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HomeMy WebLinkAboutWHOI Coastal Processes comment 1 | P a g e COASTAL PROCESSES SPECIALIST WOODS HOLE SEA GRANT | CAPE COD COOPERATIVE EXTENSION gberman@whoi.edu | gberman@barnstablecounty.org 508-289-3046 | 193 Oyster Pond Road, MS #2, Woods Hole, MA 02543-1525 December 1, 2022 TO: Brittany DiRienzo, Yarmouth Conservation Administrator CC: none FROM: Greg Berman, Coastal Processes Specialist (WHSG & CCCE) RE: Site visit to Great Island Road, Yarmouth, MA – 11/18/2022 Background: Since the inception of the coastal processes position established within WHSG & CCCE, on- site and remote technical assistance on coastal processes has been and continues to be an on-going, effective technical information communication and dissemination tool. Technical assistance relating to coastal processes, shoreline change, erosion control alternatives, coastal landform delineation, potential effects of various human activities on coastal landforms, coastal floodplains, coastal hazards and hazard mitigation analyses, and dune restoration techniques provided in the field and remotely will continue to be provided on an as-needed basis. Site visits generally address site-specific coastal processes or coastal hazards related issues. Follow-up unbiased, written technical alternatives analyses are generally provided. 2 | P a g e Site Details: This report focuses on the property of, and land immediately adjacent to, Great Island Road in Yarmouth (Figure 1). Great Island is located immediately east of the inlet to Lewis Bay and this road was established on the shoreline of Nantucket Sound. The applicant has submitted an NOI (09/19/2022) and site plans (revision 3 – 11/09/2022) for this project entitled “Great Island Road Resiliency Project”. The Yarmouth Conservation Commission Administrator requested a site visit and a follow-up report to examine the existing conditions at the site and the proposed stabilization methods. During the site visit on November 18, 2022 a series of photographs were collected. At the north of the project site, Photograph 1 shows the coastal dune is very narrow and low. It is adjacent to the only access road for Great Island. The rock groins along this shoreline are impounding large volumes (roughly four vertical feet in some locations) on their southern sides, which indicates robust transport to the north. There appears to be enough space for the heavy equipment needed to move and shape the sediment, to drive over the groins if the project creates sand ramps on the beach. The Conservation Commission may wish to confirm that there is enough distance between MLW and the existing dune so that additional permitting is not required for the Limit of Work. Further to the south the dune system has more elevation (Photograph 2), however recent erosion has created a scarp on the seaward face of the dune. While beachgrass is the predominant species, there are also a plethora of additional species of flora; other grasses and perennials, shrubs, groundcovers, and even trees. While American beachgrass can be the best species for initially stabilizing artificial dunes, it could be supplemented with a variety of other grasses and shrubs well adapted to dune protection. For example shrubs (ex. Beachplum, Bayberry, etc.) grasses/perennials (Quackgrass, Switchgrass, Little Bluestem, Seaside Goldenrod, etc.) could be included to enhance wildlife habitat (as well as further stabilize the erosion control project) by providing nesting and foraging areas for a variety of birds and mammals. Photograph 3 shows an area in which the dune is quite low and narrow. This is also an area in which the dune protects a large marsh system from wave action in Nantucket Sound. There are also existing cobbles in this portion of the site. If not artificially placed in this location, storm waves have likely moved cobbles from the beach up and onto the face of the dune. The current configuration of this dune is unlikely to prevent overwash and significant impact to the marsh system during a storm. While overwash (and subsequent burial of marsh) are natural processes in barrier beach migration, this site is now longer receiving sediment from the south that can support the barrier beach. The rock groin at the southern portion of the site (Photograph 4), referred to as Fox Point. This groin likely blocks most, if not all, sediment from rebuilding the beach and dunes to the north. Photograph 5 shows that there is significant amounts of cobble eroding from Fox Point. This material has been dispersed onto the beach and waves have transported some cobble up onto the dune. Longshore Sediment Transport: Waves do not always approach perfectly perpendicular to the shoreline, they usually strike the beach at an angle with a portion of the energy moving parallel to the shoreline. Sediments are transported along the shoreline with net movement in a single shore-parallel direction. This Longshore Sediment Transport (LST) acts like a shallow river flowing parallel to the shoreline, so erosion at one spot provides material for beaches and dunes downdrift of that position. For more basic details on LST the reader is referred to a 2011 guide (https://www.capecod.gov/wp- 3 | P a g e content/uploads/2022/03/LST.pdf). A tool that works quite well in this area is called Time