HomeMy WebLinkAboutBaxterGeotube_gbermanREVIEW_02_11_2018
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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 11, 2018
TO: Jeff Carlson (Natural Resources Coordinator, Town of Nantucket)
CC:
FROM: Greg Berman, Coastal Processes Specialist (WHSG & CCCE)
RE: Independent Review of Epsilon Annual Report for Sconset Geotextile Tube Project (SE48-2824)
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: Along the southeastern shoreline of Nantucket there are relatively few shore perpendicular
structures intended to slow longshore (shore parallel) transport. The high bluffs provide significant volumes of
sediment that forms the adjacent and downdrift beaches and dunes. After prolonged and severe erosion
along Baxter Road (Figure 1) on Nantucket the Sconset Beach Preservation Fund (SBPF) was permitted to
install geotextile tubes (aka geotubes) at the base of the coastal bank. These geotubes were constructed into
the coastal bank in order to reduce erosion, however the preservation of the upland can often come at the
expense of the beach and other coastal resource areas. When uplands are not allowed to erode sediment is
prevented from being transported to downdrift beaches and dunes. The Order of Conditions required an
annual report to review monitoring and mitigation in order to ensure these potential negative effects are not
occurring.
Extensive study on background coastal processes, which this report does not replicate, has been
performed on this area before and during this stabilization effort (installed 12/2013-1/2014). Additional
analysis on the climatologic setting was submitted in the previous independent review (by this office) of the
2016 Epsilon Report. The latest report is the “Annual Review – Sconset Geotextile Tube Project (SE48-2824)”
prepared by Epsilon Associates Inc. and dated 01/12/2018 (referred to in this document as the “2017 Epsilon
Report”). 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 evaluation of the 2017
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Epsilon Report (and associated materials) in order to assess the impacts of current and project future
conditions. The following comments are organized in the same format as the Epsilon Report section headings.
Comments on Section 4.1 Sand Delivery:
Some of the numbers in the 2017 Epsilon Report sand delivery tables do not match those in the report
for 2016. This is primarily due to the 2016 UAV volume estimates from photogrammetry being replaced by the
more accurate 2017 UAV survey data (details later in this review). The Sand Delivery and Contribution Report
(revised 1/23/2018) indicates that there is still 6,278 cy of sediment missing from the required volume as of
5/31/2017. The report indicates that the applicant would like to discuss adjusting the mitigation requirements
with the Conservation Commission, during which this missing volume may be addressed.
One potential concern is that by holding this section of shoreline in place with geotubes while adjacent
portions of the coastal bank erode naturally, the geotube array will eventually extend seaward further than
adjacent areas, making it a similar to a headland.
Typically, wave energy is concentrated on the headland
areas as wave energy is concentrated on obstacles to
wave motion along the shoreline. The diagram (to the
right) shows how waves slow down in the shallow water
in front of headlands, and move faster in the deeper
water of bays. The bottom friction changes the wave
direction (called wave refraction). As a result, the wave
front parallels the coastline and wave energy is
concentrated on the headlands. This typically leads to
more potential for erosion (depending on the sediment
type of the bank and/or armor).
Comments on Section 4.2 Bluff Monitoring:
The 2016 Epsilon report indicated that an UAV (aka drone) was used to photograph and record
topographic data across the site in April of 2016. My review provided last year on this report indicated that
data was not provided on the horizontal/vertical accuracy of the survey, grid cell resolution of the DEM, on-
the-ground horizontal/vertical control points, or even the method of topographic survey (i.e. LiDAR vs
Photogrammetry). The 2017 Epsilon report indicates that the 2016 UAV elevation data was not of high enough
quality to use in bluff volumes estimates. The table below shows the difference in estimated bluff contribution
for the periods 2013-2016 and 2013-2017. There is a very large difference between these numbers that is
probably due to the inaccuracy of the 2016 UAV elevation data. In the future, the UAV photogrammetry
survey might become accurate enough to provide seamless topographic data for the beach as well as the bluff.
Above: Graphic showing how the bathymetry near a
headland tends to converge wave energy onto the
headland. Image from //science.kennesaw.edu
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Maria Hartnett (Epsilon) indicated that the 2013 baseline data was photogrammetry collected from a
traditional aircraft and not a UAV, and the resolution and accuracy was acceptable. The Sconset Bluff August
2017 Photogrammetry Survey Report is new this year, and a welcome addition to the overall documentation.
This report indicates that “AirShark reports that the vertical and horizontal accuracy associated with the
photogrammetry survey is 3-5 centimeters (cm).” While this is more accuracy information than was provided
in the 2016 report, it would be helpful to provide additional accuracy information for the 2013, 2017, and any
future UAV surveys. Typical accuracy reporting parameters include: spatial resolution aka GSD (Ground
Sampling Distance), % image overlap, root-mean-square error (RSME) on RTK ground-truthing, etc. There are
many guidance documents on these parameters.
