Modeling Impacts of Conventional and Low Impact Stormwater Controls on Receiving Streams

This paper presents a methodology to measure the impact of stormwater controls on the receiving stream using a series of discrete events to represent the long-term hydrologic regime as a simpler alternative to continuous simulation. Hydromodification impacts on channel morphology are best evaluated using flow and shear stress duration curves, which typically require continuous simulation. The methodology proposed herein allows the development of these duration curves using discrete events. The methodology was tested in a hypothetical development in the City of Columbia, Missouri. Design assumptions were made for a typical residential development and a typical stream. Various stormwater controls were designed for this site, including conventional detention, a water quality volume and channel protection volume basin, and bioretention. The outflow from each design scenario was analyzed to determine the effectiveness of different levels of stormwater controls. The objective was to compare the effect of each scenario using duration curves to measure its ability of mimic the original hydrology. The study indicates that LID most closely replicates the original regime and that detention provides some benefits when water quality and channel protection are included, although its effect does not reproduce the original hydrology. Detention criteria based on extreme events, such as the 100-year storm event, are ineffective for this purpose.