outlined in the WPA
regulations at 310 CMR 10.06 and described below:
The Conservation Commission or DEP Commissioner has to certify the project
as an "emergency."
21597/Sconset 16 Erosion Control Recommendations for Sconset
An emergency certification can be issued only for "the protection of public
health or safety."
The Conservation Commission has 24 hours to act after receiving the
application form.
The timeframe for work to be performed is only 30 days, unless the DEP
Commissioner approves the extension.
The only work allowed is that which would "abate" the emergency.
There is an appeal period associated with the emergency action approval, but
work can occur during the appeal period.
The protection of Baxter Road for the 551-foot section from 99-105 Baxter Road may
potentially qualify as an emergency, although this is not certain. If the project were
certified as an emergency, a timeframe longer than 30 days would have to be approved in
order to allow the 8-10 weeks needed for revetment installation from 99-105 Baxter Road
or the 4 month installation timeframe required for a 1500-linear foot revetment. Typically
emergency certification is given after an emergency such as a major storm has occurred and
immediate action is needed to abate the emergency conditions created by the storm
damage. In the case of Sconset, an emergency certification would have to be granted under
a more pro-active approach where the foreseeable loss of Baxter Road and/or homes is
imminent but has not yet occurred.
3.3.2 Normal (Non-Emergency) Project Approval
The normal procedure for obtaining approval for a project under the WPA and Bylaw
involves the submission of a NOI followed by one or more public hearings, after which an
Order of Conditions (OOC) is issued. Following the issuance of the OOC, there is a 10
business day appeal period under the WPA regulations and a 20 day appeal period under
the Nantucket Wetlands Bylaw.
In order for the revetment to be installed prior to the 2013-2014 winter storm season, the
revetment must receive Conservation Commission approval during the month of July. A
NOI would have to be filed in late June or early July in order to be heard by the
Commission in July. The Conservation Commission would have to be prepared to address
any questions or concerns about the project within one hearing and to issue an Order of
Conditions soon after the close of the single public hearing.
21597/Sconset 17 Erosion Control Recommendations for Sconset
3.4 Conclusions
Immediate protection of Baxter Road is critical for the 551-foot stretch from 99-105 Baxter
Road, where the distance between the road and the top of the coastal bank is less than 45
feet (this distance ranges from 29 to 44 feet). Immediate protection of Baxter Road is also
necessary for all remaining homes along Baxter Road from 75 Baxter to the Lighthouse, to
protect both Baxter Road and threatened homes that are as close as 11-feet to the top of the
coastal bank. Most of the homes from 75 Baxter Road north to the Lighthouse would be
lost if another severe winter occurred.
A rock revetment would provide effective protection in a high-energy environment like
Sconset with no significant adverse environmental impacts; the cost of this alternative is
comparable to, or lower than, the other alternatives. Given the high erosion rates of the
coastal bank (up to 25 feet or more of the bank can be lost in one strong winter storm
season), it is critical that the revetment is installed by late fall 2013. Meeting this schedule
would require authorizing the revetment as an “Emergency Project” under the WPA or
obtaining Conservation Commission approval for the project in a single public hearing
during the month of July. Following this approval, contractor selection could occur in
August, and revetment installation could occur from mid-August or early September through
early December, depending on the length of revetment installed.
In addition to the areas requiring immediate protection, near term (2014) protection of
Baxter Road is also recommended for the remaining stretch of Baxter Road from 53 Baxter
to the Lighthouse. The significant erosion occurring along Baxter Road from 2012-2013
underscores the point that this entire section is vulnerable to erosion and should be
protected as soon as possible. Private support for those areas from 75 Baxter Road to the
Lighthouse requiring immediate protection cannot be achieved without a Board of
Selectmen commitment to support near term (2014) extension of the revetment across the
entire area from 53 Baxter to the Lighthouse
Attachment A
Figures
Figure 1 Aerial Locus Map
Figure 2 Current Setback of Road and Houses to Top of Bank, 93 to 105 Baxter Road
Figure 3 Current Setback of Houses to Top of Bank, 69 to 83 Baxter Road
Figure 4 Current Setback of Houses to Top of Bank, 109 to 115 Baxter Road
Typical Revetment Section
Figure 1Aerial Locus Map
Nantucket, Massachusetts
G:\Projects\Lighthouse\2013\aerial3.mxd
LEGEND
Basemap: 2012 Bing Aerial Imagery, ESRI °0 150 30075
Feet1 inch = 300 feet
Scale 1:3,600
Parcel Boundary
Figure 2. Current Setback of Road and Houses to Top of Bank93 to 105 Baxter RoadLot 99Lot 101Lot 105Road Setback44’Lot 93Lot 97Road Setback37’Road Setback29’House Setback8’House Setback24’
