HomeMy WebLinkAbout33 SE48_3115 SPBF Response to G Berman Review 02_07_2019
PRINCIPALS
Theodore A Barten, PE
Margaret B Briggs
Dale T Raczynski, PE
Cindy Schlessinger
Lester B Smith, Jr
Robert D O’Neal, CCM, INCE
Andrew D Magee
Michael D Howard, PWS
Douglas J Kelleher
AJ Jablonowski, PE
Stephen H Slocomb, PE
David E Hewett, LEED AP
Dwight R Dunk, LPD
David C. Klinch, PWS, PMP
Maria B. Hartnett
ASSOCIATES
Richard M. Lampeter, INCE
Geoff Starsiak, LEED AP BD+C
Marc Bergeron, PWS, CWS
3 Mill & Main Place, Suite 250
Maynard, MA 01754
www.epsilonassociates.com
978 897 7100
FAX 978 897 0099
21597/2018/Expanded Project/NOI/Conservation Commission Hearings
February 7, 2019
Nantucket Conservation Commission Via Electronic Mail and U.S. Mail
2 Bathing Beach Road
Nantucket, MA 02554
Subject: Response to Greg Berman Comments on the Expanded Baxter Road and
Sconset Bluff Storm Damage Prevention Project
(DEP File No. SE 48-3115)
Dear Commission Members:
On behalf of the Sconset Beach Preservation Fund (“SBPF”), Epsilon Associates, Inc.
(“Epsilon”) submits the following response to technical review comments by Mr.
Greg Berman, as well as a supplement to sand management reporting to clarify
certain aspects of this issue raised by the Nantucket Conservation Commission
(“Commission”) during the Public hearing process. Many of the questions raised
have been answered on numerous previous occasions and therefore many of the
responses here reference those previous responses.
TECHNICAL REVIEW COMMENTS
Mr. Berman provided technical review comments on the Notice of Intent (“NOI”),
and subsequent materials, for the Expanded Baxter Road and Sconset Bluff Storm
Damage Prevention Project (“Project”) to the Commission via correspondence dated
February 1, 2019. He has reviewed this Project and submitted comments
previously, and some of his presented below reflect points he has made in those
reviews.
SBPF was pleased to see that Mr. Berman’s review confirmed our assessment of the
Baxter Road and Sconset Bluff Storm Damage Prevention Project in many important
ways. At its essence, the Commission’s requirement is to determine whether the
historic homes and infrastructure (road, utilities, bluff walk) qualify for using a
Coastal Engineering Structure to protect them, and whether the proposed protection
is using the “best available measures” to provide that protection. Nothing about
erosion protection is easy, or simple, or perfect, and certainly not when it is being
done in one of the most dynamic coastal environments on the eastern United States.
Nantucket Conservation Commission 2
February 7, 2019
believes that the criteria for both State and local law and regulation are met here, and
that Mr. Berman’s report supports that determination.
Mr. Berman’s report confirms that SBPF’s sand nourishment numbers use the correct
formula, and also that an adaptive formula is appropriate. Moreover, Mr. Berman
confirms that principally the littoral drift defines the method through which the beach
in front of the geotubes will be impacted. The sand released from the protected bank
is a small percentage of sand resource when considering the full length of the Eastern
shore (10 miles). Nevertheless, the sand being contributed through the mitigation
program makes up for that impact. In addition, Mr. Berman points out that the timing
of the availability of that sand (whether from the bank or from mitigation sand placed
on the geotubes) into the system is not critical for protecting downdrift beaches and
dunes.
Mr. Berman confirms that there is a risk of adverse effects if the geotubes are
uncovered during a storm (but not between storms). SBPF addresses this by
maintaining a significant sand volume in the template, and by improving the protocol
for re-covering exposed geotubes by purchasing equipment and making other
changes to improve response times. That said, the sand in the template is designed
to be contributed, so some of it will be washed away during storms.
In addition Mr. Berman has raised a concern about the need to manage the ends of
the Project since continued erosion in the area adjacent to these ends, something that
we propose to address by adding soft coir rolls for return extensions as needed over
time.
Mr. Berman expresses a concern about the timeframe for which the geotube project
may be effective and over which a walkable beach in front of the geotubes effectively
can be maintained. He suggests monitoring the beach width and including a failure
trigger from the outset of the project so all parties recognize the risks. SBPF believes
that a walkable beach is maintainable and that the beach in front of the geotube
system should remain for a long time based on the mitigation sand and the sand
supplied through the littoral system from updrift beaches. However, the Commission
need not decide what the actual timeframe will be, because the proposed Project
includes failure criteria that would kick in based on the monitoring requirements.
