HomeMy WebLinkAbout04 SE48_3115 Land Council Comments 09_28_18 (2) RESEARCH AND ENGINEERING, INC.
Corporate Headquarters: 766 Falmouth Road, Suite A-1, Mashpee, MA 02649, (508) 539-3737
Gulf Coast Office: Executive Tower, 3500 North Causeway Boulevard, Suite 1480, Metairie, LA 70002
www.appliedcoastal.com
September 27, 2018
Emily Molden
Nantucket Land Council, Inc.
6 Ash Lane
Nantucket, MA 02554
Re: Preliminary Review of Siasconset Beach Preservation Fund, Notice of Intent Application for
Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project
We have conducted a preliminary review of Siasconset Beach Preservation Fund (SBPF)
application to the Nantucket Conservation Commission for the Expanded Baxter Road and
Sconset Bluff Storm Damage Prevention Project, produced by Epsilon Associates, Inc. dated
January 5, 2018. A thorough review of the engineering aspects of the proposed project could not
be completed at this time. The applicant has not submitted updated engineering plans reflecting
the latest proposed structure design. The application also lacks any discussion or analysis to
inform the Commission why design modifications are being proposed for the expansion of the
geotextile revetment relative to the design of the existing geotextile structure. Therefore, this
preliminary review will focus on sediment issues relative to the compensatory mitigation for the
existing and proposed projects.
Mitigation
The intent of the mitigation is to mimic the natural sediment contribution the coastal bluff
would provide during erosional events if the geotextile revetment was not present. This is reflected
in the guidance from Massachusetts Office of Coastal Zone Management (MCZM) the applicant
presents in the Notice of Intent (NOI):
When a coastal engineering structure, such as a seawall, revetment, or bulkhead, is
legal and determined to be the only feasible alternative, a commensurate volume of
compatible material must be periodically placed in the littoral system to compensate for
the material that is lost to the system. The volume of material to be required to be placed
in the littoral system will be based on calculation of the long-term average annual
erosion rate of the coastal landform at the site. Short-term rates can be considered in
determining the compensatory volume of material if the issuing authority determines that
the short-term rate is more indicative of current and future conditions due to alterations
along the shore (p. 23).
In the case of the Sconset Bluff, the coastal bluff is a significant contributor of sediment to
the active littoral system. The reduction or loss of this sediment source will result in significant
deficit to the sediment budget along adjacent unprotected coastlines. It is critical the
compensatory mitigation provide at a minimum an equal volume of sediment as the unarmored
bluff naturally provides. To quantify this contribution, requires that in addition to evaluating long-
term average annual erosion rates, short-term erosion rates should also be evaluated to
characterize changes in erosion rates that occur as the system becomes more sand starved. The
Page 2 of 5
long-term erosion rates provide data relative to banks movement and contributions which may
not be reflected in the short-term record due to episodic erosion that occurs along the coastal
bank. The episodic erosion occurs as the bank over steepens and then fails, such as during the
winter of 2012 and 2013 when the bank retreat 20 to 30 feet as recorded by the applicant. The
short-term average rate of change can provide indications if the rate of erosion is potentially
accelerating.
To highlight the importance of evaluating both short- and long-term erosion rates when
determining the compensatory mitigation, we can examine the change in shoreline position at
SBPF monitoring Profile 90 (Table 1), which is located at the southern end of the proposed
expansion. Analyzing the period from 2003-2017 that Epsilon utilized for long-term erosion rates
to determine mitigation volumes in the NOI, shows that the shoreline at Profile 90 retreated on
average 1.9 feet per year over that 14-year period. A quick examination of the change rates
presented in Table 1 which was obtained from the SBPF monitoring reports, shows a marked
increase in shoreline retreat beginning in 2015 and continuing through 2017. Examining the period
of time prior to the shoreline and bluff retreat rate increase, 2003 to 2014, the data shows that the
shoreline was accreting on average 1.3 feet per year. The data shows that there was no sustained
erosion taking place over the initial 11-years of the long-term average that is being utilized to
calculate compensatory mitigation. Evaluating the period once erosion of the beach and bluff
increased in 2015, the average shoreline retreat rate increased to 14.8 feet per year. Therefore,
if compensatory mitigation volume is calculated based on the long-term retreat rate from 2003-
2017 the volume of mitigation provided would drastically underrepresent the volume of material
that the coastal bank is currently providing to the littoral system. The determination of the
compensatory mitigation needs to evaluate both the long- and short-term to accurately mitigate
for the loss of the natural sediment source that the proposed expansion of the project represents.
Table 1. Shoreline change from March 2003 to May
2017. Data was obtained from Southeast
Nantucket Beach Monitoring reports prepared
by Woods Hole Group, Inc. for SBPF.
SBPF Monitoring Survey Shoreline Change (feet/year)
Mar-03 5.7
Apr-04 -5.7
Apr-05 -31.5
Apr-06 7.1
May-07 5.8
Mar-08 -3.5
Apr-09 -5.5
Sep-10 -6.1
Apr-11 -5.3
Mar-12 22.6
Mar-13 4.1
Apr-14 27.4
Apr-15 -13.3
May-16 -13.0
May-17 -18.0
Over the 5-year lifespan of the existing geotextile revetment it has been shown repeatedly
that the design of the compensatory mitigation system is underperforming and significantly flawed.
