HomeMy WebLinkAbout21 SE48_3115 BaxterGeotubeExt_GBermanREVIEW_02_01_2019
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COASTAL PROCESSES SPECIALIST
WOODS HOLE SEA GRANT | CAPE COD COOPERATIVE EXTENSION
gberman@whoi.edu | gberman@barnstablecounty.org
508-289-3046 | 193 Oyster Pond Road, MS #2, Woods Hole, MA 02543-1525
February 1, 2019
TO: Jeff Carlson (Natural Resources Coordinator, Town of Nantucket)
CC: none
FROM: Greg Berman, Coastal Processes Specialist (WHSG & CCCE)
RE: Independent Review of the Notice of Intent (01/05/2018) for the
Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project
Background: Since the inception of the coastal processes position established within WHSG & CCCE, on-site
and remote technical assistance on coastal processes has been and continues to be an on-going, effective
technical information communication and dissemination tool. Technical assistance relating to coastal
processes, shoreline change, erosion control alternatives, coastal landform delineation, potential effects of
various human activities on coastal landforms, coastal floodplains, coastal hazards and hazard mitigation
analyses, and dune restoration techniques provided in the field and remotely will continue to be provided on
an as-needed basis. Site visits generally address site-specific coastal processes or coastal hazards related
issues. Follow-up unbiased, written technical analyses are generally provided.
Image date 2/2018 from GoogleEarth
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Introduction:
The applicant’s Notice of Intent (NOI), dated 01/05/2018, indicates a proposed expansion of the existing
geotube array from 947’ to 3,820’. This 2,873’ expansion would be 895’ to the north and 1,978’ to the south of
the existing array. Mr. Carlson (Natural Resources Coordinator for the Town of Nantucket) got in touch with
the Coastal Processes Specialist (working for both the CCCE and WHSG), requesting an independent evaluation
of the application. This large project is multi-faceted, and by request this review will be focused on the
observed and potential impacts to coastal processes of the existing and proposed geotube array. I have
provided two independent reviews of annual reports for the 947’ array (4/7/2017 for the 2016 Annual Report
and 2/11/2018 for the 2017 Annual Report) and the findings will be summarized below. Background coastal
processes information was provided in these reviews as well as in the extensive studies provided by the
applicant and other parties, therefore that information will not be duplicated in this review. Another aspect of
the project which will have bearing on the Conservation Commission’s decision will be deciding which
properties might qualify as pre vs post 1978 construction, however this is being addressed by town council and
not addressed in this review.
Summary of findings from the previous 2 independent reviews:
By holding this section of shoreline in place with geotubes while adjacent portions of the coastal bank
erode naturally, the array will eventually extend seaward further than adjacent areas.
The main uses of compensatory nourishment are to: ensure the beach in the immediate vicinity of the
project does not drop and change the coastal processes of the immediate area, keep the geotube
covered so it does not interact with waves/currents, and to make up for any reduction in sediment
available for downdrift beaches.
It appears that most of the offshore area is still >25% cobble/boulder, however point data
interpolation method selection can greatly affect the outcome. Sidescan (backscatter) sonar images
would provide a more complete picture of the bottom.
During lower wave energy the geotubes stay covered with sand and have minimal negative interaction
with coastal processes. During even minor storm events portions of the geotubes are exposed, and are
likely reflecting wave energy in a similar way to a Coastal Engineering Structure (CES) during this
period.
Due to the scale of this project (947’ length) there is a high potential for currents to set up parallel to
the smooth exposed geotube during storm conditions, which can rapidly scour the end of the array.
Erosion doesn't stop in areas adjacent to a shoreline stabilization project and "holding the line" can
become more and more difficult over time. An analysis on the useable lifespan of the upland
properties and eventual retreat (or abandonment) of the array might be helpful.
