HomeMy WebLinkAbout20141023-SconsetAltsAnalysisSubmittaltoDEP_201410301429204710ALTERNATIVES ANALYSISALTERNATIVES ANALYSIS
for the
(1)Fourth Tier of the Existing Three Tier (1)Fourth Tier of the Existing Three Tier
Geotextile Tube Structure
and the
Baxter Road and Sconset Bluff Stabilization Project
(2) Return Design
Baxter Road and Sconset Bluff Stabilization Project
Nantucket, MA
Submitted to:Pd bSubmitted to:
Massachusetts Department of Environmental Protection – Southeast Region
20 Riverside Drive
Lakeville, MA 02347
Submitted by
Prepared by:
Epsilon Associates, Inc.
3 Clock Tower Place, Suite 250
Maynard, Massachusetts 01754
In Association with:Submitted by:
Siasconset Beach Preservation Fund
PO Box 2279
Nantucket, MA 02584
In Association with:
Ocean and Coastal Consultants, Inc.
October 23, 2014
i. Baxter Road and Sconset Bluff i Alternatives Analysis
Epsilon Associates, Inc.
TABLE OF CONTENTS
1.0 INTRODUCTION 1
2.0 ALTERNATIVES FOR THE FOURTH TIER OF THE EXISTING THREE TIER
GEOTEXTILE TUBE STRUCTURE 2
2.1 Alternative One: Jute or Coir Tubes 2
2.1.1 Ability to Provide Required Protection 2
2.1.2 Ability to be Constructed and Maintained 3
2.1.3 Potential Benefits and Impacts 6
2.1.4 Conclusions 8
2.2 Alternative Two: Geotextile Tube Combined with Jute or Coir Tubes 8
2.2.1 Ability to Provide Required Protection 9
2.2.2 Ability to be Constructed and Maintained 9
2.2.3 Potential Benefits and Impacts 10
2.2.4 Conclusions 11
2.3 Alternative Three: Geotextile Tube 12
2.3.1 Ability to Provide Required Protection 12
2.3.2 Ability to be Constructed and Maintained 12
2.3.3 Potential Benefits and Impacts 12
2.3.4 Conclusions 12
3.0 ALTERNATIVE DESIGNS FOR THE RETURNS 14
3.1 Original Return Design: Twenty 15-foot Circumference Geotextile Tubes 14
3.2 Alternative Return Concept One: Six 30-Foot Circumference Geotextile Tubes 15
3.3 Alternative Return Concept Two: Returns Constructed of Jute/Coir 15
3.4 Alternative Return Concept Three: Gabions or Rip Rap 16
3.5 Conclusions 17
ATTACHMENT: DRAWINGS SK-01 THROUGH SK-05
Baxter Road and Sconset Bluff 1 Alternatives Analysis
Epsilon Associates, Inc.
1.0 INTRODUCTION
The Baxter Road and Sconset Bluff Stabilization Project (EEA 15240), located at Sconset, MA,
received its Certificate of the Secretary of Energy and Environmental Affairs on the Environmental
Notification Form on October 3, 2014. The Secretary’s Certificate did not require the preparation
of an Environmental Impact report and directed the Massachusetts Department of Environmental
Protection (MassDEP) that, during the permitting process, alternative designs should be considered
for (1) the fourth tier of the existing three-tier geotextile tube structure and (2) the end returns, using
the following language:
The feasibility and potential benefits and impacts associated with design alternatives for the
fourth tier of the installation and the end returns [should be addressed during permitting.]
On behalf of the project proponent, the Siasconset Beach Preservation Fund (SBPF), Epsilon
Associates, Inc. has prepared the following Alternatives Analysis to evaluate such alternatives, for
MassDEP to use during its review.
In accordance with the Secretary’s Certificate, the Alternatives Analysis presented herein is limited
to a review of design alternatives for the fourth tier and returns for the already-installed three tiers of
geotextile tubes. This document is not intended to provide a comprehensive review of alternatives
for long-term bluff protection. It is SBPF’s current view that the most reliable, fully tested, and low
maintenance alternative for conditions in Sconset along the bluff would likely be a rock revetment.
That alternative was not proposed or evaluated here because, as noted, this alternatives analysis is
limited to a review of design alternatives for the fourth tier and returns for the existing geotextile
tube project. The existing geotextile tube project was designed to address exigent circumstances
which became emergency circumstances through a design that, in view of the demonstrated
difficulty of obtaining approval for a revetment prior to the onset of the winter 2013-2014 storm
season, could be permitted more expeditiously, and constructed quickly consistent with the
emergency timeframe under which the project was operating.
The description of the project is presented in detail in the Request for a Superseding Order of
Conditions (SOC) and is not repeated here.
