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HomeMy WebLinkAbout2013_7_24 SBPF Presentation_201405230847393335Baxter Road and Sconset Bluff Storm Damage Prevention ProjectNantucket, MassachusettsSiasconset Beach Preservation Fund 2Existing Conditions 3Existing Conditions 4Existing Conditions 5Existing Conditions 6Project Purpose and Regulatory ComplianceProtect Baxter Road and other public infrastructureProtect the entire historic residential community along ‘Sconset Bluff from storm damage 7Top at EL 26’ MLW – includes wave run-up for 100-yr storm +1’ sea level riseToe set at EL 0.0’ MLW based on scour from 100-yr stormArmor stone sized for 100-yr design stormGeotextile Filter Fabric lines the coastal bankFilter layer of 4-8” dia crushed stone2 layers of armor stoneRevetment sloped at 1.5:1Revetment Design 8Shoreline MonitoringTransects 9Phase One•8-10 weeks to install ~ 550-ft section where Baxter Rd is within 50 ft of bank edge.•~ 2 months to install ~1,300 ft section from 73 Baxter to Lighthouse. These segments would be installed to partial height to protect most severely threatened homes.•Depending on storm conditions, installation would occur from AUG/SEP to DEC.Phase Two•Partial Height Revetment would be constructed to full height in year two (2014) along with Phase Two.Construction Steps 10Construction Phasing•Highly likely to be staged from the beach with barges bringing in rocks and supplies.•Landing Barge will run up on beach at high tide.•Ramp / gangway will extend from Landing Barge.•Delivery Barge will moor at stern of Landing Barge.•Front end loaders will move rocks and supplies from Delivery Barge to Beach, then to the toe of the Coastal Bank.•An excavator will be used to excavate and place toe stones at bank / beach interface, then place stones to build up the revetment.•Beach-compatible sand will be brought in via truck from an Island pit and would be conveyed down face of bank with a conveyor device or chute. Alternatively, the sand may be brought in by barge to the beach.•Front end loaders will spread the sand on the face of the revetment and/or to adjacent downdrift areas.Construction Steps (continued) 11RevetmentsSouth Shore,Cape Cod & Islands Hull, Green Hill Revetment12About 900 feet longNo sand mitigationNo obvious downdrift impacts Scituate, Fourth Cliff Revetment13About 816 feet longNo sand mitigationNo apparent downdrift impacts Plymouth, Manomet Revetments14Over 10,000 feet of several revetmentsSome isolated sand mitigationNo significant downdrift impacts visible Plymouth, Cedarville Landing Revetments15About 1,375 feet long total for several revetmentsInitial sand mitigationSome minor downdrift impacts probably due to lack of sustained sand mitigation Mashpee RevetmentAbout 2,700 feet longProtects Pre-1978 housesSand mitigation required at bank retreat rateNo significant downdrift impacts Nantucket, Westcliff Lane Revetment17About 500 feet longProtects Pre-1978 housesRevetment was constructed in the 1990’s to replace a vertical bulkheadNo sand mitigationSome minor downdrift impacts probably due to no sand mitigation Nantucket, Capaum Pond Road Revetment18About 700 feet longProtects Pre-1978 houseNo significant sand mitigationSome minor downdrift impacts probably due to lack of sand mitigation 19Wetlands Protection Regulations•New CES “shall be” permitted (instead of “maybe be”) when required to prevent storm damage to buildingsconstructed prior to Aug.1978, including reconstructions of such buildings, if:– Designed and constructed, using best available measures, to minimizeadverse effects on adjacent or nearby coastal beaches due to changes in wave action, – No other method of protecting building is feasible.– protective planting designed to reduce erosion may be permitted. 20Local Bylaw Regulations• CES allowed to protect pre-78 structures and public infrastructure:– if no environmentally better way to control an erosion problem, including in appropriate cases movingthreatened buildings or public infrastructure, – no other alternative exists to protect structure from imminent danger.– “unless substantially improved” is questionably valid and not relevant. 21Project Purpose andRegulatory ComplianceProtected homes are pre-1978 or gap lots necessary to protect pre-1978 homes. 22ALTERNATIVES ANALYSISGeotextile TubesBeach NourishmentDewateringBreakwaterGroinSeawallDrift FenceCoastal Bank TerracesMarine Mattress & GabionsRevetment: Preferred Alternative Demolition of Bluff HouseHouse was moved back twice before being demolished as a result of bank erosion due to Sandy 24Compliance with Rules for Coastal Beaches and Banks• Sand is compatible with existing beach.• Protects the bank, which is otherwise stable and preserves bank height.• Is water dependent use.• Minimizes harm to adjacent and down-drift beaches due to supplementing sediment.• No adverse effects on marine fisheries or shellfish beds. • Enhances storm damage prevention and flood control. • Enhance the coastal bank’s function of buffering inland areas and buildings from storm damage.• Improve wildlife habitat.• Enhance the wetland scenic view. • Less construction, no debris, stable and green.• Preserves and enhances recreational trails and public access.• May loose beach at HT, but lost now to conditions. • Save and expand Bluff walk. • New stairs up/down and path across. 25New CES shall bepermitted under State and Local law: • ___Y___ Does it prevent storm damage to pre-78 structures, including reconstructions of such?• ___Y___ Does it protect pre-78 public infrastructure?• ___Y___ Does design use best available measures to minimize adverse effects on adjacent or nearby coastal beaches due to changes in wave action?• ___N___ Is another method of protecting buildings feasible?• ___Y___ Are the protective planting used to reduce erosion?• ___N___ Is there an environmentally better way to control erosion problem?• ___N___ Is it appropriate to move threatened buildings or public infrastructure?• ___N___ Is there an alternative to protect structure from imminent danger?• ___Y___ Is sand compatible with existing beach?Checklist 26• ___Y___ Are the bank and bank height being protected?• ___Y___ Is the revetment a water dependent use?• ___N___ Are there any adverse effects on marine fisheries or shellfish beds?• ___Y___ Does it enhance storm damage prevention and flood control?• ___Y___ Does it enhance the coastal bank’s function of buffering inland areas and buildings from storm damage?• ___N___ Does it harm wildlife habitat?• ___N___ Does it harm the wetland scenic views? • ___Y___ Does it preserve and enhance recreational trails and public access?Checklist (continued) 27 29The End