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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. Baxter Road and Sconset Bluff 2 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 3 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 4 Alternatives Analysis Epsilon Associates, Inc. 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, Baxter Road and Sconset Bluff 5 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 6 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 7 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 8 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 9 Alternatives Analysis Epsilon Associates, Inc. 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. Baxter Road and Sconset Bluff 10 Alternatives Analysis Epsilon Associates, Inc. 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. Baxter Road and Sconset Bluff 11 Alternatives Analysis Epsilon Associates, Inc.  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 Baxter Road and Sconset Bluff 12 Alternatives Analysis Epsilon Associates, Inc. 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 Epsilon Associates, Inc. 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. Baxter Road and Sconset Bluff 14 Alternatives Analysis Epsilon Associates, Inc. 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. Baxter Road and Sconset Bluff 15 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 16 Alternatives Analysis Epsilon Associates, Inc. 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 Baxter Road and Sconset Bluff 17 Alternatives Analysis Epsilon Associates, Inc. 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.