HomeMy WebLinkAbout2013_7_30 SBPF Submission copy_201405230851302398BAXTER ROAD AND SCONSET BLUFF STORM DAMAGE PREVENTION PROJECT
NOTICE OF INTENT
RESPONSES TO QUESTIONS FROM NANTUCKET CONSERVATION COMMISSION ASKED AT
PUBLIC HEARING ON JULY 24, 2013
Below is a summary of issues raised at the July 24, 2014 hearing for the above-referenced project,
organized by topic.
1. Sand Mitigation and Delivery
a. Description of Calculation:
The Baxter Road and Sconset Bluff Storm Damage Prevention Project intends to follow
the state standard of “Best Available Measure1,” which 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. This amount has been historically required by
the Nantucket Conservation Commission and DEP.
This number is calculated for the project by first determining the long-term erosion rate
of Sconset bluff using aerial imagery. While some erosion can be observed dating back
to 1994, the time period of 2003-2012 was utilized for this analysis because it reflects
the timeframe when most of the coastal bank was undergoing active retreat. Further,
2012 is the most recent aerial imagery available. The project area from 73-119 Baxter
Road was utilized in the calculation; south of 73 Baxter Road was excluded because the
top of the coastal bank was not yet actively retreating in 2003. Likewise, 79 Baxter
Road was also excluded from the analysis since the presence of the terraces has
considerably slowed bank retreat. This calculation yields a long-term bank erosion rate
of 3.18 ft/yr (Figure S-1). This calculated rate of bank retreat is lower than the calculated
retreat rate of 4.96 ft/yr for the previously filed Bluff Stabilization NOI because that
previous calculation was only for the coastal bank from 77 to 85 Baxter due to the
limited Pilot project area of this earlier filing.
The coastal bank contribution volume was determined by selecting the three Sections
from the project sheets (A-A, B-B and C-C at 97, 83 and 65 Baxter, respectively). These
used the 2010 LIDAR survey of the project area. Then AutoCad computer software was
used to calculate the volume that would be lost if each of these profiles retreated by the
average bank erosion rate of 3.18 ft/yr. Averaging the volume lost from each of the
three profiles yields an average bank contribution volume of 9.3 cy/lf/yr.
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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This calculated bank contribution volume corresponds well to the estimated amount
contributed by the terraces of over the past 5-7 years. Several years’ experience with
the terraces has demonstrated that, through the provision of sand mitigation
approximately 9 cy/lf/yr, (1) there is no downdrift impact to nearby beaches even when
the lower bank has been protected from erosion, and (2) there remains a significant
beach seaward of the terraces (i.e. more than 80 feet to MLW line) at 79 Baxter Road.
While the terraces are not an effective long-term solution (see discussion in Section 2.8
of the Alternatives Analysis included as Attachment E to the NOI), the experience with
the terraces strongly suggests that toe protective provided by a revetment, coupled with
a sand mitigation program of approximately 9 cy/lf/yr, will sufficiently mimic the natural
coastal bank contribution amount such that the project will not have any adverse
impacts on adjacent beaches.
b. Impact of delivering average amount of sand on system.
The amount of sediment supplied for mitigation is part of the overall sediment budget
for the coastal system. There are multiple inputs (from bluff erosion and longshore
sediment transport) and multiple outputs (also from longshore sediment transport and
also outputs to the offshore). Mitigation is supplied to equal the input to the system
from bluff erosion. Sand mitigation is typically supplied on an annual basis, usually
after the winter storm season, and that is what we propose here. Because there are
multiple inputs to the sediment budget, the impact from changing any one input (i.e.,
supplying an average amount of sand in a severe storm year) will be offset if the other
inputs remain the same.
While there are times when larger amounts of sand are removed by erosion, there are
also times when less is removed. The Best Available Measure is to use an average
volume of sand mitigation each year based on the average bank retreat rate to maintain
the average sediment budget of the system whether the previous year’s sand
contribution has been lost or not. The sand mitigation is placed on the back of the
beach and washes away during large storms, but most of this sand will remain part of
the sediment budget of this coastal system. There is not a special attempt to replicate
the loss of greater or lesser amount of sediment in a given year.
If end scour is observed at the time of this sand mitigation, sufficient sand will be
supplied to restore any localized scouring. Long term monitoring will be performed to
determine if there are longer term erosion impacts that can be attributed to the
proposed revetment. If these are measured, additional mitigation sand will be added to
the system to restore the overall sediment budget. This proposed approach is the Best
Available Measure for dealing with end scour.
