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Land Subject to Coastal Storm Flowage
Rebecca HaneyCoastal GeologistMassachusetts Office of Coastal Zone Management
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Definitions/Terminology
• Land subject to coastal storm flowage: means land subject to any inundation caused by coastal storms up to and including that caused by the 100-year storm, surge of record or storm of record, whichever is greater. (310 CMR 10.04)
• 100 year storm = storm having a 1% chance of being equaled or exceeded in a given year.
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FEMA Flood Zone Definitions• Zone VE (V1-30) – Areas of 100-year coastal flood with velocity
Wave height 3 feet or greater Wave runup depth 3 feet or greater Within primary frontal dune (first dune landward of the beach)
• Zone AE (A1-30) – Areas of 100-year flood; flood elevations May be coastal or riverine Coastal can contain up to 2.9 feet wave height Coastal flood elevations at top of wave envelope
• Zone AO – “Overwash” areas with flow depths of 1 to 3 feet Generally coastal with sloping ground Flow velocities can vary greatly Flow paths are typically not well defined
• Zone A – Areas of 100-year flood; NO flood elevations given• Shaded Zone X (B) – Areas of 500-year flood• Unshaded Zone X (C) – “Areas of minimal flooding”
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Flood Zones on the Ground
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Flood Insurance Rate Maps
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Coastal A Zones
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General Points to Consider
• Maps are graphic representations of engineering data
• Detailed information generated only at specific transects
• Effective maps may not have all current techniques incorporated
• FIRMs do not take into account future conditions (erosion, sea level rise)
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Impacts of Storms Extend Beyond Mapped Flood Zones on FIRMs
• Storm of record can affect greater area than the mapped flood zones.
• More than half the buildings in NY City affected by Sandy were outside the mapped flood zones.
• 25% of all National Flood Insurance Claims are outside the mapped flood zones.
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Salisbury Hull
PlymouthNantucket
Storm Damage
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Function: Buffer Wave Action & Ability to Erode
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Function: Sediment Supply Scituate Salisbury
Plymouth Gosnold
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Function: Ability to Erode, Shift, Move, Buffer Wave Action
ScituateRockport
Friction Force Decreased
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Function: Ability to Move, Shift, Migrate, Transport Sediment
ScituateApril, 2007
Source: Google
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Functions: Erode, Shift, Move, Migrate Buffer Wave Action,
Duxbury
Source: Google
1853
1951
2008
Orthophoto: 2011
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Function: Buffer Flood Water
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Obstructions to Flow
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Reduced Floodplain Function
Hull
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Reduced Sediment Supply/ Increased Beach Erosion
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Damage Adjacent to Seawalls
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Impacts to Roads & Utilities
Hull Westport
ScituateOak Bluffs
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Boston
NewburyMarion
Salisbury
Inundation/Moving Water
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Impacts to Public Health & Safety
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Summary
• 25% of all NFIP claims occur outside the mapped flood zones.
• Some types of land use are increasing damages to public infrastructure.
• Storm damages are increasing in lower frequency events.
• There are options to prevent this trend from continuing.
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Humarock Beach, March 2013
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Hull