Page 3 of 5
The compensatory mitigation design was proposed by the applicant to provide an equivalent
volume of sediment to the littoral system, thus replicating the contribution the coastal bank would
naturally provide. History has shown that this is not how the system has performed. The seaward
portion of the sand cover washes away in the first few hours of a storm (Figures 1 and 2), the
compensatory mitigation stockpiled above is not resupplied to the active surf zone during storms
as the coastal bank naturally would. SBPF stated in the 2016 Annual Monitoring presentation to
the Conservation Commission on average it takes 5-7 days for a storm to pass, equipment to be
mobilized, and sediment to be placed seaward of the exposed geotubes. That means that after
the tubes become exposed at the beginning of a storm, no mitigation is available to the littoral
system to offset the impacts associated with armoring the coastal bluff. The absence of
compensatory mitigation during the most critical transport periods, when sediment should be
available to minimize adverse impacts associated with geotextile revetment, results in significant
sediment deficits along the adjoining shorelines and results in increased erosion along those
shorelines. The adverse impacts are compounded during successive storm events when the
applicant cannot resupply the necessary compensatory mitigation along the revetment. Prior to
any expansion of the revetment and additional hardening of the coastal bank, the compensatory
mitigation system should be addressed and redesigned to prevent the continual erosion of the
adjoining shorelines that is associated with the applicants existing project.
The applicant has requested to use an adaptive mitigation program along proposed
expansion. The adaptive mitigation program would only replace the sediment that was utilized
from the mitigation template over the previous year. There are numerous issues with this
approach to compensatory mitigation. First, the compensatory mitigation system as designed is
not providing sediment to the littoral system during storms when it is most needed to replicate the
contribution of the coastal bluff. Therefore, when the applicant reports that substantial volumes of
mitigation remain at the end of each monitoring year, it is not surprising. The system does not
perform as designed, so during the most significant periods of the year when compensatory
mitigation should be available, that sediment is isolated from the littoral system atop the geotextile
revetment. If the mitigation sediment was available during storms to represent the contributions
of the coastal bluff and minimize the adverse impacts associated with constructing a revetment
over the coastal bank, the issues associated with excess volumes of mitigation remaining would
be addressed. The second issue with adaptive management is that it does not account for
variability in annual transport rates. As was seen in the winter of 2018, when successive
nor’easters battered the coastline of Massachusetts, there are extreme years where erosion and
transport rates are well above average. There are also years when storms are not as energetic
or frequent and erosion rates fall below average. During those quiet years, we would expect that
a portion of the compensatory mitigation would remain in front of the revetment. If in the following
year, a reduced amount of compensatory mitigation is placed, we are substantially undermining
the volume of compensatory mitigation needed to prevent adverse impacts during years that have
significant increases in storm activity. If the compensatory mitigation is allowed to be adaptively
managed, thus providing overall lower volumes of sediment, there is not a way to address the
increased erosion on adjoining shorelines that results during years of significant storm activity,
when higher than average volumes of sediment would be contributed from the natural coastal
bluff. For a compensatory mitigation system to work properly, there will be years when sediment
remains within the template to offset the years when additional sediment is required to address
more energic storm seasons.
The existing geotextile revetment was constructed along the highest eroding stretch of
Sconset Bluff. Therefore, when the applicant reports that erosion rates measured north and south
of the project are lower than the rates experienced within the project area thus proving that the
required compensatory mitigation volume is 1.5-1.8 times above the natural average bank
Page 4 of 5
contribution, it fails to account for the fact that the coastal bank within the project areas should be
eroding at a higher rate. The volume of compensatory mitigation has been discussed at length
over previous filings and monitoring updates. The mitigation volume of 22 cy/lf/yr was arrived at
using previous SBPF analysis, studies, and filings which have shown the bank does indeed
contribute at least a 22 cy/lf/yr (and potentially even greater volumes). Massachusetts Department
of Environmental Protection (DEP) specified the mitigation volume of 22 cy/lf/yr in the
Superseding Order of Conditions dated December 19, 2014 for the existing geotextile revetment.
That volume was specified by DEP because it provided an equivalent volume of sediment to the
natural coastal bank along the length of Sconset Bluff which was armored. The mitigation volume
calculation used to support the Superseding Order is consistent with mitigation requirements for
similar projects permitted by DEP and MCZM throughout the state.
Figure 1 Image of the exposed geotubes (facing the north) after a March 13th, 2018 Nor’easter. The
front of the geotubes are fully exposed.
Page 5 of 5
Figure 2 Image of the north end of the project after a March 13th, 2018 Nor’easter. The front of the
geotextile revetment is fully exposed with significant erosion at the terminal end.
Compensatory mitigation sediment remains on top of the geotubes. The lack of mitigation
during the storm interrupted the natural longshore transport of sand resulting in a sand starved
beaches adjacent to the project and contributed to the erosion seen in the photograph.