HomeMy WebLinkAboutSE48_2824 Independent Review of the 2019 Annual Report by G. Berman 12_08_2020
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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
December 8, 2020
TO: Jeff Carlson (Natural Resources Director, Town of Nantucket)
CC: none
FROM: Greg Berman, Coastal Processes Specialist (WHSG & CCCE)
RE: Independent Review of the
2019 Annual Review – Sconset Geotextile Tube Project (SE48-2824) 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.
Introduction:
After prolonged and severe erosion along Baxter Road on Nantucket the Sconset Beach Preservation Fund
(SBPF) was permitted to install geotextile tubes (aka geotubes) at the base of the coastal bank. In order to
reduce erosion a 852’ geotube array was constructed into the coastal bank and within a couple of years was
extended to 947’. The Order of Conditions for this array requires an annual report to review monitoring and
mitigation in order to ensure negative effects are not occurring. The latest report is the 2019 Annual Review –
Image date 10/2018 from GoogleEarth
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Sconset Geotextile Tube Project (SE48-2824) prepared by Epsilon Associates Inc. and dated 03/02/2020
(referred to in this document as the “2019 Epsilon Report”). Mr. Carlson (Natural Resources Director for the
Town of Nantucket) got in touch with the Coastal Processes Specialist (working for both the CCCE and WHSG),
requesting an evaluation of the 2019 Epsilon Report (and associated materials) in order to assess the impacts
of the geotube array.
For brevity, this report does not replicate the extensive study on background coastal processes that
has been performed on this area as well as the additional analysis on the climatologic setting was submitted in
the previous independent reviews (by this office). The following comments are organized in the same format
as the Epsilon Report section headings and key points are in bold.
Comments on Section 4.1 Sand Delivery:
As presented in Attachment A the May 2018 – March 2019 Sand Delivery and Contribution Report , the
Project incorporates substantial mitigation volume of 22 cy/lf/yr. Given the project’s length of 947 feet, the
total annual mitigation volume is 20,834 cy.
The May 2018 – December 2019 Sand Delivery and Contribution Report (dated February 2020)
indicates on Line 13 of Table 1 that there is 16,459 cy of sediment deficient from the required volume as of
12/31/2019. The report also indicates on Line 14 of Table 1 that if the applicant were only required to fill the
template to 22 CY/LF then would currently be a surplus instead of a deficiency. However, this type of adaptive
management has not been approved, and the surplus in the template combined with a deficit in nourishment
volume may indicate that the template may not be providing sediment to the beach system during non-storm
periods. The annual volume required was 18,744 CY (when the array was 852’) for Sand Years 2014 and 2015
and has been 20,835 CY for Sand Years 2016-2019 (to account for the 947’ array length). It should be noted
that the Sand Years are actually time periods that range from as little as 3 months to as much as 22 months
(see table below).
The volumes for the “#CY Provided” is from the May 2018 – December 2019 Sand Delivery and Contribution
Report as shown in the “Data Source” column of the table. The “Cumulative Delta” shows the surplus (in
green) changing to a deficit (in red) for the past four Sand Years. Despite irregular time periods used in
defining the Sand Years, it does not appear to significantly impact the surplus or deficit for a given year. Below
is representation of what the table would look like if regular calendar years were used for the Sand Years.
# Months Required CY # CY Provided Data Source Cumulative Delta
Sand Year 2014 Dec 13 - Mar 14 5 18,744 23,951 Table 4 Line 9 5,207
Sand Year 2015 Apr 14 - Mar 15 12 18,744 14,428 Table 5 Line 4 891
Sand Year 2016 Apr 15 - Mar 16 12 20,834 15,085 Table 6 Line 8 -4,858
Sand Year 2017 Apr 16 - Jan 18 22 20,834 15,138 Table 7 Line 5 -10,554
Sand Year 2018 Feb 18 - Apr 18 3 20,834 8,152 Table 8 Line 5 -23,236
Sand Year 2019 May 18 - Dec 19 21 20,834 23,335 Table 9 Line 8 -20,735
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It should be noted that the 20,735 deficient volume (which is less than 1% different than one year’s
requirement) calculated above does not take into account the 4,275 cy of sand in Line 10 of Table 1 which
while “not initially placed as mitigation sand, performed the same function and has been counted as such
since is sloughed off the bluff face and was pushed off the template” (email from Dwight Dunk with Epsilon). If
the +4,275 cy is added to the -20,735 cy then volume of -16,459 cy matches what is reported in Line 13 of
Table 1.
The Sand Report stops at 12/31/2019 and it is currently almost 12 months after that date. While it not
known when Sand Year 2020 will end, if it were to end on 12/31/2020 the sand deficit would likely increase.