Using the reported maximum 5cm accuracy, Figure 1 from the 2017 Photogrammetry Survey Report
could 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.
A VERY rough estimate of the accuracy of a linear foot of the coastal bluff is as follows:
1’ section of bluff * ~100’ from top to toe of bluff * 5 cm accuracy = ~0.6 cy/lf/yr
Ranges for the reported rates would be: 5.8 cf/lf/yr = 5.2-6.4 cf/lf/yr , 15.5 cy/lf/yr = 14.9-16.1 cf/lf/yr
This would also amount to +/- 575 cy of volume across the 947 linear feet of the project (2-3% of the 20,834
mitigation volume).
The 2017 Epsilon report indicates that the standard method of “multiplying the average annual erosion
of the bank or shoreline by the height and length of the shoreline protected…for the Sconset project area, a
mitigation volume of only 12.0 cy/lf/yr would be required.” The average for the past seven years is that the
armored bank is eroding 3.5cy/lf/yr more quickly than a normal nourishment calculation would provide. This
may be due to the geotube array projecting further seaward than the rest of the shoreline, leading to a higher
erosion rate as wave energy is focused on the project.
This section states that “a minimum of 22 cy/lf are available each year”. While this may be the volume
placed onto the design template each year, not all of this amount is eroding. By stabilizing the toe of the bank
with geotubes less sediment is eroding than would occur naturally. Despite some sediment being lost on top
of the geotubes, the more natural bank experiencing far more erosion (Figure 4). This seems to contradict the
findings that the project has contributed more sand (15.5 cy/lf/yr) than the unprotected bluff (5.8 cy/lf/yr),
however this is likely due to most of that erosion occurring the year of installation. The rates for 2017 for
armored (8.9 cy/lf/yr) and the unprotected area to the north (6.5 cy/lf/yr) are very similar. The unarmored
July 2013- April 2016 July 2013 - June 2017
average, distance-weighted unprotected bluff contribution volume 12.9 cy/lf/yr 5.8 cy/lf/yr
average, distance-weighted protected bluff contribution volume 18.1 cy/lf/yr 15.5 cy/lf/yr
% of design template (22 cy/lf/yr)82%70%
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areas immediately to the north (Figure 2) and south (Figure 3) appear to have minimal change in the location
of the toe of the bluff since installation of the geotubes.
Also, if 15.5 cy/lf/yr eroding each year, and the full design template is 22 cy/lf/yr, then this is over 70%
of the annual volume being contributed to the coastal system. While there have been some minor storms
during this timeframe (2013-2017), there has not been a major or even a strong moderate storm event. With
over 70% of the available sand used, during a fairly quiescent period of time, it seems unlikely that a reduction
in the sand template volume could be justified.
Comments on Section 5.1 Monitoring Program Adjustments:
The portion of section 5.1 in the 2017 Epsilon Report that applies to Aerial Bluff Monitoring appears to
be identical to section 3.1 in the 2016 Epsilon Report. The independent review from last year covers
these requested adjustments.
Comments on Section 4.3 Shoreline Monitoring:
The 2017 Epsilon Report included pages 18-27 of the Woods Hole Group, Inc. (WHG) “SOUTHEAST
NANTUCKET BEACH MONITORING October 2017 74th SURVEY REPORT” (12/2017) which covers shoreline
change trends from 1994. The WHG Report continues to use mean low water (MLW) for the vertical datum for
their shoreline change report. While the erosion rates at MLW and the toe/top of Bank are certainly linked
there is a typically convoluted correlation between them (e.g., 2’ of erosion at MLW does not immediately
equal 2’ of loss at the top of the bank). Erosion of the coastal bank can build the adjacent beach, which may
indicate accretion when looking at the wet/dry line. Profiles are not provided in these excerpted pages, only
the change in the location of MLW, however the previous WHG report showed a strong linear relationship
between MLW and beach volume. This indicates that MLW might be used as a proxy for beach volume in the
future, however MHW could also be graphed in a similar way to determine if the trend is valid for this higher
datum as well. If MHW and MLW are shown to correlate well, then this type of analysis my not be needed in
subsequent reports. The profile data were not provided, therefore the transect data (including the hard to
obtain wading shots) were not examined for this review.
