Complexities of Flood Mapping in a Sinkhole Area

Many of the flooding problems in Bowling Green, Kentucky are associated with sinkholes. Many existing sinkhole flood zones are approximate and do not conform to surface topography. An effort to update flood maps and calculate flood elevations for four large sinkholes was begun to assist the City of Bowling Green. Problems with the mapping were encountered for two of the sinkholes that lie along Nashville Road. For these two sinkholes, flood elevations based on topographically delineated watersheds were far below observed elevations. Sources of flood elevations higher than predicted could arise from two sources, (1) flooding and spillover from uphill sinkholes, and (2) back flooding from the subterranean Lost River. This project addressed the first of these two causes. The larger of the two sinkholes, Nashville Road North had direct runoff from 11 hectares, but when spillover was considered, the drainage area grew to more than 520 hectares. GIS was used to determine volumes of sinkholes at the elevation of spill points. It was found that the volume held had little correlation with the catchment draining directly to the sinkhole. The EPA Storm Water Management Model (SWMM 5.0) was used to calculate runoff and hydraulics for this complex of sinkholes. To calibrate the model with a large storm that occurred in April 1998, it was necessary to increase the losses from the runoff to a level significantly higher than recommended. This is justifiable for Bowling Green because of numerous stormwater injection wells that receive part of the runoff.