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FEMA Flood Mapping – Ulster County, NY


Gomez and Sullivan Engineers was retained by the New York State Department of Environmental Conservation (NYS DEC) as the prime on a team to perform Digital Flood Insurance Rate Map (DFIRM) production and development for Ulster County, NY. The team started work on this project in 2006 with the field reconnaissance and ground survey of approximately 29 miles of stream. The reconnaissance was used to determine floodplain features, collect information on hydraulic and/or flood-control structures, and assist in the location of cross-sections to be surveyed. During the reconnaissance phase, a total of over 200 cross-sections and structures were selected for survey. When the survey was completed in 2006, the collected stream channel topographic information was incorporated into a LiDAR (Light Detection and Ranging) based digital elevation model (DEM) of the county. Hydrologic analyses were also conducted for 96 streams and over 450 stream-miles. Stream segment flow nominations were determined for over 150 flood nomination locations to represent the hydrology.

The floodway data tables, flood profiles, and DFIRMs that were developed contain information on flooding sources in 17 communities. Hydraulic models using HEC-RAS were developed for over 280 miles of stream and the remaining stream reaches were redelineated using the effective flood elevation profiles. The results of the hydraulic and redelineation analyses were applied to the terrain (DEM) model to identify and map the 1-percent and 0.2-percent-annual chance floodplain boundaries and floodway extents.

The Gomez and Sullivan team applied GIS techniques to develop the preliminary DFIRMs according to FEMA Guidelines and Specifications. This task required development of a geodatabase consisting of vector, raster, non-spatial, and annotation data which conformed to a defined data attribute and topology rule set. The new Digital FIRMs allow users to view information in a graphical format, add or remove layers of data according to their needs, and overlay flood inundation areas on other data sets.

Gomez and Sullivan worked with the NYS DEC in attending community coordination meetings, publishing Base Flood Elevations (BFE) notices, issuing BFE determination letters, and other post-preliminary steps toward final map adoption in 2009.

In 2007, the Gomez and Sullivan team issued 54 Preliminary Flood Insurance Rate Maps (FIRMs) and a Flood Insurance Study (FIS) Report in the new county-wide format for communities in Ulster County, NY. This work was conducted under contract with the New York State Department of Environmental Conservation; a Cooperating Technical Partner (CTP) with FEMA in the development of modernized flood maps. The maps were effective as of September 2009.