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.
In the future, the UAV photogrammetry survey methods used by the applicant might become accurate
enough to provide seamless topographic data for the beach as well as the bluff. This likely would not replace
the long term profile data, but could supplement the findings and give accurate volumetric change along the
beach as well as the bluff.
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Comments on Section 5.1 Monitoring Program Adjustments:
The portion of section 5.1 in the 2017 Epsilon Report that applies to Shoreline Survey Monitoring
appears to be identical to section 3.1 in the 2016 Epsilon Report. The independent review from last
year covers these requested adjustments.
Comments on Section 4.4 Wetland Well Monitoring:
The “2017 Wetlands Wells Groundwater Level Monitoring” (10-2017) makes a reference to “50
previous well readings…from 2001-2007” helping define the expected variation (2-5’ over 6 years), however no
mention of what wells or the sampling parameters/timing are explained. If an analysis of this data were
provided and matched well with the samples collected in 2016/7 then it could be determined if the water
levels have changed since this project was constructed. Additionally, it is not only the range that is important
for wetlands. If the annual low gets lower than historic levels then there may not be as much water as there
used to be during dry times. It’s unlikely this has changed significantly, but it cannot be determined from the
information provided.
It should be noted that in 2017 the Water Level Elevations are given (opposed to just the Top of Well
and Water Depth measurements). It is much better for readers to compare these Water Level Elevations than
the other data. However the two sampling events in 2017 were in September and then again one month later
in October. This seems too close together to be meaningful. If would be better to space out the sampling
events, and also to do them about the same time each year. The 2017 report did not include the 2016 data,
however they are all converted to Water Level Elevations and included in the table below.
Comments on Section 5.1 Monitoring Program Adjustments:
The portion of section 5.1 in the 2017 Epsilon Report that applies to Wetlands Wells Monitoring
appears to be identical to section 3.1 in the 2016 Epsilon Report. The independent review from last
year covers these requested adjustments.
Comments on Section 4.5 Underwater Video Monitoring:
The first two survey dates of 2007 and 2016 are too far apart for a coherent analysis in such a dynamic
area. Additionally, the geotubes were installed in 2013/2014 and so seven years have passed between the
“baseline” study and the installation. If the baseline study were done in 2013 then perhaps some meaningful
patterns could be interpreted. The 2008 and 2013 bathymetry indicate that the underwater sand forms can be
June 28, 2016 July 28, 2016 September 12, 2016 September 1, 2017 October 3, 2017
Well
Water Level
Elevation (ft)
Water Level
Elevation (ft)
Water Level
Elevation (ft)
Water Level
Elevation (ft)
Water Level
Elevation (ft)
E-2 70.8 72.3 69.9 70.2 73.9
E-4 71.0 71.2 69.9 70.3 71.6
E-6R 67.9 68.6 66.8 68.2 70.6
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several feet high and appear to be migrating, which would have major implications on bottom type.
Additionally, according to the 2017 Underwater Survey Report, “the percent cobble/boulder coverage can vary
by 20% or more when reviewing video from locations even one or two seconds (which is a distance of about 4-
10’). When comparing the recent survey data (June 2017, October, 2016, and June 2016) there does not
appear to be a significant change that can be determined with this type of point data collection. The concern
that can be determined in this type of mapping is an overwhelming pattern of sand spreading out over top of
the cobble bottom. Across these three survey it appears that most of the offshore area is still >25%
cobble/boulder, with the overall coverage of > 50% cobble/boulder depending on just a few points and how
the data points were interpolated to form the color shaded image. Sidescan (backscatter) sonar images would
provide a much more complete picture of the bottom and could be groundtruthed with the underwater
images.
Comments on Section 5.1 Monitoring Program Adjustments:
The portion of section 5.1 in the 2017 Epsilon Report that applies to Underwater Video Monitoring
appears to be identical to section 3.1 in the 2016 Epsilon Report. The independent review from last
year covers these requested adjustments.
Comments on Section 4.6 Annual Drainage System Report:
The accumulated sediment continues to be below the threshold for cleaning (as indicated in the
Epsilon Report). The system is likely performing as designed.
Comments on Section 5.1 Monitoring Program Adjustments:
Drainage System Reporting – If the town is willing to take this monitoring program it would likely be
minimal effort.