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Sand nourishment volumes:
In order to determine nourishment volume the following parameters are needed: Length of coastal
bank, Average height of coastal bank, and Rate of erosion. To calculate compensatory nourishment
requirements the following equation is typically used:
(Length of coastal bank) x (Average height of coastal bank) x (Rate of erosion) = (Volume of nourishment)
While there seems to be relative agreement on the height of the bank and the length of the project,
the rates of erosion provided by the applicant have been contested. Erosion rates are typically calculated at
MHW as this is the datum by which the Massachusetts Shoreline Change Project mapped high water shoreline
across the entire state. These transects provide long term as well as short term data. In areas where site
specific information is not available these shoreline rates provided by the state are often used to calculate
compensatory nourishment. This project area has had extensive data collection for multiple decades, which
should provide much more accurate erosion rates for the site. The large volume of data available has lead
various parties to use different shoreline change rates (ex. bank retreat vs MHW change). While the erosion
rates at these two locations are certainly linked there is a somewhat convoluted correlation between them
(e.g., 2’ of erosion at MHW does not immediately equal 2’ of loss at the top of the bank). Erosion rates and the
related potential compensatory nourishment volumes were not calculated as part of this review.
This project, at 947’, is about 2% (or about 7% at the proposed 3,820’) of the approximately 10 miles of
the mostly unarmored eastern shoreline of Nantucket. While the erosion rates along this shoreline can be
highly variable, it is highly likely that much of the beach sediment at the site has come from updrift areas, as
opposed to the site. There is a very large natural volume of sand moving along this stretch of shoreline which
helps preserve the width of downdrift beaches and dunes. However, even with this natural volume and the
artificially placed sediment nourishment, sand cover on all portions of the geotube array appears to have been
difficult to maintain.
Most important would be finding the “right” compensatory nourishment volume and requiring that
volume be put down every year as a minimum. Then more sand may be needed if filling the sand template
requires more than that minimum volume. Ex. 8.8cy/lf/yr as a minimum to be placed each year and 22
cy/lf/yr as a minimum template volume to maintain. Whatever volumes are decided, they may need to be
adjusted based on how often the geotubes get exposed.
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Has the array been tested by storms?:
Previous comments have indicated that the project site has not experienced a significant storm event
since the installation of the geotube array. However, now data is available with the geotube array experiencing
several larger storms. While the array has not experienced a tropical storm (i.e. hurricane) of significance,
some recent winter events would qualify as “testing” the array. The initial geotube array was installed
12/2013-1/2014. Since that time 5 of the top 10 water levels have been observed at the Nantucket Tide
Gauge, since measurements began in 1965 (see yellow highlights on image below from
https://tidesandcurrents.noaa.gov/).
Using the online NOAA tool Vdatum (https://vdatum.noaa.gov/) the January 2018 storm water level of 3.72’
MHHW was converted to 5.0’ NAVD88 and the March 2018 storm water level of 3.16’ MHHW was converted
to 4.5’ NAVD88. The closest transect (#13) to the site in the FEMA Flood Insurance Study (6/9/2014, see figure
below) shows that the stillwater elevations for the 1-percent annual chance (aka 100 year flood) is 5.8’
NAVD88 and the 2-percent annual chance (aka 50 year flood) is 4.8’. Note that Base Flood Elevation includes
the Stillwater elevation and waves.
The January 2018 would correspond to between a 50-100 year storm and the March 2018 event would
be very close to a 50 year storm. The March 2018 storm was a bit of an aberration in that there was a long
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time period (6 days 3/2-8/2018) with a significant storm surge (1.5-3.2’ at the Nantucket tide gauge) as well as
another event with a 3’ surge on 3/13 (see figure below).
Effects of the geotube array:
The existing (and proposed) project
has been designed so that the sand placed
on the face of the geotubes is eroded during
storms and contributes to the littoral drift
system. This is a good feature of the design
as it provides sand to the littoral system
during storms, as during storms is when
additional sediment in the nearshore would
have the biggest impact on preserving the
upland and coastal resource areas. Another
aspect of this design is that, after the sand
on the face of the geotubes has eroded, the
array is exposed and interacting with the
waves as a CES. Also, by stabilizing the toe
of the bank with geotubes less sediment is
eroding than would occur naturally (see figure to the right). Despite some sediment being lost on top of the
geotubes, a more natural bank would not preserve the sand template above the geotubes. This would cause
the bank to erode much more sediment, compared the geotube array, and migrate landward. As there has
been sediment “left behind” in the array’s template during some years, this region may benefit by the
applicant placing a minimum yearly amount of sediment as mandatory, compensatory nourishment. This
requirement would not be affected by how much sand remains in the template. More sand than the minimum
may be required to keep the template full. Keeping the geotubes covered (i.e. maintaining the sand template)
mitigates for how the geotubes affect local wave processes, while the minimum volume would serve downdrift
beaches and dunes.
