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Mapping Internally Drained Areas to Reduce Flood Risk

Mapping Internally Drained Areas to Reduce Flood Risk

Internally drained areas are basins that act as closed watersheds because they are not connected to streams or other drainage due to their topography. They often have permanent or ephemeral wetlands or ponds at their low point.

Development in internally drained areas can result in an increased volume of stormwater runoff and increased water levels in local ponds and wetlands. Increases in runoff attributable to development in a closed watershed can increase local flood risk. If these areas are drained to a previously unconnected downstream water body, it can lead to an increase in flood risk downstream.

In 2016, the Dane County Lakes & Watershed Commission and the Capital Area Regional Planning Commission convened a joint Stormwater Technical Advisory Committee (TAC) to address concern over flooding caused by increased runoff from new urban development. The TAC ultimately produced a set of recommendations aimed at Reducing Increased Risk of Flooding. The report includes recommended standards for stormwater management in internally drained areas to protect local properties and aquatic resources located wholly or partly within closed watersheds.

One of the steps toward better managing flood risk is to identify and map the region’s internally drained areas. To accomplish this, CARPC staff produced an ArcGIS Online map, now available in our Open Data Portal, to assist communities in identifying these areas.

This map is the result of over 1,000 staff hours spent identifying and mapping culvert locations and conducting hydrologic analysis utilizing GIS tools. Used in conjunction with recommendations from the Reducing Increased Risk of Flooding report, the internally drained areas map is an important tool for improving stormwater management and reducing flood risk in the capital region.