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HomeMy WebLinkAboutMarchant Avenue Review SCS 07242023 107 A County Road, North Falmouth, MA 02556 www.sustainablecoastalsolutions.com July 24, 2023 Town of Barnstable Conservation Commission Attn: Darcy Karle Conservation Agent 230 South Street Hyannis MA 02601 Re: Review of the shore protection alternative analysis and proposal for 11 & 27 Marchant Avenue, Hyannis Port MA. Conservation Commission Chair Lee, Sustainable Coastal Solutions, Inc. (Coastal Solutions) has been conducting coastal project planning, design, environmental permitting, and implementation for more than 20 years. The work we regularly undertake utilizes state-of-science tools, including physical processes numerical models and Geographic Information System (GIS) applications. These tools are used to effectively support client needs for the development of sustainable and resilient coastal solutions. Due to our ongoing working developing a sediment budget for the Hyannis Port coastline, which identifies transport pathways, sources, stores and sinks of sediment within the budget area (littoral cell),we were asked to review the proposed coastal bank stabilization project at 11 & 27 Marchant Avenue in Hyannis Port. The review was focused on how the proposed coastal bank stabilization project could influence the littoral processes along the adjoining shorelines and if potential solutions to minimize impacts had been considered. Coastal Solutions has reviewed the supporting technical information supplied by Sullivan Engineering & Consulting, Inc. (Sullivan Engineering) developed in support of the proposed project. The project site is located on Nantucket Sound, approximately 750 feet to the west of the Hyannis Harbor Breakwater. The beginning of the littoral cell is to the west of the project site at Squaw Island. The cell extends approximately 4,700 feet eastward to Hyannis Harbor Breakwater. Sediment transport along the Hyannis Port shoreline is predominantly west-to-east. Minimal volumes of sediment enter the littoral cell from the west due to jetty at the entrance to Halls Creek trapping sediment on the west side of the inlet along Coville Beach, the significant change in shoreline orientation at Hyannis Point, and the significant erosion that has occurred over the last 200 years along the Halls Creek shoreline which feeds sediment into the Hyannis Port littoral cell. The lack of sediment entering the littoral cell is reflected in the number of groins located along the Hyannis Port Shoreline. Twelve (12) groins have been constructed to hold the shoreline position and protect the landward infrastructure. Additionally, several properties along the shoreline to the west have stone revetments to protect the coastal bank. The addition of revetments generally indicates a diminishing supply of sediment, resulting in small and lower elevation beach which in turn result in erosion along the base of the coastal bluff. It is not clear if those structures have mitigation requirements in the form of nourishment, but the increasing rates of erosion indicate that if nourishment is being placed, it is not of a significant enough quantity to balance the reductions in sediment entering the littoral cell from the west. At the east of the littoral cell, the Page 2 of 6 Hyannis Harbor Breakwater has been impounding sediment since 1913 when the Commonwealth constructed the second section of breakwater from Hyannis Port shoreline towards the western tip of the Federal breakwater which was completed in 1832. Figure 1 illustrates the impact the construction of the Commonwealth’s breakwater has had on the shoreline position between the 1890 and 1938 shoreline. The shoreline accreted seaward several hundred feet. The shoreline accretion and movement beyond 1938 slowed relative to the earlier time period, largely due to sediment being transported over the breakwater into the harbor. That can be seen in the gradual shoaling of the harbor and the accretion of the shoreline front Hyannis Port Yacht Club as shown in Figure 2, which illustrates the shoreline movement from 1938 to 2021. The accretion trend continues over the 1982 to 2021 time period, as shown in Figure 3. Evaluating the shoreline movement from 1982 to 2021 highlights the accretion inside the harbor and the erosion immediately downdrift (east) of the groin field along the Hyannis Port shoreline where there are no structures to hold sediment on the beach. The diminished supply of sediment has resulted in higher erosion rates east of the groin field relative to the erosion rates within the going field. The annualized rate of shoreline change is presented in Figure 4, which is for period from 1982 to 2021. The only section of shoreline accreting is centered on the Hyannis Port Yacht Club. At the project site, the shoreline change rate is -1.0 to -1.5 feet per year. This erosion rate corresponds to the shoreline change rate presented in MCZM MORIS GIS database from 1978 to 2014, which was calculated to be -1.4 feet/year. Examining more recent shoreline change rates from 2009 to 2021 indicate an increase in the erosion from the eastern end of the groin field eastward to the breakwater. The erosion rate at the project site between 2009 to 2021, increased to approximately -2.6 feet per year. The increase in erosion rate at the project site can be attributed to on-going lack of sediment entering the system from the west, the coastal engineering structures to the west trapping and holding the sediment that is present, and the ongoing loss of sediment from the system over the breakwater into the harbor. The coastal processes analysis and numerical modeling that has been undertaken to examine the sediment budget along the Hyannis Port shoreline indicates that on an annualized basis that approximately 5,000 cubic yards of sediment bypass over the jetty into the harbor. The analysis is based on analysis of historical shorelines, topography, in addition to other sources of data were gathered to develop and parameterize the numerical