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c. Will this project force island pits to run out of sand?
With a sand mitigation volume of 9.3 cy/lf/yr and the total build-out of the 4,200 foot
long revetment, the total sand volume per year would be 39,060 cy. Based on this
volume, the two existing island pits would have a life of over 20 years. Future sand
could be provided by opening new pits on the island or using sand sources from off
island. The cost for sand at the island pits is set based on the cost to deliver sand to the
island using a barge.
2. Design of revetment
a. Wave energy calculations, wave height data, and how revetment is designed for
this energy (Ramsey, Oktay, others)
The Project Design Parameters are provided in Section 3.1 of the NOI including:
Wave run-up elevation during 100-year storm
100-year still water elevation
Beach elevation at toe of coastal bank
Estimated maximum depth of scour at the toe of the revetment during 100-
year storm
Attached are OCC’s wave calculations and revetment stone size calculations. Please
note the original wave calculations were performed in 2010 for the mattress/gabion
project (see attached OCC Alternative Analysis Report, September 2010). The results of
those calculations are relevant to the revetment design. The wave height was increased
to 5.5 ft for this project in an effort to size the revetment stone more conservatively.
This revetment design meets the standard of Best Available Measure.
b. Front beach will narrow and if no sacrificial sand were delivered could create a hole
in front causing failure.
A sand mitigation program will be provided as discussed in 1a above. Revetment
failure is discussed in 2c. below.
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c. Failure of revetments: How do revetments fail and why, and why ours is designed
differently and will not fail.
The typical methods of revetment failure are as follows:
Cover layer failure
Pumping of fines through cover layer
Toe scour
Flanking and
Overtopping
Cover layer failure:
This can occur if the armor cover stones are not properly sized for the wave
environment. Hudson's equation was used to size the stones in the outer layer of the
rock revetment for the anticipated wave events. The Sconset revetment has been
designed to withstand predicted waves during a 100-year storm event (see attached
calculations). This is the Best Available Measure for revetment cover design.
Pumping of fines through cover layer:
This occurs when geotextile fabric and/or a smaller gradation of stones are not placed as
an underlayer to prevent the underlying soils from being pumped through the overlying
layers of a revetment when impacted by storm waves. The Sconset revetment has been
designed with a geotextile filter fabric layer at the base followed by an overlying layer of
18 inch minimum filter layer of stones to prevent failure by pumping of fines. This is
the Best Available Measure for revetment design to prevent pumping of fines through
the cover layer.
Toe Scour:
Vertical scour at the toe of a revetment can cause the underlying beach sediments to be
exposed to waves. At Sconset the toe of the revetment will be at a depth of 8.0 feet
below the beach level which is below the estimated scour from a 100-year storm. This
is the Best Available Measure for revetment design to prevent toe scour. If future
erosion of the beach is determined to pose a risk of toe scour, more stones can be
added to the revetment toe.
Flanking:
Flanking occurs when adjacent unprotected coastal bank areas continue to erode due to
storm wave attack. This erosion removes sand/soil behind the wall and if it is allowed
to continue can lead to revetment failure. Flanking can be avoided or mitigated by
extending the wall, using a return wall and/or placing mitigative sand over the end of
the wall. At Sconset flanking will be prevented or mitigated by extending or tapering
the ends of the revetment and by placing mitigative sand at the revetment ends. This is
the Best Available Measure to prevent or mitigate flanking. Also, gaps in the revetment
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will be minimized which will also minimize the number of revetment ends and the
need for flanking mitigation.
Overtopping:
Splash over the top of a revetment can lead to failure by exposing the underlying
sediments/soils to waves. This is prevented by extending the elevation of the revetment
high enough so that overtopping is not a problem. In the case of the Sconset revetment,
the revetment has been designed so that it is not overtopped during a 100-year storm
event. Wave run-up during a 100-year storm event has been calculated and the top of
the revetment has been located at +26.0 feet MLW to prevent overtopping. This is the
Best Available Measure for revetment design to prevent overtopping.
d. Revetment Slope: Is the proposed revetment slope of 1.5:1 stable and does it meet
Best Available Measures? Does it follow the existing slope or does it require
changes?
OCC has designed the revetment at the steepest stable slope of 1.5:1 to minimize the
extension of the revetment onto the coastal beach. There is obviously a balance
between the slope of the revetment and how far it would extend onto the coastal beach.