Figure 3. Current Setback of Houses to Top of Bank69 – 83 Baxter RoadLot 75Lot 79Lot 8111’ setback13’ setback18’ setbackLot 73Lot 71Lot 69Lot 83
Figure 4. Current Setback of Houses to Top of BankLots 109 - 115Lot 109Lot 113Lot 11511’ setback13’ setback18’ setbackFormer Location of Lighthouse
Attachment B
Revetment Examples
Figure B-1 Hull, Point Atherton Revetment
Figure B-2 Hull, Green Hill Revetment
Figure B-3 Scituate, Fourth Cliff Revetment
Figure B-4 Plymouth, Gurnet Revetment
Figure B-5 Plymouth, Cedarville Landing Revetments
Figure B-6 Nantucket, Capaum Pond Road Revetment
Figure B-7 Nantucket, Westcliff Lane Revetment
^_
^_
^_
^_
^_
^_^_
Gurnet Revetment, Plymouth, MA
Green Hill Revetment, Hull, MA
Point Allerton Revetment, Hull, MA
Fourth Cliff Revetment, Scituate, MA
Westcliff Lane Revetment, Nantucket, MA
Capaum Pond Road Revetment, Nantucket, MA
Cedarville Landing Revetments, Plymouth, MA
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX,
Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User CommunityFigure 1Existing Revetments
Nantucket, Massachusetts
G:\Projects\Lighthouse\2013\revetments.mxd
LEGEND
Existing Revetment Location^_°0 4.5 92.25
Miles1 inch = 9 milesScale 1:570,240
Figure B-1 – Hull, Point Atherton Revetment
Figure B-2 – Hull, Green Hill Revetment
Figure B-3 – Scituate, Fourth Cliff Revetment
Figure B-4 – Plymouth, Gurnet Revetment
Figure B-5 – Plymouth, Cedarville Landing Revetments
Figure B-6 – Nantucket, Capaum Pond Road Revetment
Figure B-7 – Nantucket, Westcliff Lane Revetment
BAXTER ROAD PUBLIC-PRIVATE PARTNERSHIP
June 7, 2013
As has been advised by Town Counsel, the Town of Nantucket has legal and political
obligations to act to protect or replace public roads and infrastructure in cases of erosion, and may also have other liabilities for damages in such cases. Dealing with these issues can be very burdensome on Town staff, public funds, and otherwise. However, interested private
parties can play an important role in easing this burden and making the process less
expensive, more efficient and more successful. There are several private entities that could
play a role, including the Sconset Beach Preservation Fund, the Sconset Trust, the Sankaty Head Golf Club, the Nantucket Preservation Trust, local property owners, and others. The Town should engage with these parties on this issue.
The Town is currently considering proposals that would reroute the access and utilities at
the northern end of Baxter Road and is also investigating options to stabilize and protect the Sconset bluff (as well as the historic homes and places, public infrastructure, public access, environmental habitats, tax valuations and other valuable resources at risk). Undertaking
these actions can be very complicated and expensive. By engaging in a cooperative process,
private parties could offer a windfall of benefits to the Town, including:
1) Engage in preliminary research and data production at private expense. 2) Engage with residents to encourage cooperation with the Town regarding legal actions that can be cooperative or adversarial. Instead of residents opposing the Town over
property rights and funding sources, a cooperative approach could be faster and less
expensive, while also avoiding potential claims against the Town for damages.
3) Develop donors and supporters to defray a significant portion of public cost for needed infrastructure and infrastructure protection. 4) Help avoid some initial and ongoing or repetitive costs to Town.
5) Potentially fund, gift or directly pay for some or all initial costs, contingent on an
agreement for the comprehensive project.
6) Avoid some burdensome or lengthy processes, like 30B bidding, as appropriate. 7) Obtain permits and act as project manager or operator, as appropriate. 8) Encourage expansions of public access to be built into the project.
9) Create tax deduction benefits for private participants.
The terms and conditions that private parties would seek from the Town would not be onerous. For example, participants would likely seek to delay installation of (not preparation for) new roads or utilities to see if those public and private burdens can be avoided. Further,
parties absorbing a significant portion of the cost of bluff protection where there are no
private homes (anymore) would want to be confident that they will also be able to protect the
bluff at habitable locations too, as soon as possible, as appropriate. However, the Town could retain its authorities and controls. The Town would also likely pay some fair share of the costs, although less than its legal liability.
If the BOS agrees that the bluff stabilization effort should move forward, it could be done
in two parts: Part 1, the emergency stabilization of the most vulnerable portions of the bluff where Baxter Road and certain homes are imminently threatened to be completed before late fall 2013. Part 1 could be funded and constructed by SBPF with private contributions. Part 2
would consist of protecting the entire bluff from Sankaty Lighthouse to the point at which
erosion has begun (approximately 53 Baxter Road).
