In July 2019, a summer thunderstorm dropped up to 5.3 inches of rain in one hour over some parts of Fairfax County (up to a 1,000-year storm event), causing a reported 149 house flooding and at least $15M in damages. Studies continually show that these high intensity, short duration storms will become more frequent with climate change. To comprehensively address flooding concerns, build resiliency into the stormwater network, and further the County’s equity policy, Fairfax County pursued a more objective and equitable approach to flood mitigation by focusing on risk.

Staff sought to develop a Geographic Information Systems (GIS) based flood risk map to meet multiple County needs.  To do so, staff identified three main causes of flooding in Fairfax County – proximity to a floodplain, located in a sump condition, and the lack of overland relief.  To identify these conditions, staff created two flood risk analysis tools.  The Potential Sump Conditions layer highlights locations of sumps, or bowl-shaped depressions, where water pools and is unable to drain without flooding a structure.  The Overland Relief Flow Accumulation Layer helps to visualize the natural overland runoff flow paths.

Using these tools, staff developed a flood risk map that analyzed four flood risk scenarios. Scenario One established a baseline by identifying all existing reported structural flooding. The second scenario highlighted structures in or within 15 feet of a mapped floodplain. Scenario three noted structures that intersected with or were within the Potential Sump Conditions Layer. The final scenario noted structures that intersected the Overland Relief Flow Accumulation Layer where the drainage area was ten acres or greater. Staff then used the Fairfax County Vulnerability Index to apply an equity lens to flood risk.

The flood risk map will help staff to select and prioritize capital improvement projects based on the frequency and impact of flooding.  Regulators can utilize the data to inform plan review and code requirements.  A public facing component could educate both residents and developers on a property’s flood risk and an associated dashboard could track flood mitigation and resiliency metrics.  Finally, the map will also build the foundation for a comprehensive flood risk reduction plan by helping County leadership and elected officials visualize the scope, cost, and timeline, to mitigate flood risk at a countywide scale.