According to the applicants there is 10,000 cy of compatible sand from Polpis Harbor that will be placed during
Sand Year 2020. This is less than half of the required amount and will further increase the sand deficit. While
there is still time in Sand Year 2020, it is likely that the site will not have received the required volume of sand
in a given calendar year since 2015. If only the 10,000 cy from Polpis are added then starting in 2021 the site
will be 27,294 cy short of the required amount (-31,569 cy plus the +4,275 cy from Line 13 of Table 1). This
will continue the trend of increasing deficits over the last five years.
2014 2015 2016 2017 2018 2019
January 23,951 4,477 2,499 4,120
February 3,936 40 2,683
March 5,506
April 6,015 2,606
May 9,320
June
July
August
September
October 2,204 5,400
November 3,156 4,266
December 15,085 1,230 2,090 1,506
Subtotal 29,966 23,498 1,230 11,410 16,011 17,975
Required CY 18,744 18,744 20,834 20,834 20,834 20,834
Length 852'852-947'947'947'947'947'
29,966 53,464 54,694 66,104 82,115 100,090 Cumulative Provided CY
18,744 37,488 58,322 79,156 99,990 120,824 Cumulative Required CY
11,222 15,976 -3,628 -13,052 -17,875 -20,735 Cumulative Delta
2014 2015 2016 2017 2018 2019
This is by calendar year instead of "sand year"
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Comments on Section 4.2 Bluff Monitoring:
The 2019 Epsilon report indicated that an UAV (aka drone) was again used to photograph and record
topographic data across the site. Previous reviews on Epsilon reports indicated that data was not provided on
the horizontal/vertical accuracy of the survey, however the current report Sconset Bluff September 2019 Aerial
Survey Report (November 2019) does an excellent job on providing information on accuracy method and
estimates. For this survey the reported accuracy was approximately 1 cm horizontally and 1.5 cm vertically.
However this reported high accuracy conflicts with the minor accretion for the area above the
geotubes of “approximately 0.37 feet (11.3 cm), which is about the vertical accuracy range of the survey
methods and likely represents no net change”. A systematic offset of 4.5 inches (11.3 cm) is much greater
than the reported 0.6 inches (1.5 cm). This has been calculated as approximately 587 cy above the geotubes
and over 1000 cy in unprotected areas. The Sconset Bluff September 2019 Aerial Survey Report postulates that
these volumes “may either: 1) represent a minor amount of sand accumulating at the toe of the bluff, or 2)
represent values that are within the vertical accuracy range of the drone survey.” The top and toe of the bank
are used for determining volume changes, the calculations utilized an elevation of 12 NAVD88 as a proxy for
the toe of the bank. The use of this proxy may also contribute to volumetric errors if the break in slope
changes between surveys. Examination of the profiles used would be required to determine if some of this
error is due to this reason.
The last independent review of this report recommended using the reported accuracy to have a
separate color for areas within the accuracy boundaries. For example, +/- 5cm would be gray instead of having
just blue (loss) and red (gain), in order to show areas that have minimal change too small to determine gain or
loss. This was not done in the Sconset Bluff September 2019 Aerial Survey Report . In addition to this
representation on the map, it has become clear that it is also important to show the accuracy in the volume
estimates tables (ex. +/- 2,000 cy, +/- 2 cy/lf/yr). The Sconset Bluff September 2019 Aerial Survey Report
indicates that the estimates are at least +/- 2.1 CY/LF/YR, which is the unlikely accretion rate determined for
the “South Unprotected Area”. At present this uncertainly is a concern for determining appropriate erosion
rates in adjacent areas, as well as an accurately reported surplus of material in the sand template.
# Months Required CY # CY Provided Data Source Cumulative Delta
Sand Year 2014 Dec 13 - Mar 14 5 18,744 23,951 Table 4 Line 9 5,207
Sand Year 2015 Apr 14 - Mar 15 12 18,744 14,428 Table 5 Line 4 891
Sand Year 2016 Apr 15 - Mar 16 12 20,834 15,085 Table 6 Line 8 -4,858
Sand Year 2017 Apr 16 - Jan 18 22 20,834 15,138 Table 7 Line 5 -10,554
Sand Year 2018 Feb 18 - Apr 18 3 20,834 8,152 Table 8 Line 5 -23,236
Sand Year 2019 May 18 - Dec 19 21 20,834 23,335 Table 9 Line 8 -20,735
Sand Year 2020 Jan 20 - Dec 20 12 20,834 10,000 from Polpis -31,569
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Comments on Section 4.3 Shoreline Monitoring:
The 2019 Epsilon Report included pages 19-28 of the Woods Hole Group, Inc. (WHG) “SOUTHEAST
NANTUCKET BEACH MONITORING December 2019 80th SURVEY REPORT” (01/2020) which covers shoreline
change trends from 1994. The natural changes in this dynamic area continue to overshadow any signal that
might be from the geotube project. No additional shoreline change can be attributed to the project at this
time with the available data provided.
