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Evaluation of Runoff Volume Nondegradation Standard
The Vermillion River Watershed Joint Powers Organization (VRWJPO) recently implemented standards for new development and redevelopment intended to prevent degradation of stream physical, chemical and biological integrity. The new VRWJPO standards incorporated development design criteria
for volume and temperature control to supplement existing peak runoff rate controls for protecting trout stream resources in the watershed, which is located in the outskirts of the Minneapolis-St. Paul, Minnesota metropolitan area. Developers are required to apply the combination of these
criteria to the stormwater management practice design to ensure that each site meets the peak runoff rate requirements for the one-, two- and 100-year return period design events and that the runoff volume and temperature control standards are met for the two-year design event. Since most
of the watershed is currently agricultural, with some woodland, grassland and existing urban development, there was a concern that the design criteria may not provide adequate protection of the water resources on a long-term basis given the added runoff volume, associated with additional imperviousness,
during larger storm events and runoff events resulting from back-to-back storms or snowmelt runoff. As a result, this project was developed to evaluate the various undeveloped and developed design scenarios, both with and without the volume and peak runoff rate control requirements, based
on continuous simulations of the hydrologic/hydraulic conditions in the XP-SWMM model. The development scenarios included typical low-density residential and high-density commercial sites. The XP-SWMM modeling also included design event simulations of each development scenario that were used
to design the hypothetical management practices and used as input for a thermal model completed under a separate project (not described here). This project also developed costs estimates for the stormwater management infrastructure associated with each development scenario. The results of
the continuous and event based simulations indicated that the VRWJPO runoff control standards will significantly improve upon the flow frequency and flow duration characteristics predicted for agricultural land uses, ensuring adequate protection of the predevelopment flow conditions and offsetting
some of the volume impacts of past watershed development. Normalizing the cost of controlling peak flows by dividing the incremental cost estimates by the peak runoff rate modeling results for the two-year design event also indicates that the volume control portion of the standards provides
a significantly more cost-effective means for reducing the peak flows for each development site. In addition, the volume control standard is the only means for reducing runoff volumes, which will reduce the additive erosive effects of flow as the spatial scale expands from individual development
sites to the overall watershed.
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