SBPF recognizes that if the success criteria included in the Project are not being met
as measured by the monitoring reports, then the Project’s failure criteria are triggered
requiring either an effective response approved by the Commission, or the potential
removal of the Project.
Nantucket Conservation Commission 3
February 7, 2019
Following are the topics and comments provided by Mr. Berman. Comments are
presented in italics typeface followed by our response in normal typeface.
1. 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)
We concur with Mr. Berman’s calculation and used that method for the proposed
extension project. See Notice of Intent (“NOUI”) Section 2.3 Updated Bank
Contribution Volume.
2. 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. … 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).
We appreciate the comment and agree there can be certain conventional advantages
to using a MHW reference. For instance, CZM recommends using MHW as a
reference datum for LIDAR and photogrammetry data because those are remote
sensing methods that have difficultly capturing data below MHW even at lower stages
of the tide. Even when these data are taken at low tide, wave action can obscure the
beach surface below the MLW line making it difficult to detect by either
photogrammetry or LIDAR. Therefore, MHW is the better reference datum for those
remote sensing technologies.
At Siasconset, however, we have actual bluff survey data that captures data along the
Coastal Bank the resource area for which sand mitigation is required. The bank
contribution volume was calculated using long-term data sources from 1994-2017,
and so incorporates data sources prior to the LiDAR data from 2000. The bank
contribution rates (as detailed in Notice of Intent “NOI” Section 2.3) uses data available
for Sconset Beach includes shoreline surveys back to 1994 and represents a much
more robust data set than is typically available for coastal projects.
The bank recession rate was determined for the area from 107-119 Baxter Road and
for the area from 71-85 Baxter Road. (71 Baxter Road is the southern extent of top of
bank erosion that is discernible on aerial photographs.) The bank recession rate was
determined using the same methodology described above, where the bank retreat
Nantucket Conservation Commission 4
February 7, 2019
distance was measured in GIS using top of bank lines digitized from 1994, 2003, and
2017. The 2017 top of bank line was derived from the August 2017 aerial survey of
the bank. As shown in Table 2-1 below [reprinted from the NOI], the distance-
weighted average annual rate of retreat for the area from 71-85 and 107-119 Baxter
Road is 2.8 ft/yr. This number is significantly less than the average annual bank retreat
of 4.6 ft/yr previously calculated for those areas covered by the Existing Project, which
reflects the higher bank erosion in the Existing Project area.
Table 2-1 Average Bank Retreat Rates, 107-119 Baxter Rd and 71-85 Baxter Rd
Average Annual Bank Retreat (1994-2017) for 107-119 Baxter Road 2.0
Average Annual Bank Retreat (2003-2017) for 71-85 Baxter Road 3.5
Average Distance-Weighted Bank Retreat 107-119 & 71-85 Baxter Road 2.8
3. 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.
The regulations require that the mitigation volume compensate for the loss of the
sediment source from the Coastal Bank that would otherwise be contributed to the
littoral system of the protection system were not in place. Therefore, the use of
Coastal Bank recession rate is the most applicable as compared to the recession rate
at MHW. The process of Coastal Bank erosion involves toe scour which leads to
over steepening of the banks, then at a certain slope the bank is too steep and it
erodes en mass. Over a long period of time the toe recession rate and the top of bluff
recession rate are essentially congruent. Therefore, the method used for the proposed
Project accurately presents Coastal Bank recession rate.
4. 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.
Nantucket Conservation Commission 5
February 7, 2019
We concur that long-shore sediment transport means that sand on the beach in front
of the geotube system has been deposited from updrift areas. The purpose of the
proposed mitigate sand volume is to compensate for the annual average volume of
sand that would be contributed to the littoral drift system if the geotube system were
not in place. Monitoring over the past several years documents that the average
annual volume of sand delivered to the template and washed off the template and
contributed to the littoral drift system was 13.5 cy/lf/yr for the period 2016 – 2018
(these three years exclude construction years and were 11.3 cy/lf in 2016; 8.9 cy/lf in
2017, and 20.3 cy/lf in 2018) which is consistent with the calculated rate of 12 – 14.3
cy/lf/yr.
Regarding the issue of maintaining sand cover on all portion of the geotube system,
the sand cover is designed to be washed away during larger storms. This is the
method through which sand that would otherwise be contributed from the natural
erosion of the bank is supplied to the littoral system. This means by definition that
the geotubes will be exposed after a storm event until they are re-covered. If the sand
on the face of the geotubes were not being washed away during larger storms the
sand template would not be functioning as intended.
See discussion in the attached memoranda regarding template sand management and
measures to maintain sand cover.
5. 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.
