Analyzing Flood Risk− Mapping a Framework to a Safer‚ Equitable‚ and Resilient Fairfax County

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. The County had also recently enacted the One Fairfax policy, which states that departments must consider equity in the decision-making, development, and delivery of future policies, programs, and services. To comprehensively address flooding concerns, build resiliency into the stormwater network, and further the One Fairfax policy, the County pursued a more objective and equitable approach to flood mitigation by focusing on risk. At the time, Fairfax County's approach to flood mitigation was largely reactive. Flooding was identified through reported complaints. The County prioritized structural flooding adjacent to county easements and infrastructure and public flood mitigation projects were mostly at the individual lot level. The County recognized there were challenges with this method- residents did not always report flooding, older neighborhoods that developed prior to stormwater management standards often lacked public stormwater infrastructure or easements, and the approach offered few options for residents living in the floodplain. The County wanted to be more proactive and ideally address drainage issues before they caused flooding. Staff sought to develop a Geographic Information Systems (GIS) based flood risk map to meet multiple County needs. The map would help staff to select and prioritize capital improvement projects based on the frequency and impact of flooding. Regulators could 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 would 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. Fairfax County covers 395 square miles and does not have a comprehensive stormwater pipe capacity model. Staff recognized, however, that many of the conditions that make properties more vulnerable to flooding could be identified through existing GIS data. Stormwater management engineers worked together with GIS staff to develop a map to assess flood risk. The first version of the map used a parcel's location and proximity to ten potential flood hazard conditions to evaluate flood risk. Criteria included the following: structures within 30 feet of a FEMA floodplain, structures within 30 feet of a County recorded floodplain, structures in a 25-foot stream buffer, parcels with flooding or drainage-related complaints, parcels located in sumps, structures located in sumps, parcels in a subdivision older than 1972 (the year the County enacted stormwater management standards), residential infill lots (where older homes were torn down and replaced with much larger homes often lacking stormwater management), parcels located in subdivisions without stormwater quantity control facilities, and parcels located in subdivisions outside of stormwater facility drainage areas. Each criterion earned a parcel a score of one, with a total maximum score of ten. Over 350,000 parcels went through the selection process and were scored. Scores for each criterion were available at the parcel level, while total scores were available at the district or county scale as a heat map where the brighter color represented a higher flood risk. After reviewing the results, staff recognized some challenges with this first version of the map. The scoring double counted some of the risk factors. For example, a structure located in the FEMA floodplain, often also received points for being located in a County recorded floodplain and within the stream buffer. The scoring method also lumped fluvial and pluvial flooding together, making it difficult to evaluate the scope and cost of potential flood mitigation solutions. Perhaps most importantly, the map did not clearly and concisely convey flood risk to a lay audience, including both residents and county officials. A revised map narrowed the risk factors to the 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 developed two flood risk analysis tools. The Potential Sump Conditions and Overland Relief Flow Accumulation Layers are the result of analysis performed on a Light Detention and Ranging (LiDAR) derived Bare Earth Digital Elevation Model. 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. Flooding can also occur when there is a stormwater inlet in the sump, but it is overwhelmed due to the magnitude of incoming flows. The layer includes the volume and three-dimensional surface area of the sump. The Overland Relief Flow Accumulation Layer helps to visualize the natural overland runoff flow paths, including paths into and out of sumps. The flow path is strictly based on the bare earth elevation. This provides a clear picture of where the water flows when there is no stormwater management in place, or the infrastructure is overwhelmed. Using these tools, staff developed a revised 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 FEMA or County floodplain. Scenario three noted structures that intersected with or were fully contained within the Potential Sump Conditions Layer, where the sump depth was greater or equal to a foot and the overland relief drainage areas was greater than an acre. 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 Vulnerability Index is comprised of scores from several indicators from the American Community Survey data and ranks vulnerability low to very high on a census block scale. By overlaying the Vulnerability Index, staff identified the most vulnerable communities that were also more susceptible to flooding. Upon identifying these flood-prone neighborhoods, staff wanted to engage with the community to better understand their flood risk. Additionally, staff sought to increase resident awareness of its programs and services in these historically underrepresented communities. To do so, staff worked with a consultant to produce an Equitable Stormwater Management Engagement Framework (Framework) that outlines a systematic process for gathering input on flood mitigation needs and establishing new community relationships. The Framework's purpose was to create more inclusive, equitable, and authentic community engagement to ensure all voices are welcome in the County's planning and decision-making process. The document established a roadmap to achieve the following objectives- equitable consideration of flooding concerns without favoring one population over another, consideration of the vulnerability index and other factors, learning community issues and priorities, building trust with the community so residents feel comfortable contacting the County when issues arise, and understanding community values and priorities as they relate to stormwater and flooding. The Framework is intended to be living document that is updated as the County gathers lessons learned from its collective community of practice. In Spring 2023, the County plans to embark on the next phase of their flood risk reduction strategy. An interdepartmental workgroup will begin outlining a comprehensive flood mitigation plan that will include recommendations for a proposed Capital Improvement Plan project prioritization methodology, updated regulations for development and redevelopment, and programmatic solutions that will lead to a safer community and minimize property damage. Staff will also pilot the Framework guidelines by spearheading a targeted flood risk community outreach and engagement campaign in a highly vulnerable and potentially flood-prone neighborhood. Staff will continue to coordinate flood risk reduction strategies with Resilient Fairfax, a program to help County residents become more resilient to changing conditions and climate change-related hazards. This shift to a more objective and equitable flood risk approach will set the County up for a safe and more resilient future.