The applicant has the requirement on the existing 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…”. The WHG Report continues to use mean low water (MLW) for the vertical datum for their
shoreline change report, which is consistent across their surveys. However, the Conservation Commission may
want to request the values at each transect from the edge of the geotube to MHW. This value will likely
determine a “walkable” beach, and by observing this value over time the Conservation Commission can
determine if this width is narrowing.
Comments on Section 4.4 Underwater Video Monitoring:
Using video (and interpolating a raster from point data) alone is not the best methodology to
determine quantitative change in bottom type. Sidescan (backscatter) sonar images would provide a much
more complete picture of the bottom (with no need for interpolation) and could be ground-truthed with the
underwater images. The underwater sand forms in the area can be several feet high are likely migrating, which
would have more significance on bottom type than any above normal contribution from the geotube sand
contribution. The bottom type appears to exhibit more seasonal change (06/2018-11/2018) than change over
time (ex. 10/2016-11/2019), as shown by the timeseries of bottom type maps below (modified from the Video
Survey Maps in the 2019 Epsilon Report).
06/2016 06/2017 06/2018
10/2016 11/2017 11/2018 11/2019
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Comments on Section 4.5 Annual Drainage System Report:
Nantucket Engineering & Survey, P.C. determined that the accumulated sediment in the drainage
system continues to be below the threshold for cleaning (as indicated in the Epsilon Report). The system is
likely performing as designed.
Comments on Section 5.0 Recommended Changes to Monitoring and Mitigation Program
The 2019 Epsilon Report indicates that “The recommended changes are the same as presented in the
December 2016 Annual Report…”. These recommended changes have also been addressed in previous
reviews of the Epsilon Reports, and are not repeated in this document.
Summary of findings from the previous independent reviews of the Epsilon Reports:
• 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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Key finding for this independent review of the 2019 Epsilon Report:
In the 2019 Sand Year slightly more than the annual required mitigation volume was delivered. This is
a positive step going forward, and hopefully an indication of future compliance with the Order of Conditions.
Less positive is the 2019 Sand Year consisting of 21 months and the previous four years of deficient sand
deliveries, which have resulted in a current net deficiency of 16,459 cy. Despite the several years of deficiency
negative impacts cannot be assumed simply from the incomplete volumes delivered. Erosion at this section of
shoreline is punctuated by large storms, when large volumes are eroded quickly and then followed by quieter
periods of reduced erosion (or even accretion). It is more important that the full volume is available during
storms than strict compliance within a year (although the sand year range of 3-22 months is already providing
some flexibility). Exposure of the geotube array for extended periods due to the lack of sand will likely result in
negative effects. Additionally, if there are direct observations of increased erosion downdrift of the geotube
array, then a portion of that erosion may be due to the reduction in volume provided by the project.
Works cited
03/02/2020. 2019 Annual Review – Sconset Geotextile Tube Project (SE48-2824). Submitted to Nantucket
Conservation Commission, Submitted by Siasconset Beach Preservation Fund, Prepared by Epsilon
Associates, Inc.
02/18/2020. Sconset Beach Underwater Video Survey Report. Submitted to Nantucket Conservation Commission,
Submitted by Siasconset Beach Preservation Fund, Prepared by Epsilon Associates, Inc. & CR Environmental, Inc.
02/2020. May 2018 - December 2019 Sand Delivery and Contribution Report Baxter Road and Sconset Bluff Stabilization
Project Nantucket, MA. Submitted to Nantucket Conservation Commission, Submitted by Siasconset Beach
Preservation Fund, Prepared by Epsilon Associates, In Association with Cottage + Castle, Inc.
01/29/2020. Letter RE: SBPF - Baxter Road Drainage System Monitoring. To Dwight Dunk, PWS, BCES (Epsilon Associates,
Inc.), By Arthur D. Gasbarro, PE, PLS (Nantucket Engineering & Survey, P.C.)
01/2020. SOUTHEAST NANTUCKET BEACH MONITORING December 2019 80th SURVEY REPORT. Prepared by Woods Hole
Group.
11/2019. Sconset Bluff September 2019 Aerial Survey Report Baxter Road and Sconset Bluff Stabilization Project
Nantucket,MA. Submitted by Siasconset Beach Preservation Fund, Prepared by Epsilon Associates.