We concur that the “right” compensatory volume be placed to maintain the template.
That is why the SBPF proposed the adaptive sand management protocol that in
summary involves:
Upon completing construction, the template would be filled to stockpile 22 cy/lf
of sand,
During the winter storm months use that stockpiled sand to maintain geotube
coverage after erosion events,
Then after spring monitoring is complete refill the template to 22 cy/lf to replace
the volume of sand contributed by the template to the littoral drift system.
Nantucket Conservation Commission 6
February 7, 2019
This protocol results in the “right” volume of compensatory sand and is adjusted on
a year-to-year basis to match what was lost off the template. Based on the historic
data, it is expected that the average amount of sand to re-fill the template will be 8.8
cy/lf, however with the adaptive approach the amount that actually is washed away
will be added based on the annual 3D survey.
6. 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 …
We agree that the existing geotube system has been tested by significant coastal
storms, and that it has held up well. It serves as good demonstration project, and
based on its performance only minor adjustments to the design of the extension
Project are proposed, most notably lowering the lowest tier to elevation -3 feet MLW
datum from 0 feet MLW datum.
As described in the attached memoranda, the March 2018 storms presented timing
challenges to re-cover the exposed geotubes in between storm events. However, the
total volume of sand delivered to the template and eroded from the sand delivery
points (i.e. sand volume contributed to the littoral drift system) was about equal to the
volume of sand that needed to be re-placed pursuant to the Order of Conditions
(“OOC”). For this reason, the littoral system was not negatively affected by the
inability, on occasion during the succession of March 2018 storms, to re-cover
portions of the geotubes in time for the next storm. The SBPF has also proposed
changes to the protocol and secured dedicated equipment and equipment operators
to overcome the timing challenges of re-covering geotubes experienced during the
March 2018 storms.
7. 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 [underlined added] 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
Nantucket Conservation Commission 7
February 7, 2019
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.
We agree with the author’s comment that this is a good design and that the geotube
system maintains the position of the bank by preventing erosion, as depicted in the
graphic on page 5 of his comments. The SBPF also agrees with, and is committed to
providing compensatory sand to maintain the template and contribute sand to the
littoral drift system. The difference is approach, 1) provide the “right” amount of sand
as the author referenced previously, or 2) as the author suggests in this comment,
provide a yearly mandatory volume of sand. We believe the proposed adaptive sand
management protocol is the best approach to place the “right” volume of sand in any
given year. The proposed protocol requires the placement of 22 cy/lf of sand on the
template when it is built, then replaces the volume of sand lost over a winter storm
season to “refill” the template to 22 cy/lf. This sand management approach meets the
criteria mentioned by the author to: 1) keep the template full, 2) provides an on-site
stockpile of sand keep the geotubes covered to mitigate for how the geotubes affect
local wave processes, and 3) supply the minimum volume needed to maintain
downdrift beaches and dunes.
8. … photographs from during and shortly after the March 2018 storm was made
available. 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.
The clay head made it impossible to build the correct returns at the northerly end of
the existing geotube system, and instead the returns were placed to abut the clay
head. Therefore, when the clay head eroded, the northern end of the system was left
unprotected as if there were no returns. It is expected that the returns for the existing
geotube system will need to be lengthened periodically as the bank adjacent to the
Nantucket Conservation Commission 8
February 7, 2019
system naturally erodes over time. This Commission recently permitted SBPF to do
exactly this type of return extension to protect the existing pilot project system, and
something similar will be needed going forward for the extension Project as well.
The northerly extension proposed for this Project will cover the area where the clay
head previously existed before it eroded away to avoid this phenomena from
happening in the future. The procedures in the attached memoranda and the
previously submitted Sand Template Protocol address measures to keep the geotube
system covered and quickly re-covering exposed tubes after erosion events.
9. 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.
See the attached March 2018 Sand Management Memorandum that describes the
work SBPF did to re-cover exposed geotubes during the March 2018 storms. A key
take away from the efforts described in this memorandum is that the volume of sand
contributed from the sand delivery point (at the southern end of the system) to the
littoral drift system was nearly equal to the sand volume required in the OOC; the
difference is that the sand was released from a single location (the southern end of
the Project) during the storms over that three week period rather than from the face
of the geotube system. That period of time, March 2018, is not a failure by the SBPF
to provide an adequate volume sand to littoral drift system during this sequence of
storms, rather it shows the commitment by SBPF and C+C to persevere during an
intense storm season to comply with the intent to the OOC. March 2018 did however
identify a logistical challenge to re-cover expose geotubes when there is a succession
of winter storms. To address that issue SBPF and C+C has revised their template
management procedures to more quickly access and re-cover exposed geotubes. See
the attached Future Maintenance Procedures Memorandum for details about
dedicated equipment and personal. Please note the procedures described in the
Future Maintenance Procedures Memorandum supplement the Template Sand
Protocol previously submitted to the Commission.
