11/29/2023 HARBOR MANAGEMENT COMMITTEE Minutes

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Woods Hole Group: Dack Stuart,from Woods Hole Group, was unable to join the meeting due to scheduling conflict. Mrs. Fisher presented findings to the Committee, and invited Dack Stuart to attend a future meeting. Seconsett Island Causeway The following recommendations were made to the Town after thoroughly surveying the area commonly known as the " Seconsett Island Causeway" : In the Short term,the Town is limited to maintenance work to keep the structure stable and functional. The Town should continue to place larger chinking stone(4-6"or 6-12")to fill the gaps between stones and prevent washout.The use of small pebbles/cobbles should be discontinued as this material is not being retained.The Town should consider staking in fiber rolls and installing plantings along the top of the slope/revetement to prevent material washout at the crest of the structure,which could require limited permitting. In the long term,the Town should consider reconstructing the revetment to modern standards (layering/fabric)that would prevent washout from storm and roadway runoff from undermining it.Woods Hole Group offers the following recommendations: •The crest should be at a higher elevation and a buffer zone could be created with sand/fill& plantings. •Toe stones need to be buried below the depth of scour. •The armor stones are likely of sufficient size that much of the material could be reused.This would save on materials costs reusing the stone so long as the engineering design analysis determines that they are of sufficient size. •Consider placing sacrificial sand on the face of the structure to provide a nourishment source. •The structure will have to be carefully joined to the structures on adjacent private parcels to the north and south. •Consider alternatives to the current method of dinghy storage,possibly by creating a structure with other structural components that can receive a boat,such as a rack. It appears that the portion of the revetment in the worst condition is at the low point(s)in the road where stormwater collects in the roadway and drains over the face of the structure.The Town will need to consider roadway stormwater improvements as part of this project to realize the full benefits.This could include installation of catch basins with drains that discharge through the revetment closer to the base. Raising the causeway grade will result in an expansion of the causeway width and encroach into the adjacent coastal and wetland resources,which will trigger additional and possibly difficult permitting. Therefore,it would be difficult to raise the grade of the road within its current footprint without using a vertical structure(sheet pile/concrete)to go vertical.A raised causeway consisting of a bridge on piles could also be constructed and likely would be a better option if the roadway needs to be raised more than a few feet from its current elevation.However,it will have to minimize adverse impacts to the private properties on either end of the structure.Also, a new dingy storage/docking solution would need to be developed.Costs to raise the road are very expensive and not part of this memo. The opportunities to capture and treat the stormwater runoff from the roadway are limited due to the low elevation of the area compared to the underlying water table and the lack of space outside of the adjacent wetland resource areas.The installation of cape cod berm along the easterly paved edge with waterways that lead to deep sump catch basins spaced along it that discharge into shallow subsurface leaching fields installed beneath the roadway pavementwould offeran improvement to the existing stormwat er treatment situation.The paved waterways could allow excess runoff that exceeds the system capacity to discharge towards the salt marsh as it does now. The drainage system's treatment capacity would be dependent on the percolation rate of the underlying soils and the size and type of leaching system.Capturing and treating the more frequent,less intense