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Available on the Town of Nantucket’s project webpage, a PDF (Sconset
Bluff Images 03_04_18 to 03_16_18 from Ian Golding.pdf) of photographs from during and
shortly after the March 2018 storm was made available (3 images to the
right). Some of these images show, what appears to be, classic images of end
scour (top and middle images) and flanking caused by a hard structure
interacting with the waves. What complicates this location is the clay
outcrop (lower right corner of middle image). The area between the clay
and geotube is “between a rock and a hard place” with the unconsolidated
sand of this portion of bank eroding much more quickly due to increased
wave turbulence on each side. There is little doubt that the exposed
geotube is partly to blame for the scour, but the natural clay headland likely
shares the blame.
Also of concern is the multiple tiers of the geotube array (5-10’ high)
that were exposed along, what appears to be, the entire length of the array
during the storm (shown in the bottom image). As mentioned in previous
reports when the geotubes are exposed they are likely reflecting wave
energy in a similar way to a CES. The early March 2018 series of storms was
long time period (for 6 days the surge was over 1.5’) with a high maximum
storm surge of 3.2’ as well as another event with a 3’ surge less than a week
later (3/13/18).
According to the “Baxter Road and Siasconset Bluff Stabilization
Project January 2018 – March 2018 Work Log” the first work performed in
March of 2018 was a sand delivery of 434 CY on 3/12. An additional 4,312
CY was delivered between 3/14-3/16. This means that after the geotubes
were exposed (likely on 3/3/18) they continued to be exposed and interact
with the series of storms for another 5 days, and then the shorter event on
3/13 likely interacted with the geotubes as well. While the exposed
geotubes might not be visually appealing, they cause relatively little harm
during quiet periods. It is when the geotubes interact with storm waves and currents that they have the most
negative impacts on coastal resource areas.
Feasibility of maintaining a beach width:
The applicant has the requirement on the existing (and presumably the proposed) geotube array to
“…maintain adequate beach width in front of the Bank…” with failure defined as “…MHW migrates landward to
the seaward edge of the second tier of geotextile tubes for any two consecutive surveys…”. The applicant also
has to “…maintain a walkable beach in front of the geotextile tubes…” with failure defined as “…the seaward
side of the coastal bank is not passable by foot…”.
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While maintaining a beach in front of a Coastal Engineering Structure is theoretically possible, at some
time in the future (likely tens of years, not hundreds) it will not be feasible. As was previously mentioned, the
proposed project would be about 7% of the mostly unarmored eastern shoreline of Nantucket and much of the
beach sediment in this area has likely come from updrift areas, as opposed to being placed at the site. The
compensatory nourishment volume described in a previous section is intended to put the amount of sediment
into the system that would have eroded naturally. This is the volume of material that would be coming out of
an eroding bank. This is not the volume of material that would be needed to prevent the bank from eroding
any further.
If a geotube expansion is approved erosion will continue in adjacent areas, as is occurring now with the
current extent of the geotube array. With erosion continuing to occur (or made worse) at the end of the
structure, properties adjacent to the structure will often request an extension of the CES to cover their
property (aka “chasing erosion”). The geotube array has been designed with returns so that it is not
compromised by scour. One of the dangers of “holding the line” with a CES is that the array will artificially
protrude further seaward than the rest of the shoreline. Flanking may occur if adjacent properties continue to
erode naturally, while the project site maintains a shoreline position further seaward to protect the homes.
Flanking could require returns to be extended landward over time in order to protect the house, allowing the
property to protrude further seaward than the rest of the shoreline (see image below).
The above image an example from Hardings Beach, in Chatham, of an armored property (CES) that was
allowed to exist further seaward than the rest of the shoreline properties. This configuration has major
implications for wave energy and sediment transport, as well as a lack of “walkable beach” at high tides. Wave
reflection can exacerbate erosion on the adjacent beach and the property may affect sediment transport
parallel to shore (similar to a groin). While this type of setting is different from Baxter Road, it is provided to
illustrate the potential ramifications of “holding the line” for too long.