Siasconset Beach Preservation Fund
Impact of Bluff Erosion on Baxter Road Real Property Taxable Base
Introduction: In order to approximate the impact that bluff erosion is having on town revenue,
Windwalker Real Estate looked at two snapshots of assessed value for the properties on the
ocean side of Baxter Road -- one in 1991 and the other in 2013. The properties were separated
into two groups: the “South” (#5-63), where erosion has had limited impact, and the “North”
(#65-119) which are the properties most impacted by erosion. Land and Building assessments
were reviewed separately.
Key Findings:
In 1991, the average assessed value of land and buildings between the North and South
were nearly identical. After 22 years of erosion exposure, this value differential widened
substantially (see below chart) such that in 2013 there was a stark difference between the
average assessed value of properties on the North and South side of Baxter Road.
If the properties in the North had appreciated at the same rate as the properties in the
South, the Town of Nantucket would realize $230,000* in additional tax revenue from
the $63 million in land and building value that has been “lost” to erosion. Over the next
ten years, this would equate to approximately $2.3 million in Town revenue.
If all of the properties along the eastern side of Baxter Road were lost to erosion, the
town would lose $134 million* in assessed value, or $473,000* in tax revenue. Combined
with the $63 million already lost, nearly $200 million* (or $734,000* in revenue) would
be lost to the Town’s tax base from the east side of Baxter Road alone. Substantial
additional losses would be seen from the west side of Baxter Road and elsewhere in
‘Sconset. (*Based on 2013 assessment values and residential mil rate).
Analysis:
LAND
THEN: In 1991, the average assessed value of land for lots located on the south* end of Baxter Road was
$825,000 while those located on the north* end of Baxter Road averaged $618,000.
Land located on south Baxter was, on average, assessed at 1.3X the value of land located
on north Baxter.
NOW: In 2013, the average assessed value of land for lots located on the south end of Baxter Road was
$3,806,000 while those located on the north end of Baxter Road averaged $329,000.
Land located on south Baxter was, on average, assessed at 11.6X the value of land located
on north Baxter.
BUILDINGS
THEN: In 1991, the average assessed value of buildings located on the south end of Baxter Road was
$190,000 while buildings on the north end of Baxter Road averaged $175,000.
Buildings located on south Baxter were, on average, assessed at 1.1X the value of
buildings on north Baxter.
NOW: In 2013, the average assessed value of buildings located on the south end of Baxter Road was
$1,050,000 while buildings on the north end of Baxter Road averaged $190,000.
Buildings located on south Baxter were, on average, assessed at 5.5X the value of
buildings on north Baxter.
Tax Impact:
Between 1991 and 2013, the average assessed value of land and buildings on the southern portion of
Baxter Road increased by 361% and 453% respectively, while the assessed value of land and
buildings on the northern portion declined by 47% and increased by only 9% respectively.
Hypothetically, had erosion been controlled, ceteris parabis, and the assessed values of the
northerly properties enjoyed the same appreciation as the southerly properties, in 2013, the Town
of Nantucket would have had additional revenue of:
Land: $175,000
Buildings: $55,000
TOTAL: $230,000
Holding all economic conditions constant, this yearly revenue loss would only increase over time as
erosion worsens on the properties presently affected, and more properties, south and west of Baxter
Road, are exposed to erosion.
On this date _________, the Town of Nantucket, by and through its Board of Selectmen,
finds the following:
1) Baxter Road is a Public Way that provides the sole source of legal access and frontage for most of the abutting lots, and also contains within it public and private
utilities, including water and sewer lines.
2) The northern portion of Baxter Road is in danger of forced closure or physical breach
due to seasonal erosion of the bluff.
3) The danger to the public infrastructure and risk to public safety is imminent because
the distance from the top of the bluff to the road is less than 30 feet, and is declining
significantly in regularly occurring storm events.
4) Baxter Road is a historic place with historically important structures on both sides
that are at risk of loss, and some have been lost already.
5) The loss of any portion of Baxter Road, or of any homes, poses an imminent and
significant threat to public infrastructure and public health and safety, to Nantucket’s historic structures and settings, and to the its finances (for both liability and taxes).
6) This erosion is also reducing public accesses to the beach, public access along the
Bluff Walk, and other recreational and environmental resources.
Therefore, the Town and its staff shall engage cooperatively with the public and
stakeholders to develop and implement an emergency and long term plan to protect
public roads, public buildings, or other public assets from imminent destruction, to allow
for the protection of the historic structures and settings of Siasconset, to protect and
enhance the tax base, to limit public liability, and to provide for alternatives and replacement of the most endangered portion(s) of the at risk public infrastructure in the
event of a failure of such protection, as appropriate. Such a plan shall include permitting,
construction and public and private funding mechanisms and schedules to be approved by
the Board of Selectmen and shall encourage public benefits, as appropriate. It should
also work out the details of alternative access routes and utility replacement plans. The BOS support a public/private partnership for immediate and long term bluff stabilization,
authorizes the filing of a Notice of Intent for such, and urges the Conservation
Commission to review the matter on an emergency basis so as to save the public
infrastructure and historic homes, as appropriate.