Older neighborhoods in the City of Fort Worth, TX are experiencing significant redevelopment with zero lot lines and higher impervious surfaces. This has caused increased runoff and flooding conditions that were not there previously. There is concern that current zoning, land use, and maximum impervious cover percentages may not be adequate to mitigate the cumulative impacts of development on flooding. Analysis was needed to assess whether modifications to City procedures used for cumulative impact evaluation, design, and zoning are needed. Tetra Tech assisted the City to study the cumulative impacts of impervious cover changes on stormwater runoff volumes and peak flows in two developed watersheds - Central Arlington Heights (CAH) and Linwood Bailey (LB). Tetra Tech calculated average cumulative impervious surface and associated methodologies for each of the three (3) scenarios – baseline existing, per the City’s Stormwater Design Manual, and full buildout. Then 2-D XPSWMM models were developed for each watershed and each scenario for the 1-year, 5-year, and 100-year 24-hour rain event. Each return rainfall runoff and flooding volume was compared per rain event against each scenario to determine if both code and future conditions were adequate. The resulting data showed the maximum depth of inundation increased on both private properties by over 5 feet, on roads by just over 0.5 feet. The SWMM model also increased number of flood prone properties by over 100 with an increase in cumulative runoff between 5.7 and 5.9 inches.