In the figure below a green line in the left panel represents the Massachusetts Shoreline Change Project
high water shoreline for 1978. If a CES was installed 40 years ago (1978) at this location the returns would
extend hundreds of feet (middle panel). To maintain a dry beach for this type of structure would require
infeasible volumes of sand.
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The shoreline position of comparison was selected as 1978 because, as the year the state WPA went into
effect, this would be the most recently a structure could have been built without taking these regulations into
account. Inversely, looking 40 years into the future (a reasonable expectation for a house to survive) the
geotubes will need extensive returns as the top of the bank will have retreated 80’ (using the applicant’s rate
of 2.0’/yr for the area north of the existing project) and 140’ (using the applicant’s rate of 3.5’/yr for the area
south of the existing project). If erosion continues at these rates, the returns will have to be extended
significantly over this time frame (right panel). Note that if the calculated erosion rate of 4.6’/yr for the
existing 947’ geotube array is used the returns would extend much further. Note also that the road would be
compromised in this 40 year timeframe if the geotube array is not expanded, however this potential future
configuration will make it extremely difficult to keep a dry beach during all high tides. Any project undertaken
at this location should have both short and long term plans, which would include the eventual criteria for
retreat (of the homes, road, and/or array). Having a planned trigger for eventual removal of the geotubes is
reasonable foresight, and beach width is a valid indicator of the likelihood of negative impacts. If it is the
intention for the beach width to be the trigger for removal of the geotubes then this information should be
included in any long term plans.
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Works cited and helpful references
04/16/2018. Submission of Work Logs (January – March/early April 2018) SE48-2824, Sconset Bluff Geotextile Tube
Project. Submitted to Nantucket Conservation Commission, Submitted by Epsilon Associates, Inc.
02/11/2018. Independent Review of Epsilon Annual Report for Sconset Geotextile Tube Project (SE48-2824).
Submitted to Nantucket Conservation Commission, Submitted by Greg Berman, Coastal Processes
Specialist (WHSG & CCCE)
01/12/2018. 2017 Annual Review – Sconset Geotextile Tube Project (SE48-2824). Submitted to Nantucket
Conservation Commission, Submitted by Siasconset Beach Preservation Fund, Prepared by Epsilon
Associates, Inc.
01/05/2018. Notice of Intent: Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project.
Submitted to Nantucket Conservation Commission, Submitted by Siasconset Beach Preservation Fund,
Prepared by Epsilon Associates, Inc., In Association with W.F. Baird & Associates Ltd.
04/07/2017. Independent Review of Epsilon Annual Report for Sconset Geotextile Tube Project (SE48-2824).
Submitted to Nantucket Conservation Commission, Submitted by Greg Berman, Coastal Processes
Specialist (WHSG & CCCE)
12/13/2016. 2016 Annual Review – Sconset Geotextile Tube Project (SE48-2824). Submitted to Nantucket
Conservation Commission, Submitted by Siasconset Beach Preservation Fund, Prepared by Epsilon
Associates, Inc.
Thieler, E.R., Smith, T.L., Knisel, J.M., and Sampson, D.W., 2013, Massachusetts Shoreline Change Mapping and
Analysis Project, 2013 Update: U.S. Geological Survey Open-File Report 2012–1189, 42 p.,
http://pubs.usgs.gov/of/2012/1189/.
Massachusetts Shoreline Change Project
https://www.mass.gov/service-details/massachusetts-shoreline-change-project
NOAA - Nantucket Tide Gauge
https://tidesandcurrents.noaa.gov/
https://tidesandcurrents.noaa.gov/stationhome.html?id=8449130
NOAA - Vdatum tool
https://vdatum.noaa.gov/
FEMA- Flood Insurance Study
https://www.fema.gov/flood-insurance-study
https://msc.fema.gov/portal/availabilitySearch?addcommunity=250230&communityName=NANTUCKET,%20TOWN%20O
F#searchresultsanchor
Town of Nantucket’s project webpage, a PDF of photographs Ian Golding
https://www.nantucket-ma.gov/DocumentCenter/View/22939/Sconset-Bluff-Images-03_04_18-to-03_16_18-from-Ian-
Golding