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2.0 ALTERNATIVES FOR THE FOURTH TIER OF THE EXISTING THREE
TIER GEOTEXTILE TUBE STRUCTURE
In accordance with the Secretary’s Certificate, the following alternatives were considered for the 4th
and/or 5th tiers of the coastal protection structure. Each alternative includes the same base of the
existing three tiers of 45-foot circumference geotextile tubes:
(1) Alternative One: Jute/Coir Tube(s)
(2) Alternative Two: Geotextile Tube Combined with Jute/Coir Tube(s)
(3) Alternative Three: Geotextile Tube
Each alternative is evaluated below according to its ability to provide the required protection,
ability to be constructed and maintained, potential benefits, and potential impacts. Alternatives are
also evaluated according to the Wetlands Protection Act Regulations as listed in Section 310 CMR
10.30(3)(a): “a coastal engineering structure or a modification thereto shall be designed and
constructed so as to minimize, using best available measures, adverse effects on adjacent or nearby
coastal beaches due to changes in wave action….” “Best Available Measures” are defined in 310
CMR 10.04 as: “the most up-to-date technology or the best designs, measures or engineering
practices that have been developed and that are commercially available.” These standards make it
clear that coastal engineering structures should minimize adverse effects on adjacent beaches by
using the best engineering practices and designs.
The design elevation for the geotextile tube installation needed to provide protection during a 100-
year storm is +24.1 feet on the Mean Low Water (MLW) datum, as presented in the memo
provided by Ocean and Coastal Consultants included as an attachment to Exhibit Z in the Request
for an SOC. The existing three-tier geotextile tube structure ranges in elevation from +18.9 MLW
to +21.3 MLW (with an average elevation of +20.4 ft MLW), so up to an additional 5 feet of
vertical protection are required for the upper tier(s) of the geotextile tube installation. The need for
up to an additional 5 feet of vertical protection guided the selection of the size of the tubes utilized
in the three alternative design concepts.
2.1 Alternative One: Jute or Coir Tubes
In this alternative, two jute/coir tubes would be stacked on top of the existing three-tier
geotextile tube structure. The use of two stacked tubes is required to reach the design
elevation. Each jute/coir tube would be 30-foot circumference, with an approximate height
of 3.5 feet. The tubes would be stacked so as to provide some overlap (see attached
drawing SK-01).
2.1.1 Ability to Provide Required Protection
Jute and coir fiber mats, bags, or envelopes have been used in select locations along
Sconset Bluff and Beach in the past and are referred to as “terraces” due to their stepped
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design. The long history with the use of coir/jute terraces at Sconset has demonstrated that
these biodegradable materials are not well suited to withstanding wave impacts associated
with the Atlantic Ocean. The terraces are designed to release their sand and fail during
storm events, leaving the coastal bank vulnerable during major, successive, or multi-day
storm events. The terraces simply cannot withstand the storm conditions experienced
regularly at the Project site. The failure of the jute terraces during storms means that all or
part of the terraces must be replaced one to three times per year.
The demonstrated failure of the jute/coir terraces and their subsequent need for frequent
replacement also means that the coastal bank is left vulnerable during major, multi-day or
successive storms. This was readily apparent at 79 Baxter Road, where the jute terraces had
been regularly maintained for over 5 years. The jute envelopes had been replaced
completely one to three times each year since they were installed. This frequent
maintenance had led to reduced erosion of the coastal bank in this location until the jute
terraces failed and could not be reconstructed during successive storms in the winter of
2012-2013, resulting in the loss of approximately 30 feet of the bank at the north end of the
property. This experience demonstrates that jute is not an adequate material to withstand
the extreme forces and successive storms that occur at the Project site.
Additional engineering analysis of the use of coir/jute for the upper tiers demonstrated that
these materials would be expected to fail during storm conditions (presented in the memo
from Ocean and Coastal Consultants [OCC] dated March 13, 2014 included as an
attachment to Exhibit U in the Request for an SOC), consistent with what has already been
demonstrated through the historical use of coir/jute terraces at the Project site:
[I]t is expected that the jute bags will be torn open under wave overtopping
pressures of the 100-year storm event….[I]t is not advisable that jute/coir be used as
substitution for proper geotextile tubes (geotubes).
The project area does not have the capacity to absorb an additional 30 feet of bank loss, so
the use of jute/coir is not advisable. In particular, pre-1978 homes are as close as four feet
from the top of the bank (as shown on Exhibit Y in the Request for an SOC) and Baxter Road
is as close as 29 feet from the top of the bank (as presented in Exhibits L and Y in the
Request for an SOC). It has been suggested that the placement of additional nourishment
material would allow jute/coir tubes to “stand up to more severe storms.” During severe
storms, unconsolidated sand would be eroded and the jute/coir tubes would become
exposed and deflated, allowing erosion and collapse of the bank. In conclusion, Alternative
One does not provide the required storm protection.
2.1.2 Ability to be Constructed and Maintained
Construction and maintenance of jute or coir materials poses significant concerns related to
the ability to drive heavy equipment on the materials, the length of time required to replace
the jute/coir, and the required removal or inability to replace the sand template for a period
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of six-eight weeks any time the jute/coir are required to be replaced. These concerns are
detailed below:
Jute/Coir Subject to Degradation. The jute/coir can be expected degrade relatively
quickly. The type of jute typically used at the site will degrade after 1-2 seasons.
The use of multiple (three) layers of coir, or the use of multiple (three-four) layers of
jute with a tighter weave but still porous nature, and/or the use of chemical
coatings, may be able to extend the lifespan of the materials to approximately 3-4
years. Additionally, the jute/coir can be expected to require replacement after
major storms. It is therefore anticipated that the jute/coir tiers would need to be
completely replaced on a regular basis due to degradation or storms, likely every
one-three years.
