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FEMA REGION III COASTAL HAZARD ANALYSES AND DFIRMS UPDATE
Jeff Gangai, CFM2
Robin Danforth, PE, CFM1
Elena Drei-Horgan, PhD, CFM2
1FEMA Region III, 615 Chestnut Street, 6th Floor, 19106 Philadelphia, PA – 215-931-5573
2RAMPP, 8401 Arlington Boulevard, 22031 Fairfax, VA – 703 849 0215
The Federal Emergency Management Agency (FEMA) Region III encompasses Delaware, Maryland and Virginia coastal counties. As part of FEMA’s Risk Map Program, an identification and assessment of the coastal flooding risks will be performed for all Region III coastal counties and will ultimately produce updated Digital Flood Insurance Rate Maps (DFIRMs). New coastal hazard analyses, initiated in the Fall of 2009, will utilize the 1% annual chance stillwater elevations obtained from the storm surge modeling of the Atlantic Ocean, Chesapeake and Delaware Bays, concurrently being performed by the USACE for FEMA Region III.
A coastal flooding risk update is needed mainly because of the age of data shown on the effective FIRMs. In particular, the topographic data used for modeling and mapping shown on effective FIRMs dates back to the mid-1970s and mid-1980s from USGS contours. The stillwater elevations go back to a 1978 Virginia Institute of Marine Science study for the Chesapeake Bay and to a tidal gage analysis for the Atlantic coasts. Effective FIRMs do not account for wave setup, and the overland wave determination was computed with a methodology that dates back to the late 1970s. Dune erosion was accounted for in isolated locations only.
In addition, major changes in NFIP policies and methodologies have occurred since the effective date of many FIS studies in the area, creating the need for an update that would reflect a more detailed and complete hazard determination. New guidelines for the Atlantic Coast and Gulf of Mexico, released in 2007, propose updated methods for the computation of wave setup; dune erosion should be applied following the 540 sq ft rule for dune volume criteria, and wave runup has been updated from a mean wave runup to a 2% of the highest wave runup value. Additional guidelines were released for sheltered coasts in 2008, and new policies such as the Limit of Moderate Wave Action (LiMWA) were introduced to limit wave damages to structures.
Within the new coastal study for the Delaware, Maryland and Virginia coastal counties the analysis will be performed along transects located at a higher density than old studies, reducing interpolation of results, to favor more precise and detailed products.
In addition to producing update DFIRMs, FEMA Region III aims at an extensive outreach program to bring to light methodologies and results through a variety of efforts. These efforts entailed identifying and engaging stakeholders within the study area through a series of announcements, mailings, web sites and public meetings to discuss results and plans for DFIRM updates.
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