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HomeMy WebLinkAboutMarchant Sullivan Eng to NHESP Letter 07-13-2023July 13, 2023 Amy Hoenig Division of Fisheries & Wildlife 1 Rabbit Hill Road Westborough, MA 01581 RE: SE3-6097 / NHESP 23-2190 Babcock Holding LLC 11 & 27 Marchant Avenue, Hyannisport MA Dear Ms. Hoenig, Per your request, we offer the following additional more detailed information: Existing Conditions: The project site is located at 11 & 27 Marchant Avenue in Hyannisport abutting Nantucket Sound. The sites are near the eastern end of a nearly 1 mile stretch of shoreline westerly from the tip of Squaw Island easterly to the Hyannis Harbor Breakwater. Page 2 of 7 This overall shoreline is along the open ocean with a narrow beach that allows wave energy to travel to the bank face. The net sediment transport along this shoreline is from west to east in this area. Approximately 13 stone groins armor this stretch of shoreline, with some dating back to at least the early 1900’s. Most of the homes along this stretch of coastline have additional coastal engineering structures protecting the coastal bank and homes. There is little to no new sediment feeding the narrow beach, and the existing beach sediment is slipping through the Hyannis Breakwater into the Harbor at a rate of approximately 5,000 CY per year. We do not believe the shoreline is ideal habitat for nesting shorebirds given the lack of an upper beach, limited elevation above high tide, and susceptibility to wave action At the project site there remains a stretch approximately 60 linear feet of unprotected coastal bank between the existing stone revetments on the western portion of the property and the existing revetment on the property to the east. The revetment to the west was permitted as existing in 1977 (SE3-0269) with repairs permitted in 1991 (SE3-2312) and 2014 (SE3-5186). Erosion has remained consistent along the down drift edge of unprotected bank . The dwelling at #61 Dale Avenue (circa 1925) is now approximately 20’ from the scarp wrapping around the side of the structure. Page 3 of 7 Nourishment with fencing and / or fiber rolls were implemented at #27 Marchant around 1993 (DA-93070), 2012 (SE3-5001 / 700 CY+/-), 2013 (SE3-5120 / 165 CY+/-, 2015 (SE3- 5261 / 200 CY +/-), 2017 (SE3-5468 / 200 CY +/-), and 2018 (SE3-5573 / 700 CY +/-). These efforts were unable to withstand minor coastal storm events given the narrow beach, and the property was ultimately largely armored through the efforts in 2013 and 2018. The existing dwelling (circa 1927) is located approximately 50’ from the western end of the armoring which is being flanked by the erosion. Page 4 of 7 Beach access stair and landing at #11 Marchant were constructed in 2010 under SE3-4879. The stairs have been severely undermined, which is consistent with the erosion rates outlined below in Estimated Sediment Loss. The dwelling at this property dates to circa 1797. Alternatives Analysis: No Action – The No Action alternative would see continued erosion of the bluff, undermining of the adjacent coastal engineering structures, and shortly threaten the dwelling at #61 Dale. The No Action alternative is not acceptable and will shortly result in the construction of additional coastal protection measures to protect the home at #61 Dale and eventually #27 Marchant. Nourishment / Soft Solution – The Nourishment and / or Soft Solution alternative such as bioengineering projects is not viable at this site given the open ocean exposure and narrow beach which allows wave energy to travel to the face of the coastal bank . Between 2012 and 2018, approximately 2,000 CY of sediment were added to the upper beach and coastal bank at #27 Marchant as various soft solutions were implemented. Two of the projects included engineered soft stabilizations systems coupled with nourishment. Each of these nourishment efforts was lost within months of implementation. Coastal Engineering Structure – A revetment is the only viable Coastal Engineering Structure solution at the site. The revetment face would be sloped to match the slope of the existing revetments (~1.75H:1V). There are two options for placement of the revetment. The first would to tie the proposed revetment to the existing returns of the adjacent revetment structures. Following the existing exacerbated erosion face, this would result in a nearly perpendicular revetment return to west, which would protrude out from the natural shape of the shoreline similar to a groin. According to the Massachusetts Applying the Coastal Wetlands Regulations: A Practical Page 5 of 7 Manual for Conservation Commissions to Protect Storm Damage Prevention and Flood Control Functions of Coastal Resource Areas, leaving a nodal point in this configuration has “the potential to refocus wave energy, which exacerbates erosion of the beach and reduces the longevity of the structure”. The second option for laying out the revetment would be too reset/repositioning the returns on the existing adjacent revetments. The proposed revetment could be positioned in alignment with the revetment to the east and smoothly transition into the revetment to the west. This layout for the proposed stone revetment would follow the natural shape of the shoreline and prevent the creation of a shore perpendicular face which would trap sediment and interrupt the littoral dynamics along the shoreline. The arc of the proposed revetment connection is concaved, so that the tangent point is approximately 5-feet landward of the adjacent structures. We believe utilizing this approach with the proposed design is in accordance with the Coastal Manual and will minimize impacts to the Coastal Beach, and should be considered the preferred alternative. Preferred Alternative: A revetment is the only alternative that offers long-term protection for #11 Marchant. Nature based soft solutions with and without nourishment have been tried and do not offer as functional design life of a year. A revetment will offer years of protection and by aligning the structure with the adjacent revetments will minimize the impacts to the littoral system by not introducing a shoreline perpendicular return which would act like a groin. Nourishment has been added to the design as discussed below and will preserve the littoral contributions to the system that the coastal bank is currently providing. Page 6 of 7 Estimated Sediment Loss / Proposed Nourishment Mitigation: According to the Massachusetts Coastal Zone Management Shoreline Change Program (MORIS), the transect at the project site shows an erosion rate of -0.3 feet/year along Transect 782 from 1846 to 2014. The Shoreline Change database also presents a more contemporary shoreline change rate from 1978 to 2014 of -1.4 feet/year. Using the larger short term erosion rate of -1.4 feet/year, the annual amount of sand that would no longer be eroded from the Coastal Bank is 28 CY ((1.4 ft/yr x 60 ft length x 9 ft high / 27 ft3/CY)). As mitigation for the covering the Coastal Bank, a beach compatible sand nourishment has been incorporated into the project design above Mean High Water. This will provide 140 CY of sediment and would equate to approximately 1-foot of nourishment over the proposed revetment connection and beach area. Page 7 of 7 Post construction the shoreline should be monitored annually. The beach access stairs at the project site, and existing groin will serve as visual baselines. The shoreline at the adjacent parcels by their beach access stairs should also be monitored as baseline beyond the project area. The proposed beach profile may, but is not required, to be maintained through continued nourishment in consultation with Conservation Commission Staff. If the Mean High Water Line reaches the beach interface of the proposed revetment connection between the stairs and revetment (approximately a 3 foot lowering of the beach profile), notification should be provided to Conservation Commission, frequency of inspection increased to a quarterly cycle, and nourishment considered. If the beach lowers by 4 feet, nourishment should be implemented and / or other plans to continue to provide adequate protection to the toe stone of the revetment connection as designed. I trust this meets your present needs. Very truly yours, John O’Dea, P.E. Sullivan Engineering & Consulting, Inc.