Jute/Coir May Not Support Required Equipment Loading; Special Equipment
Required. Heavy equipment is needed to drive on top of the sand template to
spread the substantial volume of the mitigation sand. This equipment includes an
excavator (324EL excavator – 26 tons) and/or a bulldozer (D6 dozer – 18 tons).
Engineering analyses (as presented in the memo from OCC dated March 13, 2014
included as an attachment to Exhibit U in the Request for an SOC) suggest that the
porous coir or jute bags could not support the required equipment loading and
shear stresses. This issue is especially critical after major storms when it can be
expected that part or all of the 4th and 5th tiers may be exposed. Therefore, in order
to rebuild the jute/coir tubes and spread the sand template, special equipment (in
the form of a long reach excavator) would be required to rebuild and recover the
jute/coir tubes from beach level. No long reach excavators are available on the
island of Nantucket, so the excavator would need to be rented and mobilized to the
site by ferry any time replacement of the jute/coir tubes is required. Without the use
of a long reach excavator, only a smaller piece of equipment such as a skid steer
could be utilized; this equipment would be prohibitively slow given the length of
the installation and the substantial volume of the sand template.
Six-Eight Weeks Required to Replace Jute/Coir. The jute/coir are sewn together by
hand at the site, which results in a lengthy construction process. Additionally, as
described in the preceding bullet, the use of a long reach excavator would be
required to allow reconstruction of the jute/coir from the beach level; such an
excavator would need to be mobilized to the island since none are available on
Nantucket. Rebuilding just under 900 feet of jute/coir would likely require
approximately five weeks; this estimate includes the time required to mobilize the
long reach excavator to the site. This timeframe would start once the beach had
recovered enough from the storm activity that equipment could access the site. In
the past, equipment access can usually occur within three days of the end of a
storm; however, it has been as long as eleven days. Additionally, any storm activity
during the reconstruction of the jute/coir tubes would delay reconstruction efforts,
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especially since such reconstruction would be occurring from the beach level.
Therefore, a period of six-eight weeks or longer is considered realistic to allow the
reconstruction of the jute/coir tubes (including the overlying sand template) and
account for any storm delays.
Lack of Protection During Replacement of Jute/Coir. The bank would be
completely unprotected from major storms when the 4th and 5th tiers need replacing.
The effort to rebuild the jute/coir would take approximately six-eight weeks or
longer; during this timeframe, the bank would be left partially or completely
vulnerable to major storms that would result in wave run-up above the existing
three tiers of geotextile tubes.
Six to Eight Week Delay in Replenishing Sand Template/ Removal of the Top of
Sand Template. Any time the jute/coir need to be replaced due to degradation or
depletion/failure during major storms, any remaining sand template on the top of
the third tier of geotextile tubes would need to be removed. If needed, this would
be accomplished using a bulldozer (for the upper portion) while a high-pressure
slurry pump would be needed to spray all remaining sand off of the third tier of
geotextile tubes in order to not risk tearing the top of the third tier geotube
accidentally (since the top elevation of the geotextile tubes varies by about two and
one half vertical feet along the length of the installation). Crucially, the top of the
sand template could not be replaced until the 4th and 5th tiers of jute/coir tubes are
reconstructed. If this effort is occurring post-storm, the existing sand template
would likely have been quite depleted, with only minimal sand remaining in the
template. Post-storm replacement of jute/coir in the winter would likely require six-
eight weeks or longer, during which time all or part of the sand template would not
be available. (During replacement efforts, one segment of the jute/coir tubes would
be repaired and then covered with sand; then the next section would be replaced
and covered with sand; this process would repeat until the entire length of the
installation was completed.) In contrast, with the geotextile tube as described in
Alternative Three, SBPF intends to inspect and re-grade the sand template by
pushing sand from the top of the template onto the seaward face of the template
(and more sand can also be added from the top of the bank via conveyor belt if
needed); this effort is relatively straightforward and can be accomplished within just
a few days.
Typical Construction Methods Aren’t Feasible. The methods used to fill the existing
jute envelopes would not work for a longer (approximately 900-foot) project that
involves a 22 cy/lf/yr sand cover. The existing jute envelopes are substantially
shorter than the approximately 900-foot long installation and only involve minimal
sand cover, so they are filled using a small piece of equipment known as a skid
steer. For the current project, the proposed fourth and fifth jute/coir would need to
be filled from below with a long-armed excavator which would be extremely
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arduous and subject to interruption given anticipated wave runup during winter
conditions. A bulldozer or skid steer would work in tandem with the long reach
excavator to rebuild the jute/coir tubes and replace the sand template.
2.1.3 Potential Benefits and Impacts
The purported benefits of jute/coir are stated to be related to the slower release of sand
during storm events and the associated reduction in wave energy and storm damage. A
review of these potential benefits suggests that they are both negligible and not exclusively
dependent upon the use of jute/coir material.
Quantity of Material Provided. A single 30’ circumference jute/coir tube can hold
1-1.5 cy of sand per linear foot; two 30’ circumference tubes would hold no more
than 2.5-3 cy/lf. This material would only be available during a large storm when
wave runup heights reached the elevation of the fourth or fifth tiers (this would
require a storm greater than a 20-year storm). This small volume available to the
littoral system from the two jute/coir tubes included in Alternative One - which
would only be available during major storms - needs to be considered when
evaluating the purported benefits of the use of jute/coir.