In addition, it should be noted that the amount of sand contributed off the template,
even in this above average erosion year, exceeded the amount of sand contributed
Nantucket Conservation Commission 9
February 7, 2019
from the adjacent unprotected bank to the north and south. In other words, even
though there were a number of days during March 2018 when a number of sequential
large storms occurred before the geotubes could be re-covered with sand, thereby
potentially reducing the amount of sand that would have washed away on those days,
and even after factoring in the sand washed away from the southern delivery point,
the total volume of sand that did wash away over the 12 month period (as measured
by 3-D survey) was still greater than the volume of sand that would have been
contributed by this portion of the bank if it were not protected by the geotubes.
10. 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.
As the author describes this reach of the shoreline represents about 7% of the eastern
Nantucket shoreline (3,820 feet / 50,280 feet). That means 93% of the shoreline
remains as is to provide sediment to support the long-shore sediment transport system.
Additionally, the compensatory sand volume eroded off the template will be
contributed to the littoral drift system by cross-shore, diffusion, and long-shore
mechanisms to contribute the same volume of sand on an average annual basis that
would have been contributed to the littoral drift system from the 3,820 feet of
unprotected Coastal Bank. The recent surveys by the Woods Hole Group, Inc. show
that as the sand migrates off the template it builds up on the Coastal Beach until finally
being washed out to the near-shore environment. This movement of sand off the
template by cross-shore and diffusion mechanisms plus the sand contributed from the
updrift shoreline by long-shore transport will help to maintain the width of the Coastal
Beach going forward in time.
As described herein as well as in the NOI and at the Public Hearings, the mitigation
volume of sand is contributing sand to the littoral system at a greater volume than the
unprotected bank, but the Project has no effect on the updrift shoreline’s capacity to
supply sediment to the Project area. Given that, 1) the sand eroded off the template
mitigates for the protected bank, and 2) the updrift landforms continue to supply sand
equal to the present rate, the beach in front of the geotube system should remain for
years to come.
Nantucket Conservation Commission 10
February 7, 2019
The Coastal Beach profile in the Project area is unique formed by the dynamic and
erosive nature of the wave climate at Siasconset, the beach has a very steep foreshore
slope offshore typically to about -7 to -10 feet MLW before becoming more gradual.
There is not a typical equilibrium beach profile with a berm/bar configuration. This
profile and the area’s wave action will have more of an effect on beach width going
forward in time than the presence or absence of the geotube system. Continued
placement of compensatory sand is expected to slow the beach loss and maintain a
walkable beach longer than if no mitigation sand is contributed to the system, and as
described above the primary source of sand in front of the geotube system comes
from updrift landforms and is expected to continue to be deposited in front of the
geotubes maintaining a walkable beach over the long term.
11. 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).
By definition the Coastal Bank stabilization provided by the geotube system only
protects that reach of the Coastal Bank from erosion, adjacent unprotected reaches of
Coastal Bank will continue to erode. SBPF expects there will be the need to extend
returns in the future as the immediately adjacent bank erodes. In fact, the existing
OOC was recently amended to extend returns for the pilot project for exactly this
reason, and we suggest that similar return extensions be a part of the proposed 4,000
linear foot system as well.
The length of the proposed geotube system, approaching 4,000 linear feet also helps
to ameliorate the potential flanking as compared with sorter shoreline stabilization
installations intended to “hold the line” for a short shoreline reach. The author
included a photo of a single house protected by a revetment and protruding seaward
from the adjacent shoreline. This is an extreme example and depicts the “groin effect”
that a short system can cause. Although included as an extreme example, it really
does not illustrate the conditions along the Siasconset Bluff Project area. First, the
proposed extended geotube system “holding the line” for about 8/10ths of mile is not
Nantucket Conservation Commission 11
February 7, 2019
expected to result in a “groin effect” due to its length whereby long-shore sediment
transport will continue to move sediment along an elongated bulge as the adjacent
shoreline erodes landward. Continued placement of compensatory sand is expected
to yield shallow sloped junctions between the shoreline in front of the geotube system
and the shoreline in front of eroding shoreline, instead of sharp nearly right angle
transitions. Second, the example does not appear to include compensatory sand
mitigation. The continued placement of sand mitigates the “starvation” effect caused
by groins and mitigates the effects of flanking by diffusion.
12. If erosion continues at these rates, the returns will have to be extended
significantly over this time frame … 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.
