PRE- AND POST-RETROFIT MODELING OF A STORMWATER BASIN IN FARGO ND

Many holistic techniques are now available to manage stormwater runoff from urban areas and include low impact design, permeable pavements, and other green infrastructure practices that encourage infiltration, evapotranspiration, and abstraction of water. Although these techniques can decrease runoff volume and pollutant loads, many highly urbanized cities were developed to include detention/retention basins that simply collect and attenuate stormwater runoff. Furthermore, these detention/retention basins are often designed to reduce peak flows of 10- and 100-year rainfall events and include concrete low-flow channels. Therefore, small but frequent storms (< 1-yr) are mostly un-attenuated and larger storms (> 1-yr and < 10-yr) are released at a much higher rate compared to pre-development rates. While many retrofit detention/retention studies focus on increased attenuation and ponding time via outlet control, which can ultimately increase evaporation and sediment fallout, other studies utilize a more holistic approach by substituting the concrete-channel and mowed grass for an earthen-channel and native vegetation, which increases potential for infiltration, evapotranspiration, and attenuation. “The Fargo Project” located in Fargo, North Dakota is an 18-acre stormwater detention basin that collects runoff from highly urbanized areas. Before being retrofitted in 2017, the detention basin consisted of a concrete low-flow channel and mowed grass within the floodplain, while it now includes an earthen channel, sediment forebay, and native vegetation. Goals of this study were to assess how the post-retrofit earthen-channel performs relative to the pre-retrofit concrete-channel in terms of conveyance of small storms, and to estimate infiltration and evaporation during various storm sizes and intensities. Monitoring of the post-retrofit basin took place between May 1, 2018 and September 31, 2018 via Onset HOBO water level loggers and weather station, a manual survey of the earthen-channel and sediment forebay, and a UAV flight to estimate a DEM. HEC-RAS models were created for both the pre- and post-retrofit channels, and water level data were used to estimate post-retrofit channel flow to compare with pre-retrofit channel conveyance capacity and flooding. ArcMap was used to estimate ponded volume, area, and average depth within the post-retrofit basin for various storm sizes and intensities at one-hour times steps. HYDRUS-1D was then used to estimate infiltration and evaporation.