Purported Benefit of Reducing Storm Energy. It has been asserted that the use of
jute/coir would provide sand continuously during a storm, and that the presence of
such sand would reduce storm wave energy. During storm activity, sand can be
provided, and indeed is provided, to the littoral system from offshore sand sources
such as shoals, from longshore sand transport - from adjacent beaches or other
eroding landforms along the eastern shore of Nantucket, and from the mitigation
template (which would take a major storm to be completely exhausted). The
volume of sand that would be in the water column during major storms due to
inputs from these various sources (offshore shoals, longshore sand transport, and
from the mitigation template itself) is at least an order of magnitude greater than the
volume provided by the use of two jute/coir tubes. When considered in this
context, the up to 2.5-3 cy/lf provided by two jute/coir tubes does not play a
meaningful role in realizing, and is not necessary to realize, the purported benefit of
reducing storm wave energy by supplying sand to the water column during storms.
Purported Benefit of Reducing Damage to Downdrift Areas. SBPF has provided
information in Exhibit Z (see Section III.b) of the Request for an SOC that indicates
that the mitigation sand is even more available to the littoral system than the
existing coastal bank. During the March 27 storm, more sand (1.5-2.5 cy/lf) eroded
from the sand template than from the jute terraces (0.25 cy/lf/yr) or unprotected
bank (no or minimal erosion). This “over contribution” during small or moderate
storm events can be expected to reduce damage to downdrift areas during major
storms. Likewise, this regular “over contribution” can be expected to offset any
limited times when the sand template has been completely exhausted; this
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condition is only expected be reached at the end of a major storm that occurs only
every few years. (As previously stated, SBPF intends to promptly re-grade and, if
necessary, replenish the sand template after any significant storm activity.) Finally,
the volume of sand potentially to be contributed during a major storm that would
reach the 4th and 5th tiers is a maximum of 2.5-3 cy/lf; this volume is minimal when
compared to the volume of sand that can be expected to be in the littoral system
during a major storm, when sand would be supplied by the up to 22 cy/lf mitigation
template, longshore sand transport from adjacent areas, and offshore shoals.
Purported Benefit of Mimicking Natural Release of Sand. It has been asserted that
the natural bank contributes sand continuously during a storm and that the
mitigation sand needs to mimic this pattern in order to be effective. During storm
activity, wave action erodes the toe of the coastal bank until such point as the upper
bluff is destabilized and collapses. This pattern of gradual toe erosion followed by
collapse of the upper bank may repeat itself one or more times during a given
storm. Under natural conditions, then, the contribution of sand from the bank is not
a continuous release of a constant volume of sand, but involves a sudden release of
a large quantity of sand. This sudden release of a large quantity of sand is more
closely mimicked by the sand template, which can collapse down as wave runup
reaches higher elevations, than it is by the gradual release of a small volume (2.5-3
cy/lf) of sand from two jute/coir tubes.
Purported Benefit of Immediacy. It has been asserted that the use of jute/coir would
allow release of sand during storms for immediate transport to and protection of
downdrift areas. During major storm activity, the 2.5-3 cy/lf of sand within both
jute/coir tubes would be released to the coastal environment. Some fraction of this
would immediately move into the longshore transport system and some fraction
would potentially move offshore, so it is likely that something less than 2.5-3 cy/lf
would actually be immediately available to the littoral system.
Purported Benefit of Additional Sand Transport to the North During Storms. It has
also been asserted that the use of jute/coir may specifically benefit the Quidnet
Squam area, which is located to the north. Previous sediment transport studies have
concluded that sand eroded from the project area tends to move both north and
south depending on particular tidal and wind conditions.
The potential impact of using all jute/coir for the fourth and fifth tiers of protection is that
these tubes would only become exposed to wave energy during a major storm (greater than
a 20-year storm). Such a major storm is likely to completely deplete the jute/coir bags,
causing the bank behind the bags to fail and resulting in additional coastal bank loss. Such
a storm is also likely to significantly deplete the sand template. A delay of six-eight weeks
or longer would be required to completely replenish the sand template, which is a serious
disadvantage since the project site is known to experience successive storms. Given this
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circumstance, any potential minor benefit provided by the small volume of sand contained
in the jute/coir tubes is offset by the delay in re-establishing the sand template.
In contrast, the sand template volume is a substantial 22 cy/lf/yr, which has been shown to
be 1.5 times the average annual bank contribution rate (see Attachment A to Exhibit F of the
Request for an SOC and “Responses to Comments Received on ENF for EEA 15240,” dated
September 29, 2014, Response to Nantucket Land Council Comment #4). This substantial
mitigation volume compensates for any minimal benefit provided by jute/coir. This finding
is supported by MassDEP’s previous conclusion, in its cover letter dated December 10,
2013 that approved the Emergency Certification Request for the four tier geotextile tube
system, that the increased volume of mitigation sand will mitigate any potential downdrift
impacts and will address the differences between geotextile and coir/jute tubes:
The implementation of the nourishment plan will mitigate any potential difference
in down drift impacts between the four Geotube design and the hybrid design
approved in the Town's Certification.