Machine, developed by Google Earth Engine and Carnegie Mellon University’s CREATE Lab. Working with Sea Grant, The Barnstable County Cape Cod Cooperative Extension’s Coastal Processes team has used Time Machine to create a virtual tour of Cape Cod’s sandy shoreline, revealing just how dynamic they are. You can access the tool under the “Shifting Shorelines” tab at https://www.capecod.gov/departments/cooperative- extension/programs/coastal-processes/, then navigate to the site in one of the interactive maps. From 1984-2020 it is clearly visible that some sediment makes it around the groin at Fox Point and moves north along the shoreline. This sediment eventually makes it to the jetty at Seagull Beach, where it becomes impounded or works it’s way over/around the jetty to fill the navigational channel, which requires regular dredging. Please note that the robust Coastal Processes Analysis (Draft April 2022) has not yet been reviewed and is not discussed in this report. Depending on the timeline for this project, and the needs of the Conservation Commission, the Coastal Processes Specialist is willing to perform this review. Sand Nourishment: The existing rock groins would act to slow down the transport of the new nourishment to the east. While these structures would likely be very difficult (if not impossible) to permit now, they would actually be serving an important role in the project. If they were not present there would be more of a concern about the speed at which the placed material would migrate to Seagull Beach and potentially affect navigation. In the long term, if nourishment is continually applied this may require more frequent dredging at the inlet. Once aspect of the project in the long term could be “backpassing” where sediment is excavated from Seagull beach, right before it flows into the inlet. This material could then be taken back to Fox Point, which could be considered the “beginning of the conveyor belt”. The project at Dead Neck Sampson Island is a local example of this process. A potential concern might be that the volume of the sand nourishment may impact eelgrass. This concern could have been addressed in the Coastal Processes Analysis, which has not yet been reviewed. Additionally, the sand at the western side of the project may overwash onto the marsh. In this area a dune core of cobbles is proposed. This is a good aspect of the design as it may keep the sand on top of the cobbles from rapidly flowing onto the marsh. This area has been highly impacted by the groins to the west. The barrier beach is being prevented from the natural flow of sediment which would sustain it. If nothing is done the barrier beach will breach very soon and the marsh will be negatively impacted. Natural barrier beach migration does include breaches and overwash, however this section of shoreline is no longer acting in a natural way. The Conservation Commission may want to request additional detail about the cobbles. While there are some natural cobbles at the site they are relatively small (mostly < 4”), and the grain size class for cobble is quite large (2.5-10”). Cobbles at the biggest end of the spectrum may not be appropriate for portions of this barrier beach. Additionally, if the cobble dune core is permitted, the Conservation Commission could consider putting triggers in the Order of Conditions such that, if ?? linear feet of the cobble is exposed, it would need to be recovered with sand within ?? days to reduce the reflected wave energy. 4 | P a g e Figure 1. Location of site. 5 | P a g e The red arrow on the image to the right indicates the location and direction of the photograph at the bottom of this page, and the other photograph in the series is indicated in yellow. Photograph 1. The coastal dune at the north of the site is very narrow and low adjacent to the only access road for Great Island. 6 | P a g e The red arrow on the image to the right indicates the location and direction of the photograph at the bottom of this page, and the other photograph in the series is indicated in yellow. Photograph 2. Further to the south the dune has more elevation, however recent erosion has created a scarp on the seaward face of the dune. 7 | P a g e The red arrow on the image to the right indicates the location and direction of the photograph at the bottom of this page, and the other photograph in the series is indicated in yellow. Photograph 3. The dune is quite low and narrow where it protects a large marsh system from wave action in Nantucket Sound. There are also existing cobbles in this portion of the site. 8 | P a g e The red arrow on the image to the right indicates the location and direction of the photograph at the bottom of this page, and the other photograph in the series is indicated in yellow. Photograph 4. The rock groin at the southern portion of the site, referred to as Fox Point. This groin likely blocks most, if not all, sediment from rebuilding the beach and dunes to the north. 9 | P a g e The red arrow on the image to the right indicates the location and direction of the photograph at the bottom of this page, and the other photograph in the series is indicated in yellow. Photograph 5. There are significant amounts of cobble eroding from Fox Point. This material has been dispersed onto the beach and waves has transported some cobble up onto the dune.