OCC is of the opinion that the slope of 1.5:1 is the Best Available Measure for the slope
of this revetment.
The revetment will only be on the lower 16 feet of the coastal bank slope, whereas the
coastal bank height ranges from about 58 feet in the south to about 80 feet in the north,
thus the percentage of bank covered will range from about 28 percent in the south to 20
percent in the north. Some minor grading will be required to install the revetment as
shown on the project Proposed Sections A-A, B-B and C-C. In coastal bank areas where
the bank is oversteepened at the top, there may be some continued slumping of the
upper slope after revetment construction until a stable angle of repose is reached.
e. “Unaware of any revetment in this wave energy”. (Ramsey, others.) Example of
other revetment with this wave energy.
Most of Outer Cape Cod facing the Atlantic Ocean is within the Cape Cod National
Seashore where no new development and protective structures are allowed.
The South Shore coastline of Massachusetts from Hull to Plymouth is exposed to
Northeasters which tend to be the most severe storms for our coastal area as they can
last several days over multiple high tides as was experienced during the Blizzard of ‘78.
Wave energy along the South Shore during Northeasters can be quite severe and
comparable to Sconset during these storms and consequently this shoreline has been
protected with many revetment structures (see Figure S-2).
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The North Shore coastline consists of more bedrock and pocket beaches such as the
coastlines of Rockport and Gloucester. More developed beach areas tend to be
protected with seawall structures such as in Swampscott, Lynn, Revere and Winthrop.
There are many shoreline areas in other parts of the United States and worldwide that
experience wave energies equal or greater than those experienced at Sconset and the
revetments in those areas have withstood those wave forces for decades without failing.
For example, there are numerous coastal revetments along the Pacific coast of the U.S.,
including those in Washington, Oregon and California. In numerous places where the
Pacific Coast Highway runs along the shoreline of the Pacific Ocean in California
revetments have been used to protect this major north-south highway system. One such
section of this highway that is protected by a large revetment which is about 4,000 feet
long is shown on the attached Figure S-3.
f. Discuss success/limitation of jute bags
See Attachment E (Alternatives Analysis) to the NOI, where the terraces are discussed in
Section 2.8.
3. How does bluff erosion occur and what is the role of run-off and downslope failure?
The driving force for coastal bank retreat is erosion of the toe of the coastal bank by storm
waves. Greatest storm erosion occurs during Northeasters which may last more than one
day and over several high tide cycles. Waves lower the beach elevation and remove the
toe of the coastal bank creating an undercutting of the bank. This produces an unstable
bank situation which leads to upper bank failure in the form of slumping and downslope
movement of sediment. The amount of bank erosion can vary greatly from year to year
depending on the frequency and severity of coastal storms. If there are storms in rapid
succession where the beach does not have time to rebuild in between the bank will be
more vulnerable to toe erosion. It also does not provide time for contractors to rebuild the
terraces, thus making the terrace-protected bank areas susceptible to toe erosion and failure
of the upper bank as well. Other erosion of the coastal bank, such as that due to rain and
wind, plays a minor role in the overall erosion problem. This other surface erosion can be
prevented using vegetative plantings as are being proposed as part of the proposed project.
4. Impact of revetment on adjacent beaches
Several years’ experience with the terraces has demonstrated that, through the provision of
sand mitigation in the amount of approximately 9 cy/lf/yr, (1) there is no downdrift impact
to nearby beaches even when the lower bank has been protected from erosion, and (2)
there remains a significant beach seaward of the terraces (i.e. more than 80 feet to MLW
line) at 79 Baxter Road. While the terraces are not an effective long-term solution (see
discussion in Section 2.8 of the Alternatives Analysis included as Attachment E to the NOI),
the experience with the terraces strongly suggests that toe protective provided by a
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revetment, coupled with a sand mitigation program of approximately 9 cy/lf/yr, will
sufficiently mimic the natural coastal bank contribution amount such that the project will
not have any adverse impacts on adjacent beaches.
Notwithstanding that no adverse impacts are expected based on site-specific experience,
monitoring will be conducted. If end scour is observed at the time of this sand mitigation,
sufficient sand will be supplied to restore any localized scouring. Note that coastal banks
adjacent to the revetment will continue to erode or retreat. This normal coastal bank
erosion is not “end scour”. End scour refers to additional erosion at the end of a revetment
that is due to wave reflection off the revetment or sediment starvation that can be attributed
to the updrift or adjacent revetment.
