Marne Creek Flood Mitigation	Platte Eastside Drainage Project	Tea Storm Sewer Project	Willow Ridge Drainage Study

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Willow Ridge Drainage Study

Yankton, South Dakota

A new subdivision, Willow Ridge, began expanding in Yankton in a low-lying area of land to the north of the city. Eisenbraun and Associates was consulted to address the storm drainage issues in this new housing development. Part of the scope of the study was to determine the feasibility of a wet detention pond that would function both as a flood control structure and aesthetic enhancement for the development. Primary parameters that needed to be considered were a normal pond depth of eight foot to deter algae growth, and also sufficient storage capacity to limit flows resulting from a 100-year storm to historic values. Eisenbraun and Associates’ Willow Ridge Drainage Study utilized the computer model, TR-55 Urban Hydrology for Small Watersheds, to assess the impact that the 180-acre Willow Ridge Development would have on stormwater flows originating from a 1.1 square mile area. Flood frequency intervals ranging from 5 to 100 years were analyzed to determine channel configuration and to determine minimum building elevations for the subdivision. The E/A design team determined that their client should abandon the wet detention pond concept for fiscal reasons even though technically it would be feasible to construct.
Instead, the constructed flood control project consisted of a grassed trapezoidal channel that bisects a shallow 4-acre basin. This basin is dry except during storm events and provides a visually pleasing green space within the development. Minor drainage events exit the development through a 48 foot RCP culvert beneath Donohoe Boulevard, which is the lower boundary of the project area. An outfall structure at the lower end of this basin in conjunction with an engineered flood spillway across Donohoe Boulevard limits storm flows leaving the development to predevelopment levels. Both water quality improvement and flood control were accomplished by the implementation of this project.