The three main causes of flooding in Fairfax County include the following conditions:

1. Proximity to a floodplain.
2. Location in a sump condition.
3. Lack of overland relief. 

To identify these conditions, Land Development Services (LDS) staff developed two new Flood Risk Analysis Tools as part of the Floodplain Viewer and the Drainage Basin Delineation Tool. The new data layers:

• Potential Sump Conditions Layer.
• Overland Relief Flow Accumulation Layer.

These two layers are the result of analysis performed on a Light Detection and Ranging (LiDAR) derived Bare Earth Digital Elevation Model (DEM).

The Potential Sump Conditions layer highlights locations of sumps, or bowl-shaped depressions, where water pools and is unable to drain until the depression fills and overtops. Where structures are within such sump areas those structures may be subject to flood risk. Flooding can also occur when there is a stormwater inlet in the sump, and it becomes overwhelmed due to the magnitude of incoming flows. This layer includes the volume and three-dimensional surface area of the sump areas.

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.   

Fairfax County uses the Flood Analysis Tools to better assess and review flood risk. Applications for the tools include: 

• Assessing existing site conditions when responding to service requests for structural, yard and road flooding.
• Evaluating existing site conditions and potential impacts of localized changes in topography during redevelopment on adjacent properties.   
• Assessing potential impacts of overwhelmed stormwater inlets in sumps on adjacent structures due to ponding of stormwater runoff.   
• Determining topographic features of the sump area including inflows, outflow and extents.  
• Ensuring that proposed elevations of entry points into new structures (i.e., areaways, walk-in basements, window wells) are adequately protected or situated away from the sumps.

The Potential Sump Conditions and Overland Relief Flow Accumulation layers can be found in Floodplain Viewer and the Drainage Basin Delineation Tool. The Potential Sump Conditions layer is available for download in the Fairfax County Open Geospatial Data site. The image service for the Overland Relief Flow Accumulation is located on our server and can be added to ArcGIS Pro and ArcMap. Please visit the Flood Risk Analysis Tools story map for more information on these new datasets.

Please review the Fairfax County GIS Division disclaimer. These layers are provided to improve awareness of flood risk but are not engineering analysis nor hydraulic analysis.

For questions about any LDS GIS apps or data, including the two new layers, please email LDS GIS.