Long term monitoring will be performed to determine if there are longer term erosion
impacts that can be attributed to the proposed revetment. If these are measured, additional
mitigation sand will be added to the system to restore the overall sediment budget.
5. Impact on plants and biological community
Due to the dynamic wind, wave, and current conditions, the biological community on and
within high-energy coastal beaches, such as Sconset Beach, is “depauperate,” (i.e., lacking
in numbers or variety of species), when compared to communities on either rocky or
muddy shores. The regular, localized elimination of the inhabitants of the backshore beach
community, and the renewal of that habitat, is a common event resulting from wave
activities. For instance, major storms can be assumed to have swept the shoreline clean of
any resident plants and animals. Therefore, the proposed project is expected to do little to
alter the biologically restrictive conditions that normally exist within and below the work
area. In this setting, it is important to recognize that the project area profile and associated
biological community present at a given point in time are the result of conditions created by
the most recent storm event that washed the Sconset area.
Sand fleas and mole crabs both exist in this challenging environment and are adapted to the
inherent “hardship” of a dynamic environment. Neither species forms a stabile
community; rather, they survive in and naturally adapt to dynamically unstable conditions.
The loss of members of a population is quickly compensated for by new colonizers
migrating in from adjacent populations as they disperse or are carried by the wind and
water currents into the area. Therefore, the project will not adversely affect the biological
community of the back beach or the intertidal zone.
Additional information specific to the mole crab was submitted to the Conservation
Commission on July 24, 2013 (see technical memo from Mike Ludwig dated January 23,
2012).
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During a review of the marine mattress and gabion project’s impacts on the beach
community), the Commission’s Independent Consultant, Nicolle Burnham of Milone &
MacBroom, stated in a letter dated December 29, 2011: “The organisms currently
inhabiting this shoreline area are adept at moving/migrating within the intertidal zone on a
daily basis, so minimal impact from the proposed shoreline stabilization is expected on said
organisms.” She concluded “the overall impact is expected to be minimal.” The impact
from the revetment will be similar as that from the previously proposed marine mattress and
gabion project; therefore, no adverse impact to these organisms is expected.
The Notice of Intent fully addresses the project’s consistency with the performance standard
requirements related to wildlife habitat – see Sections 2.2, 4.0, and 5.0. the project meets
all applicable state and local performance standards.
Regulatory Compliance
a. Dirk- legal theory of WPA section- 310 CMR 10.30 (7)
Dirk didn’t provide the full language of the regulation. He only picked out section (7)
which by itself does not include the full language.
First, there is a preamble:
“WHEN A COASTAL BANK IS DETERMINED TO BE SIGNIFICANT TO STORM
DAMAGE PREVENTION AND FLOOD CONTROL BECAUSE IT IS A VERTICAL
BUFFER TO STORM WATERS, 310 CMR 1030 (6) through (8) SHALL APPLY:”
Then section (7) “ Bulkheads, revetments seawalls, groins or other coastal engineering
structures may be permitted on such a coastal bank except when such a coastal bank is
significant to storm damage prevention or flood control because it supplies sediment to
coastal beaches, coastal dunes, and barrier beaches.”
So it is important to first read the preamble and understand that section (7) only applies
to vertical buffer type of coastal banks. Then section 7 indicates the exception is for a
coastal bank that supplies sediment. Thus, this refers the reader to the section of the
coastal bank regulations which then deal specifically with those banks that supply
sediment which states as follows;
WHEN A COASTAL BANK IS DETERMINED TO BE SIGNIFICANT TO STORM
DAMAGE PREVENTION OR FLOOD CONTROL BECAUSE IT SUPPLIES SEDIMENT
TO COASTAL BEACHES, COASTAL DUNES OR BARRIER BEACHES, 310 CMR
10.30(3) though (5) SHALL APPLY:
Section (3) reads as follows:
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(3) No new bulkhead, revetment, seawall, groin or other coastal engineering structure
shall be permitted on such as coastal bank except that such as coastal structure shall be
permitted when required to prevent storm damage to buildings constructed prior to the
effective date of 310 CMR 10.21 through 10.37 (August 10, 1978)….” This is
grandfathering provision which we meet.
So it is important to understand that Sconset bluff is both a vertical buffer type of
coastal bank and a coastal bank that supplies sediment. The vertical buffer coastal bank
regulatory language refers the reader to the sediment supply coastal bank regulatory
language which we comply with.