- Technical Review of FEMA Flood Mapping
- Preparation of HEC-HMS hydrologic and HEC-RAS hydraulic model to define base flood elevations (BFEs)
- Coordination with FEMA
- eLOMA Preparation
- Removal of site from Special Flood Hazard Area
Challenge
GZA was contracted by a private client who wished to expand their existing facility in Stoughton, Massachusetts to re-evaluate Federal Emergency Management Agency's (FEMA) Flood Insurance Rate Mapping (FIRM) in the vicinity of the Site. The facility was located within a delineated Special Flood Hazard Area (SFHA), Zone A in the FEMA FIRM. FEMA defines a Zone A SFHA as “Areas subject to inundation by the 1-percent-annual-chance flood event generally determined using approximate methodologies. Because detailed hydraulic analyses have not been performed, no Base Flood Elevations (BFEs) or flood depths are shown. Mandatory flood insurance purchase requirements and floodplain management standards apply.”
Solution
One approach used by GZA for such properties is to perform a detailed technical analysis to better define the limits of the base flood elevation and its resulting floodplain. Since the existing SFHA was delineated using approximate methods, no prior FEMA hydrologic and hydraulic analysis existed. Therefore, GZA performed a hydrologic analysis using procedures outlined in FEMA’s technical reference Appendix C: Guidance for Riverine Flooding Analyses and Mapping. Our results for the ungaged watershed compared favorably to available regression methods, and a Town study from 1976. GZA then developed a hydraulic model for a reach of Lovett Brook USACE’s HEC-RAS and GeoHEC-RAS models. The model included roadway crossings and culverts, and the effects of sedimentation on culvert capacity. A new BFE was established for the study reach of Lovett Brook using the results of our analysis. The results of our work, in addition to the results of our review of the 1976 Town study, were compiled and summarized into the FEMA eLOMA (electronic Letter of Map Amendment) request process. This process was selected by the Client based on GZA’s coordination with the FEMA Map Service Center, in lieu of a more complex Letter of Map Revision request.
Benefit
FEMA’s approval of GZA’s work product resulted in the Site’s removal from the SFHA and floodplain. The removal of the site from the SFHA allowed the Client to proceed more expeditiously in the expansion of its facility and saved the Client the cost of additional permitting and flood insurance.