2.1.4 Conclusions
In summary, Alternative One would result in depletion or failure of the jute/coir tubes
during major storms, followed by additional failure and collapse of the upper bluff.
Alternative One does not provide the required level of protection, which renders this
alternative infeasible because the project area does not have the capacity to absorb
additional bank loss. Additionally, construction and maintenance of Alternative One would
be problematic, as it is likely that the jute/coir materials cannot support the required
equipment loading and shear stresses necessary to spread the substantial volume of the
sand template. Subsequent reconstruction of jute/coir would require removal of the sand
template and would result in delays of six-eight weeks or longer in re-establishment of the
sand template while the jute/coir tubes were rebuilt. Further, the purported benefits of the
use of jute/coir tubes are minimal and are compensated for by the substantial mitigation
volume of 22 cy/lf/yr. Any small benefit offered by this volume is strongly offset by the
certain failure of the bank during major storm events. For all these reasons, the use of
coir/jute for the fourth and fifth tiers of protection continues to be considered infeasible.
2.2 Alternative Two: Geotextile Tube Combined with Jute or Coir Tubes
In this alternative, the upper tiers would include a geotextile tube to provide the required
protection, combined with jute/coir tubes. Various configurations of a geotextile and
jute/coir combination were evaluated. It was determined to reduce the size of the
geotextile tube from 45-foot circumference to 30-foot circumference to allow the use of a
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larger jute/coir tube.1 Alternative Two contemplates having the fourth tier consist of a 30-
foot circumference geotextile tube fronted by a 15-foot circumference jute/coir tube, and
topped by a fifth tier consisting of a 30-foot circumference jute/coir tube (see attached
drawing SK-02). The fifth tier is needed to reach the design height along the entire
installation, in addition to allowing for additional use of jute/coir. Each 30-foot
circumference tube (whether geotextile, jute, or coir) would have an approximate height of
3.5 feet.
2.2.1 Ability to Provide Required Protection
By including a geotextile tube at its landward side, this alternative would generally provide
a greater level of protection than Alternative One, although, as noted above, the geotextile
tube alone would not reach the design elevation along the entire installation. Given its 30-
foot circumference, it would also provide substantially less horizontal depth than the 45-
foot geotextile tube in Alternative Three. The jute/coir tubes on the 4th and 5th tiers would
be expected to fail during major storms and the geotextile tube would not fully provide
protection to the 100-year storm elevation. Alternative Two therefore would fall short of
providing the full required protection.
2.2.2 Ability to be Constructed and Maintained
As detailed above in Section 2.1.2, construction and maintenance of jute or coir materials
poses significant concerns related to the ability to drive heavy equipment on the materials,
the length of time required to replace the jute/coir, and the required removal or inability to
replace the sand template for a period of six-eight weeks any time the jute/coir are required
to be replaced. These concerns are detailed below:
Jute/Coir Subject to Degradation. See discussion in Section 2.1.2 above.
Jute/Coir May Not Support Required Equipment Loading; Special Equipment
Required. See discussion in Section 2.1.2 above.
Six-Eight Weeks Required to Replace Jute/Coir. See discussion in Section 2.1.2
above.
Six to Eight Week Delay in Replenishing Sand Template/ Removal of the Top of
Sand Template. Any time the jute/coir tubes need to be replaced due to
degradation or depletion/failure during major storms, any remaining sand template
1 If a 45-foot circumference geotextile tube were used, there would only be enough room to fit in a 15-foot
circumference jute/coir tube in front of it. (The 45-foot geotextile tube could not be moved farther landward
without cutting into the bank.) While viable from a design perspective, this concept was considered to only
provide minimal incorporation of jute/coir and so was not considered further for this alternative.
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on the fourth tier of geotextile tube and on the seaward side of the third tier of
geotextile tubes would need to be removed. If needed, this would be accomplished
using a bulldozer (for the upper portion of the sand template) and a high-pressure
slurry pump, which will be needed to spray all remaining sand off of the third and
fourth tiers of geotextile tubes. Crucially, the top of the sand template could not be
replaced until the 4th and 5th tiers of jute/coir tubes are reconstructed. If this effort is
occurring post-storm, the existing sand template would likely have been quite
depleted, with only minimal sand remaining in the template. Post-storm
replacement of jute/coir in the winter would likely require six-eight weeks or longer,
during which time all or part of the sand template would not be available. (During
replacement efforts, one segment of the jute/coir tubes would be repaired and then
covered with sand; then the next section would be replaced and covered with sand;
this process would repeat until the entire length of the installation was completed.)
In contrast, with the geotextile tube as described in Alternative Three, SBPF intends
to inspect and re-grade the sand template by pushing sand from the top of the
template onto the seaward face of the template (and more sand can also be added
from the top of the bank via conveyor belt if needed); this effort is relatively
straightforward and can be accomplished within just a few days. Additionally, the
sand template which sits on top of the geotubes is never completely depleted except
in the most extreme storms, allowing it to serve almost perpetually as a work
platform for heavy equipment as needed.
Typical Construction Methods Aren’t Feasible. See discussion in Section 2.1.2
above.