models. The topographic and bathymetry dataset were used to develop a two- dimensional wave model (SWAN) of the Hyannis Port nearshore region. The wave model was used to drive one-line longshore transport numerical model to simulate the evolution of a shoreline through time. An example of the numerical output from the models is present in Figure 6. The figure shows the calculated east and west directed sediment transport rates along the net transport rate for the entire shoreline. The influence of the groins can be seen in the Figure 6 as the sediment transport rate changes at each structure. Page 3 of 6 Figure 1. Historic shoreline positions adjacent to the Hyannis Harbor Breakwater, with a 2021 aerial in the background. The project site is outlined in red. Figure 2. November 1938 USGS aerial with April 2021 shoreline (blue dashed line). The movement in shoreline position at the Hyannis Port Yacht Club dock highlights the accretion that has resulted from sediment movement over the breakwater into the harbor. Page 4 of 6 Figure 3. March 1982 NPS aerial with April 2021 shoreline (blue dashed line), illustrating accretion at the Hyannis Port Yacht Club due to sediment movement over the breakwater into the harbor. Figure 4. Annualized shoreline change between 1982 and 2021 (39 year period). Page 5 of 6 Figure 5. Annualized shoreline change between 2009 and 2021 (12 year period). Figure 6. Calculated annualized alongshore sediment transport rates along the Hyannis Port shoreline. Reviewing the Alternatives Analysis provided by Sullivan Engineering illustrates the limited options that exist for stabilizing the shoreline at the project site. With erosion rates of over 2.5 feet per year it is not surprising that soft engineering solutions tried over the last decade have failed. The ongoing erosion over has lowered the elevation of the beach profile, exposing the upper beach and base of the coastal bank to regular storm surge inundation and wave attack during extreme high tides and minor coastal storms. For soft engineering solutions to be effective, the systems need to be located at elevations that are above regular tidal induction elevations and minor surges. The systems are designed to moderate erosion during significant events, with the anticipation maintenance would be required after the storm. However, when maintenance and repair is required on a regular basis (i.e. monthly if not more frequently with the beach elevations at the project site) the natural fiber materials reach the end of design life quickly. The materials are not suited to withstand the wave energy/forces that are regularly encountered and thus fail. The only practical stabilization solution to protect the landward infrastructure at the project Page 6 of 6 site is a traditional hardened coastal engineering structure. Sullivan Engineering has proposed to construct a rock revetment to prevent the ongoing erosion of the coastal bluff. The properties to the east and west of the project site already have stone revetments in place along the face of the coastal bank. The revetment immediately to the west of the project site, extends out onto the beach with lateral arms extending landward to minimize the potential for erosion and waves to bypass the structure. The property immediately to the east is also protected with a rock revetment. This structure has been covered with sediment and then vegetated. Therefore this revetment is bridging the approximate 50-foot gap between the two existing structures. Reviewing the design plans provide by Sullivan Engineering illustrates that the proposed revetment has been designed to smoothly tie into the face of the existing revetments. This design approach will prevent the creation of a shoreline perpendicular structure face on the western edge of the project. If the proposed revetment was positioned further landward to follow the eroded coastal bank face and tie in the terminal end of the revetment at 61 Dale Ave, it would create shoreline perpendicular harden structure face, approximately 30-feet in length, which would act in a similar manner to a groin. While the sediment transport predominantly west-to-east, during significant storm events when sediment transport is reversed (i.e. nor’easters), the structure face would trap sediment further protecting 11 & 27 Marchant Avenue but at the detriment to the downdrift shoreline. The design approach to smooth the structure face across the three properties is the best approach from a sediment dynamics perspective and minimize the impacts to the beach east of the groin field. The Alternatives Analysis also discusses the proposal to add sediment to the beach in the form of a nourishment to offset the impacts associated with armoring the coastal bank. The addition of sediment to the system is an advantageous design choice since the nourishment will help maintain the beach and thus help dissipate wave energy during storm events and thus aid in the protection of the landward infrastructure. The nourishment will also slow the ongoing erosion of the beach. While this beach would not represent productive nest habitat for piping plovers due to the narrow width and low elevation relative to high tides, preserving the beach would prolong the forging habitat along this section of shoreline. One suggestion relative to the determination of the nourishment volume would be to utilize the more recent shoreline change rates from 2009 and 2021 in the calculation. This would increase the annual contribution volume from 28 cubic yards to 50 cubic yards. As designed the proposed revetment has incorporated all the available measures to minimize impacts to the littoral system and thus the environment resources located along the Hyannis Port shoreline. If the Commission has any questions or needs additional information, please feel free to contact me via email (truthven@coastalengineer.us). I can also be reached by phone at 508-864-5652. All the Best, Trey Ruthven Sustainable Coastal Solutions truthven@coastalengineer.us 107A County Road North Falmouth, MA 02556