The SBPF understands this may not necessarily be a permanent fix as compared to
longer-term erosional trends. It is expected to be a positive shoreline protection
program that should last for decades, and as such has a meaningful and positive
impact on an otherwise threatened historic residential community. The Project need
not be permanent to be worthwhile, in the same way that an effective medical
procedure is considered worthwhile if it adds to a person’s lifespan and quality of life.
The bank had been stabile for a long period of time and as documented by the
Massachusetts Office of Coastal Zone Management in a 2013 comment letter,
increased erosion of the Siasconset shoreline has been observed since he mid-1970’s.
The reason for this change is unknown, hypotheses include ideas such as a shift of
offshore shoals, or a shift in currents, or even the dredging of kelp beds – all
potentially effecting location, directions, velocity of currents. The construction and
maintenance of the geotube system is proposed for as long as success criteria, as
defined by the Nantucket Conservation Commission, can be maintained. Existing
OOC (SE48-2824) Special Condition Number 36 requires addressing any failures and
if necessary removing the geotube system. Thus, the SBPF in accepting the OOC has
accepted the potentiality that at some unknown time in the future that retreat of homes
and the road may be required.
In closing, the SBPF has provided herein responses to Mr. Berman’s technical
assistance review, and has addressed lingering issues regarding the template
maintenance that we thought Commission members may still have. This
Nantucket Conservation Commission 12
February 7, 2019
correspondence plus the NOI and several additional information / response to
comments submissions throughout the Public Hearing process, we believe, provides
the Commission with all the information it needs to make a permitting decision. The
record for this filing (SE48-3115) describes the Project purpose, proposed Project
design, alternatives considered in lieu of the proposed Project, anticipates potential
impacts and provides mitigation measures based on results of monitoring the existing
geotube system (SE48-2824), and documents compliance with the performance
standards for Coastal Bank and Coastal Beach. Therefore we respectfully request that
the Commission issue an Order of Conditions authorizing construction and
maintenance of the proposed geotube extension Project with practical special
conditions to protect relevant interests of the Wetlands Protection Act and the
Nantucket Wetlands Protection By-Law.
Please contact me at (978) 897-7100 or via email at ddunk@epsilonassociates.com
with questions of comments regarding this matter.
Sincerely,
EPSILON ASSOCIATES, INC.
Dwight R. Dunk, LPD, PWS, BCES
Principal
cc: MassDEP-SERO
J. Posner, SBPF
A. Gasbarro, Nantucket Eng. & Survey
S. Cohen, Cohen & Cohen Law, PC
G. Wood, Ruben and Rudman, LLP
G. Thomson, W.F. Baird & Assoc.
L. Smith, Epsilon
R. Hamilton, Woods Hole Group
encl. 1. Expanded Baxter Road and Sconset Bluff Storm Damage Prevention
Project (DEP File No. SE 48-3115) – March 2018 Sand Management
06 February 2019
2. Expanded Baxter Road and Sconset Bluff Storm Damage Prevention
Project (DEP File No. SE 48-3115) – Future Maintenance Procedures
Memorandum, 06 February 2019
M E M O R A N D U M
Date: 06 February 2019
To: Nantucket Conservation Commission
From: J. Feeley, Cottage + Castle, Inc.
D. Dunk, Epsilon Associates, Inc.
Subject: Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project
(DEP File No. SE 48-3115) – March 2018 Sand Management
During the Public Hearing process for the Expanded Baxter Road and Sconset Bluff Storm Damage
Prevention Project (“Project”) the Nantucket Conservation Commission (“Commission”) and
members of the public asked about template sand management both with regards to the March 2018
sequence of storms and going forward, relative to managing a longer sand template. We believe this
topic has been addressed thoroughly during the lengthy Public Hearing process. In closing out the
Public Hearing however, we take this opportunity to again address these topics in a manner that: 1)
summarizes the information previously submitted to the Commission; and 2) answers any lingering
questions about template sand management relative to the existing template (SE48-2824) and the
proposed extension (SE48-3115). This memorandum is submitted to describe efforts to replenish the
sand template during March 2018, and the Future Maintenance Procedures Memorandum dated 06
February 2019 addresses procedures to manage the existing sand template and the longer template
for the proposed Project.
March 2018 Sand Management
The work records describing the March 2018 period were provide to the Commission in April 2018
in accordance with Order of Conditions (“OOC”) reporting requirements. These records were also
summarized in response to comments on the extension Project (SE48-3115) as part of the response
to comments letter dated 02 November 2018. Below we re-present this same information in a slightly
different format to better describe the SBPF’s response to the sequence of coastal storms in March
2018.