2.2.3 Potential Benefits and Impacts
The proposed use of the geotextile tube landward of the jute/coir tubes should provide
adequate protection during most conditions, but not during a 100 year storm when the
existing natural bank would suffer losses. Moreover, the purported benefits of jute/coir
have been reviewed in detail in Section 2.1.3 above and were determined to be negligible
and not exclusively dependent upon the use of jute/coir material.
Quantity of Material Provided. The 15’ circumference jute/coir tube can hold less
than 0.6 cy/lf of sand; the 30’ circumference jute/coir tube can hold a 1 -1.5 cy of
sand per linear foot. In total, Alternative Two would provide approximately 2 cy/lf
of sand inside jute/coir tubes. This material would only be available during a large
storm when wave runup heights reached the elevation of the fourth or fifth tiers (this
would require a storm greater than a 20-year storm).
Purported Benefit of Reducing Storm Energy. See above discussion in Section 2.1.3.
Purported Benefit of Reducing Damage to Downdrift Areas. See above discussion
in Section 2.1.3.
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Purported Benefit of Mimicking Natural Release of Sand. See above discussion in
Section 2.1.3.
Purported Benefit of Immediacy. See above discussion in Section 2.1.3.
Purported Benefit of Additional Sand Transport to the North During Storms. See
above discussion in Section 2.1.3.
The potential impact of using jute/coir for part of the fourth tier and the fifth tier of
protection is that these tubes would only become exposed to wave energy during a major
storm (greater than a 20-year storm). Such a major storm is likely to completely deplete the
jute/coir bags, and to significantly deplete the sand template. A delay of six-eight weeks or
longer would be required to completely replenish the sand template, which is a serious
disadvantage since the project site is known to experience successive storms. Given this
circumstance, any potential minor benefit provided by the small volume of sand (~2
cy/lf/yr) contained in the jute/coir tubes is offset by the delay in re-establishing the sand
template.
In contrast, the sand template volume is a substantial 22 cy/lf/yr, which has been shown to
be 1.5 times the average annual bank contribution rate (see Attachment A to Exhibit F of the
Request for an SOC and “Responses to Comments Received on ENF for EEA 15240,” dated
September 29, 2014, Response to Nantucket Land Council Comment #4). This substantial
mitigation volume compensates for any minimal benefit provided by jute/coir. This finding
is supported by MassDEP’s previous conclusion, in its cover letter dated December 10,
2013 that approved the Emergency Certification Request for the four tier geotextile tube
system, that the increased volume of mitigation sand will mitigate any potential downdrift
impacts and will address the differences between geotextile and coir/jute tubes:
The implementation of the nourishment plan will mitigate any potential difference
in down drift impacts between the four Geotube design and the hybrid design
approved in the Town's Certification.
2.2.4 Conclusions
In summary, Alternative Two is preferable to Alternative One, but does not fully provide the
required level of protection. Construction and maintenance of Alternative Two would be
both problematic and burdensome, as regular replacement of the jute/coir would be
required. Such replacement would require six-eight weeks or more and the use of special
(off-island) equipment, since the jute/coir materials cannot support the required equipment
loading and shear stresses necessary to spread the substantial volume of the sand template.
During this six-eight week timeframe, all or part of the sand template would not be
available to the littoral system. The delay in replenishing the sand template is a substantial
disadvantage, especially compared to the small volume (2 cy/lf) provided by the use of
jute/coir. Further, the purported benefits of the use of jute/coir have been shown to be both
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minimal and compensated for by the substantial mitigation volume. For all the preceding
reasons, Alternative Two is not recommended.
2.3 Alternative Three: Geotextile Tube
In this alternative, the fourth tier would consist of a single 45-foot circumference geotextile
tube. The design elevation would be reached along the length of the entire installation (see
attached drawing SK-03).
2.3.1 Ability to Provide Required Protection
This alternative would provide the required protection, along the entire length of the
installation, from a 100-year storm.
2.3.2 Ability to be Constructed and Maintained
One-time, temporary removal of the top of the sand template would be required; however,
this work could probably be scheduled in the summer when it is unlikely that the sand
template would be contributed to the littoral system. The geotextile tubes can support the
required equipment loading and shear stresses associated with the heavy equipment needed
to spread the sand template. Unlike the jute/coir material, no regular repair, replacement,
or maintenance of the geotextile tube is anticipated. Occasional post-storm repairs may be
required in discreet locations.
2.3.3 Potential Benefits and Impacts
The fourth-tier geotextile tube, with its substantial sand cover of 22 cy/lf, is expected to
avoid adverse impacts to downdrift areas. (Sections 2.1.3 and 2.2.3 above describe how
the purported benefits associated jute/coir are negligible.) This amount is equivalent to 1.5
times the average bank contribution amount (see Attachment A to Exhibit F of the Request
for an SOC and “Responses to Comments Received on ENF for EEA 15240,” dated
September 29, 2014, Response to Nantucket Land Council Comment #4). No regular or
lengthy removal of the sand template would be required for ongoing maintenance of the
geotextile tubes. Likewise, no regular use of heavy equipment on the beach for
maintenance is anticipated. Finally, there would be no increase in wave reflection
compared to the use of jute/coir. This is because, at the higher elevations of the fourth
and/or fifth tiers, the impacts are from wave runup and not from direct wave attack. As
such, wave reflection would be expected to be negligible for either material (geotextile or
jute/coir).
