Examining the Impact of Detention Basins on Floodplains in Lenexa, Kansas

The impacts of stormwater controls within headwater streams are rarely accounted for when Federal Emergency Management Agency floodplains are delineated even though stormwater controls, and specifically detention ponds, are often designed to control peak discharges from 10-, 25- or 100-year runoff events. Inconsistency in the maintenance of stormwater controls impacts their reliability for control of peak discharges, but it is hypothesized that if these facilities are properly and consistently maintained, their impacts on floodplains should be considered in their delineation. The City of Lenexa, Kansas is currently undergoing a study to develop a strategy for maintaining and improving existing detention facilities with the goal of refining floodplain boundaries, providing long-term stream stability, and improving water quality. As part of this project, an assessment was conducted to inventory existing detention basins within the study area by reviewing aerial/topographic mapping and as-built information combined with a field reconnaissance. It was found that a significantly greater number of basins existed within the study area than was originally anticipated. The impacts of the detention basins were evaluated using HEC-HMS and HEC-RAS models to better understand the function and necessity of the existing facilities, to determine the level of service of the existing basins and to determine their combined impacts on receiving streams from both a flooding standpoint and a geomorphologic standpoint. For the hydrologic analysis, an existing HEC-1 model was converted to a HEC-HMS model, and then modified to provide storage nodes that reflect the existing detention ponds. For the hydraulic analysis, HEC-RAS models were used to simulate the 2, 10, and 100-year design storms using the peak flow rates estimated by the HEC-HMS model. These analyses will be used to create a detention basin strategy that maximizes flood reduction potential for the 10 and 100-year design storms through improved maintenance of existing detention basins, retrofitting of existing basins, and the identification of locations that would be beneficial for new detention basins.