There were four storm events in March 2018 that caused major wave action along the Siasconset
Bluff and existing geotube Coastal Bank stabilization system. The following is a brief description of
the site conditions prior to, and after, the storms and the actions to re-cover the geotube system.
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Photos are also included below which document the area of exposed geotubes in March, all of which
have been previously submitted.
March Storm Events and Maintenance:
March 3, 2018 Major Storm – Prior to the March 3rd storm, the geotube system was covered, and
an additional 2,500+ cubic yards (“cy”) of sand was available for contribution to the littoral drift
system at the southern end of the sand template below the sand delivery point. This storm exposed
the entire length of the second and third tier of geotubes, the southern and northern returns and
approximately 100 feet (“ft”) of the fourth tier of the geotube structure. The storm also removed the
2,500+ cy of sand from the southern end of the template (that was contributed to the littoral system).
Please note, this 2,500 cy of sand is more than what would be contributed from the front of the entire
template when the second (and maybe third) tiers(s) are exposed during a new storm.
March 8, 2018 Minor Storm – The geotubes were exposed following the March 3rd storm, in the areas
described above, and they remained uncovered up to and during the March 8th storm because the
beach was inaccessible, and work conditions were unsafe, for maintenance due to wave run-up.
Therefore, this storm occurred at a time when the geotubes were uncovered so that any sand that
would have been contributed off the face of the system during this storm was not available at that
time.
March 12 to March 16, 2018 Sand Deliveries – During the period of March 12th to March 16th, 4,746
cy of sand was delivered to the sand template, via the Baxter Road delivery area, for template
replenishment (see Table 1. SBPF Sand Deliveries 12 March 2018 to 16 March 2018 below). Re-
grading to cover the exposed geotubes did not occur during this period because C+C’s heavy
equipment subcontractors were not able to perform the work due to scheduling conflicts caused by
the high volume of building that was occurring on the island. A large percentage of the 4,746 cy of
sand delivered to the sand template at the southern end of the system was, however, made available
for contribution to the littoral drift system during the next storm event (see March 16th delivery photo
below).
Table 1. SBPF Sand Deliveries 12 March 2018 to 16 March 2018
Vendor Invoice # Date Quantity (CY)
PM Reis Trucking 133673 3/12/2018 434
PM Reis Trucking 133683 3/14/2018 1,020
PM Reis Trucking 133696 3/15/2018 1,754
PM Reis Trucking 133710 3/16/2018 1,538
Total 4,746
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March 13, 2018 Major Storm – For the reasons stated above, the geotube system was still exposed
as it had been prior to the March 8th storm in the areas described above. However, the 434 cy of
sand delivered to the southern end on March 12th were available and consumed in the storm.
March 18 – 20, 2018 Template Maintenance – Template maintenance commenced on March 18th
and continued through March 20th. During this period:
the northern ramp was rebuilt,
the northern and southern returns were covered, and
approximately 550 ft of the southern portion of the geotube structure (second and third tiers)
was covered (approximately 400 ft of the northern portion of the structure remained un-
covered).
Maintenance ceased on March 20th due to another storm that was to make landfall on the island on
March 21st. The second and third tiers of the northern portion of the structure were exposed over
approximately 400’ due to the work stoppage. A large percentage of the 4,746 cy of the sand
delivered to the template at the southern end of the structure, during the period March 12th to March
16th was available for contribution to the littoral drift system. It should be noted that the amount of
extra sand still at the southern end and claimed by the storm approximates the amount of sand that
would have been claimed from the northern section of the geotubes if they had been re-covered.
March 21 to March 22, 2018 Minor Storm – The storm re-exposed approximately 550’ of the face of
the second tier of the geotubes on the southern portion of the template that had been covered during
the period March 18th through March 20th. The storm also claimed a portion of the 4,746 cy of sand
delivered to the southern end of the system.
March 26, 2018 to April 11, 2018 Template Maintenance – Maintenance recommenced on March
26th and was fully completed on April 11th. It took longer than normal to re-cover the full-length of
the geotube structure because the remaining large volume of sand delivered between March 12th and
March 16th had to be evenly distributed over the full length of the Project. The succession of March
storms and the unavailability of C+C’s heavy equipment subcontractor in March had delayed the
maintenance and sand distribution.
See the attached March 2018 calendar presents a succinct summary of events, and identifies the
storm dates as well as sand delivery and template management dates.
