Alternate: Stormwater Forecasting – A New Indicator to Manage Basin-Scale Urban Runoff Volume

Since 2001, the Metropolitan North Georgia Water Planning District (District) has focused on water supply, wastewater, and watershed planning through an integrated Water Resources Management Plan (Plan) for the 15 counties and 95 cities that make up the Metro Atlanta Region. As part of the watershed planning efforts, the District has coordinated strategies to protect watershed conditions and manage stormwater in conjunction with existing regulatory requirements. However, despite these efforts, urban stormwater runoff remains a leading cause of nonpoint source pollution and flooding in the District, leaving watershed managers with ongoing challenges related to water quality, streambank erosion, and nuisance flooding. Historically, managers have focused on strategies to address water quality rather than water quantity, even though they are fundamentally linked. For these reasons, the District has developed a novel water quantity-based indicator, called the Stormwater Forecast (Forecast), as part of the integrated 2022 Water Resources Management Plan. The Forecast is a planning-level estimate of the total potential runoff management volume from development, calculated at the basin scale using site-scale post-construction stormwater performance standards. The calculation estimates runoff volume in cubic feet for three post-construction stormwater performance standards used in the District: Water Quality Volume (1.2-inch storm event), Channel Protection Volume (1-year, 24-hour storm event) and Overbank Flood Protection Volume (25-year, 24-hour storm event). The Forecast provides a planning-level estimate of runoff volume from all historical and existing development through 2019 (using the most recent USGS land cover data) and calculates future runoff volume through 2040.With this approach, the Forecast represents total potential runoff management volume from development with the potential to be managed by Stormwater Control Measures (e.g., stormwater ponds or bioretention basins) if current post-construction stormwater management standards were fully in place. The goals of the Forecast are to provide additional information for improved decision making at both the site-scale and basin-scale, to support a better understanding of overall needs, and to help generate new solutions and policies that address ongoing stormwater management challenges. Future use of the Forecast is intended to complement and support the implementation of existing water quality and flood reduction goals in the District. This presentation will focus on the basic framework of the Stormwater Forecast as well as an overview of the technical approach and future implementation.