Urban Plains Water Quality Management Plan and Stormwater Modeling

The Urban Plains by Brandt development will eventually cover an entire section of currently undeveloped land in southwest Fargo. The development plan includes a unique water feature intended to provide a local attraction and stormwater retention. When fully developed, the water feature will be approximately one mile long with a concrete retaining wall along one side and a meandering sloped shoreline along the other. The water feature will be surrounded by beaches, boardwalks, and other park lands intended to allow residents of the community to utilize the water feature for recreational purposes.

Stormwater Retention

In order to assess the performance of the proposed Urban Plains retention system, a pre- vs. post-development runoff analysis was performed for the 2-yr, 10-yr, and 100-yr hydrologic events. In order to make the comparison, a HydroCAD® model was setup to replicate the existing on-site conditions. Existing topography, soils, and land use data was examined and entered into the software. The software was then used to generate peak flows for each event that were utilized for comparative purposes.

To assess the proposed system conditions, the Urban Plains Main Pond Watershed was broken down into 19 individual subwatersheds and each was assigned a Tc and a CN based on each area’s assumed individual characteristics. A model was then set-up using XP-Storm Stormwater Management Model. A trunk stormwater system was designed to handle the 2-yr flows with little or no surcharging (i.e. manning’s open channel flow design). In addition to the 2-Yr Design Event, 10-Yr and 100-Yr event analyses were performed on the system to determine peak elevations and flows throughout the system for each event. The peak discharges for the post-development conditions were then compared to those of the pre-development model to assess the performance of the proposed flood control.

Water Quality Management

In order for the water feature to be both functional and aesthetic, AE2S was retained to perform a water quality management plan. The water quality evaluation included estimating the nutrient and sediment loading to the main pond from the proposed land uses, determining the appropriate water quality control strategies, and providing water quality control recommendations for construction of the water feature. A smaller “water quality rain event” was developed for the area through a statistical analysis of historical rainfall records. The water quality event was then modeled to develop an understanding of water quality control parameters, and to determine low-flow performance of best management practices (BMP’s) to ensure an attractive water feature.

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