FEMA Letter of Map Revision (LOMR) and Modeling for Muddy Brook

Challenge

In 2017, GZA was retained by the Town of Westport to prepare and submit a Letter of Map Revision (LOMR) to FEMA for one of the town’s urban streams – Muddy Brook. According to the current FEMA Flood Insurance Study (FIS) for Fairfield County (last revised October 16, 2013), FEMA’s existing mapping for Muddy Brook was prepared during the 1980s.

Solution

GZA updated the hydrologic analysis using Snyder Method rainfall-runoff model. The Snyder Method uses a nearby gaged stream to calibrate watershed properties, which are then translated to the Muddy Brook watershed. GZA subdivided Muddy Brook’s watershed into 7 subwatershed and used the Snyder Method to calculate a peak flow for each subwatershed. 

GZA routed the calculated peak flows through a 1-dimensional HEC-RAS 5.0.7 model. The HEC-RAS cross sections were developed using recently acquired terrain data from the Town.  GZA performed a field assessment of the brook’s accessible stream crossings (bridges, culverts, and dams) and acquired drawings and inspection reports for those structures that were inaccessible. The model includes a total of 228 cross sections, 29 bridges/culverts, and 2 dams. GZA used HEC-RAS version 5.0.7 GIS Mapper program to geospatially identify ineffective flow locations. In addition, GZA had prepared a HEC-RAS 2-dimensional model of Muddy Brook for a separate study for the Town and used the 2-dimensional results to help identify proper cross section placement. The 10-, 50-, 100-, and 500-year steady state floods were modeled. GZA then performed a floodway analysis for the 100-year flood. GZA modeled and mapped 4.8 miles of Muddy Brook. The floodway analysis was performed for 3.5 miles. GZA’s LOMR submission included a detailed H&H report and the necessary FEMA forms.  Map revisions were prepared by GZA’s GIS specialist using ArcMap. GZA maintained communications with a FEMA representative over the course of the project to verify that GZA’s approach was acceptable.
 

Benefit

The LOMR was submitted in May 2018 to FEMA. The inundation extents calculated by GZA’s analysis differed from the existing FEMA Flood Insurance Rate Maps. In some locations, GZA’s analysis found the floodplain larger, whereas in other locations, GZA found the floodplain narrower. In August 2019, FEMA requested additional information for the LOMR. GZA resubmitted the LOMR with the additional information in November 2019.