Comments on Section 5.2 Mitigation Volume Adjustments:
This is a fairly open and sandy coastline, and the beaches in this area are being naturally restored by
the large volumes of sediment that are being eroded from the nearby coastal banks. If many of these sources
become armored, without making up for the sediment lost to the system, the beaches will not be as robust.
Note that the volume of sediment coming from the coastal bank is considered significant if it will “play a role”
(310CMR10.04) in building up the beach, and 15-20,000 cy per year would certainly play a role. Secondly, the
hard nature of a CES (i.e. geotube) allows for more reflection of wave energy as the waves strike the CES. This
can result in turbulence which will mobilize the sediment from the base of the structure and allow for easier
transport offshore. In this way the beach elevation at the toe of a structure often lowers over time. The beach
elevation in front of the structure is likely more dependent on erosion of the coastal banks updrift of the site
and subsequent transport to this area. As more and more of the shoreline gets armored, progressively less
material will be available to sustain Nantucket’s beaches. Careful thought should be given to what direction
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sediment moves when examining this project in order to make sure that sediment isn’t deprived from an area
that needs it.
There are two main uses of compensatory nourishment. First, the project will need to address the
beach adjacent to the proposed structures (i.e. trigger volume). This is so that the beach in the immediate
vicinity of the project does not drop and change the coastal processes of the immediate area. Additionally, for
this project it is essential that the geotube remain covered at ALL times. As soon as the geotube is uncovered
it will be acting as a CES. Geotubes (even ones that are intended to be covered) are considered a coastal
engineering structure (CES) as they always have the potential to be uncovered (which has happened
repeatedly each year since installation)…especially during a storm. The 2nd concern is to make up for any
reduction in sediment available for downdrift beaches (i.e. annual volume) due to the slowing or stopping of
the coastal bank erosion. This type of dual-purpose nourishment style is often recommended by WHSG/CCCE
reports.
A re-analysis of the annual nourishment requirement was not undertaken for this report as it is
understood that all parties have agreed to the 3 main components: erosion rate, height of the landform, and
length of the stabilization project. The current requirement contains a 50% factor of safety, in addition to
replacing the amount of material that would have been eroded from the coastal bank at long-term rates. A
safety factor is a good idea for this kind of project as if the geotube gets exposed it will act as a CES with all the
associated potential negative impacts to coastal resource areas. The volume currently required is to place 22
cy/lf/yr (20,834 cy), without this safety factor it would be 13,542 cy/yr (from 2017 Epsilon Report showing the
upper range of average annual bank contribution from 1994-2013 as 14.3 cy/lf/yr).
It should be noted that less than 14,000 cy has been contributed to the littoral system every year
except for the year of installation (see table below). Accuracy of the UAV surveys would be about +/- 575 cy.
12/2013 – 3/2014 4/2014 – 3/2015 4/2015 – 3/2016 4/2016 – 5/2017
21,758 cy 13,966 cy 10,723 cy 8,550 cy
This reduced volume of sediment entering the littoral system could be due to the relatively mild storm
seasons over the last few years. This is supported by the most recent year’s contribution of 8.9 cy/lf/yr being
similar to the adjacent unarmored areas (6.5 cy/lf/yr to the north and 3.2 cy/lf/yr to the south). It is
reasonable to assume that the project should be contributing more sediment than adjacent areas, since it
protrudes further seaward than the unarmored areas. This discrepancy will likely increase in the future unless
the array is moved landward to match the rest of the shoreline.
The current practice is to place 22 cy/lf/yr (20,834 cy) of sediment on the bluff each year, regardless of
how much erosion has occurred during the previous year. The Epsilon report requests a switch to a new
“adaptive mitigation” program, in which the sand template will be refilled each year to 22 cy/lf. Since there
was about 17,000 cy of sand left in the template in 2017, this new program would only require 3,834 cy of
sand to be placed. While there may be a large volume (i.e. 17,000 cy) “available” it may not yet be
“contributed” to the littoral system…this is due to erosion not being steady. Except for a few nor’easters in
early 2014…there have not been many large storms to hit the project site during this monitoring period.
Depending on the frequency and intensity of storm events this requirement could very well be more than the
safety factor currently in effect.
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The Conservation Commission may want to consider the adaptive mitigation program, however the
program could include a method to put the minimum of 13,542 cy/yr (from 2017 Epsilon Report showing the
upper range of average annual bank contribution from 1994-2013 as 14.3 cy/lf/yr) to provide sediment to
downdrift beaches, and then additionally filling the template back to 22 cy/lf each year.