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Summary and Conclusion:
Following the March 3rd storm, the second and third tier, the southern and northern returns and
approximately 100’ of geotube structure’s fourth tier were exposed. This condition persisted, for the
reasons stated above, until March 18th (14 days total) when the northern and southern returns and
550’ of the southern portion of the geotube structure were re-covered entirely. However, it is
important to remember that there was a large additional volume (2,500 cy) stored at the southern
delivery point and available to erode off the template in to the littoral system before the initial March
storm plus additional sand was delivered to the template during this period on four different days
totaling an additional 4,746 cy. Approximately 400 ft of the second and third tiers on the northern
portion of the structure that were originally exposed during the March 3rd storm remained exposed
until maintenance was completed on April 11th.
In addition to the contribution from the sand that was on the face of the structure before the March
3rd storm, and following partial maintenance completed before the March 21st storm, the 2,500+ cy
of sand available at the southern end of the structure below the delivery point was contributed to the
littoral system. This volume plus a significant portion of the 4,746 cy that was delivered to the
template between March 12th and March 16th, estimated to be 60% or approximately 2,850 cy of
sand, eroded off the template at the sand delivery point. Combined is equates to 5,550 cy of sand
contributed off the template from the delivery point instead of the face of the template. These
contributions helped mitigate the fact that a portion of the face of the geotubes were exposed in
March 2018.
Special Condition 32.(b) of the Order of Conditions (SE48-2824) reads in part;
“ … Throughout the winter, place additional sand on an as-needed basis, in
accordance with the replenishment trigger in the Milone & MacBroom November 12,
2013 letter (i.e., if half the vertical height of the lowest Geotube is exposed, place a
minimum of2 cy/If). …”
SBPF worked during March 2018 to recover exposed geotubes as quickly as possible after each
erosion events. The existing template is +950 feet long, thus pursuant to the Special Condition 32.(b)
after each erosion event approximately 1,900 cy of sand should have been placed on the template
following each erosion event to re-cover exposed geotubes.
Review of the attached calendar shows there were three storm periods – March 3 major storm, March
8 minor storm, and March 13 major storm – between March 1st and March 20th. As described above
during this time approximately +7,250 cy of sand was prepositioned (2,500 cy) and delivered to the
template (4,746 cy) during that same time period. During those three storm events approximately
+5,550 cy of sand was eroded out of the delivery point and contributed to the littoral drift system.
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This equates to placing the required volume of sand on the template 2.98 times (approximately 3
times during March 2018) [5,550 cy / 1,900 cy = 2.98]. Therefore, the overall contribution of sand
volume required by the OOC Special Conditions 32.(b) [2 cy/lf following each of the three erosion
events] was contributed to the littoral system from a single point rather than along the face of the
template following those three storm periods. Thus, although the SBPF was not able to re-cover the
geotubes prior to each storm due to un-safe work conditions and equipment availability, the criterion
of the replenishment volume was met during March 2018 because of the availability and contribution
of sand from the plume of sand at the southern delivery point.
Last, following after the storms of March 21 and 22, template maintenance was performed over the
period March 26th to April 11th to fully re-cover the exposed geotubes. There were no storms during
this period and maintenance to re-cover the tubes was completed on April 11th. Grading maintenance
continued during the period April 12th to April 30th to level the template on top of the geotube
structure and evenly distribute the sand.
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SCONSET BLUFF PHOTOS MARCH 2018
Conditions Prior to 3/3/18 Storm
Northern ramp and return Northern end viewing south
Midpoint viewing north Midpoint viewing south
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Southern end viewing north South ramp and return
Post Storm Photos 3/4/18 – The beach in front of the dune and Project was not accessible.