2.3.4 Conclusions
In summary, Alternative Three is preferred because it would provide the required protection
for the project area while also maintaining the sand supply to the littoral system. The
geotextile tubes can support the required equipment loading and shear stresses associated
Baxter Road and Sconset Bluff 13 Alternatives Analysis
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with the heavy equipment needed to spread the sand template. Alternative Three does not
require regular intensive maintenance, and does not involve delays of six-eight weeks or
longer to rebuild any jute/coir tubes and replace the entire sand template. The benefits of
jute/coir have been shown to be minimal and are mitigated by the substantial sand template
volume of 22 cy/lf. Therefore, Alternative Three is preferred.
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3.0 ALTERNATIVE DESIGNS FOR THE RETURNS
The Secretary directed that the project look at design alternatives for the returns, in accordance with
the comment letter received from the Massachusetts Office of Coastal Zone Management (CZM).
This comment reads:
[CZM] recommends that the proponent use alternative methods for reducing end scour that
would reflect less wave energy than the proposed 15’ circumference geotextile tubes. In
order to mitigate the end scour and avoid extending it onto adjacent areas, CZM typically
recommends that softer options that reflect less wave energy be considered (e.g. sand-filled
coir bags and nourishment). The end scour protection should also taper in elevation and
slope to minimize the amount of reflected wave energy and the amount of associated
erosion.
In accordance with this comment and the direction in the Secretary’s Certificate, SBPF and its
engineering consultant, OCC, developed three proposed alternative return design concepts that
would reduce the amount of reflected wave energy. Each alternative design concept is discussed
below.
As in Section 2.0 above, alternatives are evaluated according to the Wetlands Protection Act
Regulations as listed in Section 310 CMR 10.30(3)(a): “a coastal engineering structure or a
modification thereto shall be designed and constructed so as to minimize, using best available
measures, adverse effects on adjacent or nearby coastal beaches due to changes in wave action….”
“Best Available Measures” are defined in 310 CMR 10.04 as: “the most up-to-date technology or
the best designs, measures or engineering practices that have been developed and that are
commercially available.” These standards make it clear that coastal engineering structures
(including returns) should minimize adverse effects on adjacent beaches by using the best
engineering practices and designs.
3.1 Original Return Design: Twenty 15-foot Circumference Geotextile Tubes
The original return design (see attached drawing SK-04) included twenty geotextile tubes,
each 15-foot in circumference. The geotextile tubes were to be placed in a stacked design,
extending from elevation +3 or +4 MLW up to the level of the top of the fourth geotextile
tube at +26 to +28 MLW.
The original design incorporated several features to reduce wave reflectivity and end scour.
Specifically, the proposed design utilized smaller geotextile tubes where the lower tiers
were in a somewhat stepped configuration, which would result in decreased wave
reflectivity as compared to the use of larger (45-foot circumference) geotextile tubes in a
more vertical configuration. It was not anticipated that this design would result in end
scour; however, three alternative return concepts have been developed in order to be
responsive to the comments received from CZM.
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3.2 Alternative Return Concept One: Six 30-Foot Circumference Geotextile Tubes
Alternative Return Concept One includes six 30-foot circumference geotextile tubes (see
attached drawing SK-05). Each geotextile tube is approximately 4 to 5 feet in height and 13
feet in width. The geotextile tubes are placed in a stepped configuration from elevation +3
or +4 MLW to elevation ~+24 to +26 MLW. The slope is approximately half as steep as
in the Original Return Design. The 30-foot circumference size was selected after discussion
with the manufacturer, where it was determined that the use of 15-foot circumference tubes
was not advisable because it would result in less stable fill proportions. Per the
manufacturer’s recommendations, the geotextile tubes for the returns are placed in
alignment with the main geotextile tube structure: the top of the lowest tier of the return
aligns with the top of the lowest tier of the main installation; the top of the third tier of the
return aligns with the top of the second tier of the main installation; and the top of the fifth
tier of the return aligns with the top of the third tier of the main installation. Following this
approach, the sixth (and top) tier of the returns will be somewhat lower in elevation than
the main installation. Accordingly, Alternative Return Concept One achieves both a
shallower slope and a reduced elevation compared to the Original Return Design, both of
which will decrease wave reflectivity.
The option of further tapering the height of the return was investigated; however, this option
would require using shorter geotextile tubes for the top tiers of the returns. Given their
smaller (30-foot circumference) size, these shorter geotextile tubes would not be stable in a
high energy environment like Sconset and could potentially be mobilized during storm
events, becoming a source of marine debris. Therefore, this variation that involved using
shorter tubes was not evaluated further.
In summary, Alternative Return Concept One utilizes 30-foot circumference geotextile
tubes and would provide the required protection during storm activity. This alternative
reduces the number of geotextile tubes required for the returns, achieves a shallower slope,
and results in a reduced elevation. All these factors will result in reduced wave reflectivity.