The 2017 Epsilon Report indicates that the large volume of sand in the template is making the access
ramps too steep. The required sand volume could be placed outside the footprint of the template (ex. toe of
bluff, adjacent to the geotubes). In this way the sediment will rapidly provide material to downdrift beaches
that may be deprived due to the geotubes acting as a shoreline armor.
If maintenance of the array is discontinued the geotextile will become exposed rapidly and affect
coastal processes in a way similar to a CES. If a biodegradable alternative was used the nourishment could
cease with little long term negative effects to the coastal resource areas. The negative impacts would be on
the upland property due to erosion re-occurring. One of the benefits of a biodegradable “soft” (aka non-CES)
alternative is that failure and abandonment is acceptable (from a coastal resource area point of view). If
maintenance efforts are stopped, the coastal bank will not be worse off than before the project. Once the
natural erosion rate is restored the bank will provide the same natural landform functions to protect the
system as other un-armored coastal banks.
Additional Considerations (repeated from 2016 review):
During lower wave energy the geotubes stay covered with sand and have minimal negative interaction
with coastal processes. During minor storm events portions of the geotubes are exposed, and are
likely reflecting wave energy in a similar way to a CES for a short time period. The project site has not
experienced a significant storm event since the installation of the geotube array. Until data is available
from the geotube array experiencing a larger storm (for example with Stillwater elevations intersecting
the geotube array), the Conservation Commission may want to carefully deliberate before removing
conservative controls on the project (ex. high volume of nourishment and monitoring).
CESs (including geotubes) have the potential to alter wave, tidal or sediment transport processes while
protecting upland structures. General negative effects include: exacerbating beach erosion, damaging
neighboring properties, impacting marine habitats, and diminishing the capacity of landforms to
protect inland areas from storm damage. Reflected wave energy can erode the fronting beach,
potentially undermining the revetment while lowering the height of the beach. This erosion may also
result in a loss of dry beach at high tide, reducing the beach’s value for storm damage protection,
recreation, and wildlife habitat. Specific to this site, it is difficult to determine if these negative effects
are occurring as the sand appears to be highly mobile in this section of shoreline. Typical evidence of
reflection of wave energy includes; a lower beach than adjacent areas, and/or a larger grain size
(cobble) lag left behind after erosion of the small grain size.
Due to the scale of this project (947’ length) there is a high potential for current to set up parallel to
the smooth exposed geotube during storm conditions with oblique waves. This type of current can
rapidly scour the end of the array, even with a well-built return.
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One of the dangers of “holding the line” with either a CES or softer alternatives (i.e. coir
envelopes/fencing) is that the stabilization array will eventually, 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 than necessary to protect the
house. Flanking could require returns to be extended landward over time in order to protect the
house, which would allow the property to protrude further seaward than the rest of the shoreline and
affect the coastal processes (erosion and sediment transport).
Erosion doesn't stop in areas adjacent to a shoreline stabilization project and "holding the line" can
become more and more difficult over time. Eventually there will be a time when the landward retreat
of the array, to be more compatible with the surrounding shoreline, will be the preferred course of
action. Some potential indicators will likely exist when it is time to retreat: slumping of the top of the
coastal bank, loss of vegetation, frequent maintenance, loss of the high tide beach, etc. Many of these
will likely be present after a significant storm event. A section in the Work Protocol on the eventual
retreat (or abandonment) of the array might be helpful and inform monitoring activities to support the
long-term longevity of stabilization methods being utilized at this site.
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Figure 1. Overview map illustrating the location of Baxter Road in relation to Nantucket, MA.
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Figure 2. Map showing a series of Google Aerial Images for the period (2014-2017) of the northern end of the geotube array. The dashed red line is
the approximate 2014 toe of coastal bank in each image.
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Figure 3. Map showing a series of Google Aerial Images for the period (2014-2017) of the southern end of the geotube array. The dashed red line is
the approximate 2014 toe of coastal bank in each image.
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Figure 4. A crude graphic showing the theoretical potential for erosion during a storm. The three images on
the left are a natural coastal bank, while those on the right are a bank stabilized with geotubes. T1, T2, and T3
all imply a time series where T1=before/during the storm, T2=immediately after the storm, and T3=long
enough after the storm for the bank to equilibrate. Despite some sediment being lost on top of the geotubes,
the more natural bank experiencing far more erosion as shown in T2, and then retreats to a more stable angle
in T3.
T1
T2
T3
T1
T2
T3