Development of Design Criteria for Regional Stormwater Management Facilities to Maintain Geomorphic Stability in Cedar Creek

Abstract

The Cedar Creek Watershed located in Lenexa, Kansas is presently undeveloped, but plans for residential communities and an urban business park are currently being designed for this rapidly growing Kansas City metropolitan suburb. The City of Lenexa, while wishing to accommodate responsible development, is committed to maintaining the natural setting and geomorphology of the stream corridors that will receive the drainage from these new urban developments. They propose to do this through a combination of on-site BMPs and regional detention basins. This paper discusses the procedures that were used to determine the design criteria for the regional detention basins so that the current geomorphic condition in the natural stream system would be preserved to the maximum extent practicable. Fifty-three years of historical precipitation data were applied to the USEPA SWMM model to conduct continuous hydrologic and hydraulic simulations, generating continuous stream flow hydrographs in the receiving stream channels. On-site BMP and regional detention criteria were selected to allow post-development replication of pre-development flow frequency curves and the critical portions of the shear stress duration curves. The critical portion of the shear stress curve is described as the duration of shear stress magnitudes above which stream bed and bank erosion are expected to take place. Critical shear stress values were identified during field surveys. Critical flow thresholds were extrapolated from critical shear stress values. These critical flow thresholds were used to identify return frequencies above which the duration of stormwater must be controlled to pre-development levels. Instream continuous stage data generated by the SWMM model were used to examine erosion potential through the use of an erosion potential index. This index calculated increases in excess shear between pre-development and various post-development management control scenarios, allowing identification of the stormwater management scenarios that allowed erosion potential to remain constant under future development conditions to the maximum extent practicable.