The Impact of Smaller Detention Basins on Flood Hazard Areas in Lenexa, Kansas

In the early 1990s, the City of Lenexa, Kansas (City) adopted a flood control standard which required new developments to limit post-construction 1-percent-annual-chance flow rates to 2.79 cubic feet per second (cfs) per acre. In 2003, the City adopted new standards (American Public Works Association Specification 5600) that removed the 1-percent-annual-chance requirement but included matching the 100-percent-annual-chance and 10-percent-annual-chance post-development flow rates to pre-development flow rates (Lenexa City Code, January 2008). As a result of these requirements, many detention basins have been constructed in the past two decades have not been included as part of a city-wide flood hazard analysis. To remedy this omission, the City initiated the East Lenexa Detention Study (ELDS) to quantify the effect of the small detention basins on peak flow rates and water surface elevations. The purpose of this study was to evaluate the impacts of the smaller detention basins on flood hazard areas in Lenexa using HEC-HMS and HEC-RAS models. For the hydrologic analysis, HEC-HMS models were used to simulate peak runoff from the 50 percent, 10 percent, and 1 percent annual chance storms. For the hydraulic analysis, HEC-RAS models were used to estimate water surface elevations using the peak flow rates estimated by the HEC-HMS model.