3.3 Alternative Return Concept Two: Returns Constructed of Jute/Coir
In this alternative, the same size and configuration of tubes as Alternative Return Concept
One (six tiers of 30-foot circumference geotextile tubes) would be selected given its noted
benefits in reducing wave reflectivity compared to the Original Design. However, in
Alternative Return Concept Two, all or part (the top three tiers) of the six tiers of 30-foot
circumference tubes would be constructed out of jute/coir. Either approach (use of jute/coir
for all of the returns, or for just the top half of the returns) would have similar
considerations, and so both options are evaluated together below.
Alternative Return Concept Two would not be expected to provide the required level of
protection during major storms. As noted above in Section 2.1.1, jute or coir cannot
withstand the storm forces regularly experienced at the site. Jute or coir would be expected
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to fail during major, multi-day, or successive storms, leaving the main installation subject to
flanking and end scour. As noted above in Section 2.1.2, the use of jute or coir for all or
part of the returns would be problematic and disadvantageous. The jute or coir would be
anticipated to require regular replacement, probably every one-three years. Rebuilding the
returns would require two-three weeks; during this timeframe; the ends of the installation
would be completely or partially unprotected. This is considered a serious disadvantage, as
successive storms are known to occur at the project site. The jute/coir could not support
the required equipment loading and shear stresses, so special equipment would need to be
brought on-island or only smaller equipment (such as a skid steer) could be used to rebuild
the returns. This option would also cause serious obstruction to the spreading of the sand
template. Given its substantial volume, the sand template is typically spread by heavy
equipment (excavator and/or bulldozer) that access the main installation by sand ramps
placed over the returns. The use of jute/coir for all or part of the returns would likely
remove this source of access for the heavy equipment, requiring the use of special or
smaller equipment and resulting in long delays in replacing the sand template after storms.
Further, the use of jute/coir in Alternative Return Concept Two may not have a
meaningfully different wave reflectivity compared to Alternative Return Concept One, since
both involve the same slope and geometry.
In summary, Alternative Return Concept Two would not provide the required level of
protection. Alternative Return Concept Two would also require regular and time-
consuming replacement of the returns, which would leave the ends of the installation
vulnerable to flanking and end scour during successive, multi-day, or major storms.
Further, Alternative Return Concept Two would obstruct and delay the replacement of the
sand template. Finally, Alternative Return Concept Two may not have a meaningful
difference in wave reflectivity compared to Alternative Return Concept One. For these
reasons, Alternative Return Concept Two is rejected.
3.4 Alternative Return Concept Three: Gabions or Rip Rap
Alternative Return Concept Three is a preliminary concept that considers the use of gabions
(HDPE baskets that are approximately four feet high x five feet wide x ten feet long and are
filled with 4- to 6 –inch stone) or rip rap for the returns. Given the porous nature of gabions
or rip rap, either of these materials would be expected to have the lowest wave reflectivity
compared to the Original Design, Alternative Return Concept One, and Alternative Return
Concept Two. Additionally, given their modular nature, the gabions or the rip rap likely
could taper in height from where they abut the main geotextile tube structure down to their
ends. The gabion or rip rap returns would likely provide the required level of protection;
however, design considerations would include (1) ensuring that suitable backfill could be
placed and compacted to support the gabions or rip rap, and (2) ensuring that shifts in the
rip rap or gabion returns during storms would not damage or puncture the adjacent
geotextile tubes, likely through “shrouding” the geotextile tubes by placing an additional
layer of geotextile material over the existing geotextile tubes to provide additional tear
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protection. Because SBPF believes opponents of the project would use any proposal for
rock-filled gabions or rip rap as an occasion for unwarranted controversy and extended
objection, and due to the previously mentioned design considerations, Alternative Return
Concept Three is not considered further at this point. If MassDEP prefers this alternative
SBPF is prepared to provide further design details and parameters.
3.5 Conclusions
Alternative Return Concept One is preferred because it would provide the required
protection during storm activity and also reduces the number of geotextile tubes required
for the returns, achieves a shallower slope( the slope is reduced to approximately half of the
slope in the Original Design), and results in a reduced elevation. Alternative Return
Concept One is also responsive to the comments received from CZM because the shallower
slope and reduced height would reduce wave reflection and associated erosion. Finally,
Alternative Return Concept One would not require frequent replacement and can support
the anticipated equipment loads required to spread the sand template in a timely fashion.
The Original Design is no longer recommended because it involves a steeper slope than
Alternative Return Concept One, is less stable than Alternative Return Concept One, and
involves the use of more geotextile tubes (twenty for the Original Design vs. six for
Alternative Return Concept One).
Alternative Return Concept Two is rejected because it would not provide the required level
of protection and cannot support the required equipment loads, as heavy equipment needs
to traverse sand ramps constructed on top of the returns in order to spread the substantial
volume of the sand template along the main installation. Alternative Return Concept Two
also results in increased vulnerability of the ends to flanking and scour during major,
successive, or multi-day storms that would be expected to deplete any jute/coir bags and
would subsequently require two-three weeks for reconstruction.
Alternative Return Concept Three was reviewed to show that the use of rip rap or rock-filled
gabions may result in the greatest reduction in wave reflectivity. Because SBPF believes
opponents of the project would use any proposal for rock-filled gabions or riprap as an
occasion for unwarranted controversy and extended objection, and due to design
considerations that highlight the complexity of using different materials for the returns than
for the main installation, Alternative Return Concept Three is not considered further at this
point.