Exposed third tier viewing south Northern end
8
Northern returns Midpoint viewing south
Southern returns Southern end viewing north
9
3/6/18 Post Storm Photos
Northern end viewing south Northern return and ramp
Northern end viewing south Midpoint viewing north
10
Midpoint viewing south Southern end viewing north
South return and ramp area
11
3/16/18 Sand Delivery to Southern End of Structure
3/20/18 Maintenance Photos Taken Prior to 3/21 to 3/22/18 Storm
Northern return and ramp Northern end viewing south
12
Midpoint viewing north Midpoint viewing south
Southern end viewing north Southern return and ramp area
13
Post Storm Photos 3/23/18
Northern return and ramp Northern end viewing south
Midpoint viewing north Midpoint viewing south
14
Southern end viewing north Southern return and ramp area
Post Partial Maintenance Photos 3/26/18
Northern return and ramp Northern end viewing south (1)
15
Northern end viewing South (2) Southern return and ramp area
Post Maintenance Photos 4/11/18
Northern return and ramp Northern end viewing south
16
Midpoint viewing north Midpoint viewing south
Southern end viewing north Southern return and ramp area
MARCH 2018
SBPF MAINTENANCE ACTIVITIES
SUN MON TUE WED THU FRI SAT
1 2 3
Geotubes covered
Geotubes covered;
~2,500 cy sand
stockpiled below the
southern delivery point
MAJOR STORM
+/- 950 ft geotunes
exposed
4 5 6 7 8 9 10
Beach Inaccessible Beach Inaccessible Beach Inaccessible Beach Inaccessible
Minor Storm
Beach Inaccessible
+/- 950 ft geotubes
exposed
Beach Inaccessible
11 12 13 14 15 16 17
434 cy of sand
delivered to geotubes
MAJOR STORM
+/- 950 ft geotubes
exposed
1,020 cy of sand
delivered to geotubes
1,754 cy of sand
delivered to geotubes
1,538 cy of sand
delivered to geotubes
18 19 20 21 22 23 24
Template
maintenance
Template
maintenance
Template
maintenance
Minor Storm
+/- 400 ft geotubes
exposed
Minor Storm
25 26 27 28 29 30 31
Template maintenance
760 cy of sand
delivered
Geotubes covered
M E M O R A N D U M
Date: 06 February 2019
To: Nantucket Conservation Commission
From: J. Feeley, Cottage+Castle, Inc.
D. Dunk, Epsilon Associates, Inc.
Subject: Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project
(DEP File No. SE 48-3115) – Future Maintenance Procedures
Because of the unforeseen subcontractor scheduling conflicts, and to avoid template maintenance
delays in the future Cottage+Castle, Inc. (“C+C”) retained an independent heavy equipment
operator as a dedicated operator for the Project, and it purchased a Caterpillar D6N XL Dozer, with
a Trimble Computerized Grade Control System in May 2018. The following sections present the
protocol details that Sconset Beach Preservation Fund (“SBPF”) will implement for future maintenance
of the sand template in accordance with the Order of Conditions (“OOC”). This supplements the
Template Sand Protocol previously submitted to the Commission (see Attachment 1 to the response
to comments package dated November 2, 2018).
To ensure sand template maintenance requirements can be accomplished within ten business days
following erosion events and sand deliveries, C+C purchased a Caterpillar D6N XL Dozer, with a
Trimble Computerized Grade Control System, and retained an independent heavy equipment
operator, and backup operators, who are assigned as dedicated operators for the SBPF sand template.
The primary dedicated operator is also an experienced diesel mechanic. Additionally, a maintenance
protocol was developed to ensure sand template maintenance can be accomplished rapidly,
efficiently and without interruption for the existing sand template and future expansion. As stated
above, the following is information regarding the equipment and maintenance protocol, and
supplements the Template Sand Protocol submitted to the Commission on November 2, 2018:
Equipment
In May 2018 after the March 28 to April 11 maintenance work, , C+C purchased a Caterpillar D6N
XL Dozer (“D6N”), with a Trimble Computerized Grade Control System for dedicated use at the
Sconset Bank sand template. The D6N is a large dozer that is highly versatile and well suited to
perform the maintenance and grading work that is required at the existing and expanded template.
The Trimble Dual Mast Grading System uses laser points and a computer system to automatically lift
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and tilt the dozer’s blade to accurately establish a desired grade and pitch to achieve design grades.
These features allow for rapid and precise grading over long distances which will significantly further
assist in the efficiency of covering exposed geotubes following erosion events as soon as practical.
See Photos 1 and 2 below.
Dedicated Personnel
C+C has retained Dan Baird Construction, Inc. (“Baird Construction”) on Nantucket to serve as the
Project’s dedicated heavy equipment operator. This arrangement ensures that storm maintenance
will not be delayed by scheduling conflicts that occurred in March 2018 when non-dedicated heavy
equipment subcontractors were used for site maintenance. In the event Baird Construction cannot
not provide maintenance services, AH Construction also on a Nantucket company, is the dedicated
backup heavy equipment subcontractor.
Alternative Equipment
In the event C+C’s D6N is temporarily out of commission, Baird Construction has its own grader
and excavator that may be used to perform post storm maintenance, both of which have been used
in the past. The equipment can be quickly mobilized if required. Additionally, AH Construction has
loaders and excavators which could be called in to assist in an emergency.
Photo 1. Caterpillar D6N XL Dozer Photo 2. Trimble Dual Mast Laser Grading System
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Post Storm Inspections
Following storm events, or abnormally high tide events that cause run-up to the base of the geotube
structure, inspections are performed within 24 hours of the event to determine what maintenance is
required. Immediately thereafter, Baird Construction who is already on alert, is notified to schedule
and perform the maintenance as soon as practical within the proposed ten business days after the
geotubes are exposed.