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 over 95 cities that make up the Metro Atlanta Region. As part of the watershed planning efforts, the District coordinates strategies to protect watershed conditions and manage stormwater in conjunction with existing State regulatory frameworks such as the Municipal Separate Storm Sewer System permit program.
Despite efforts to implement these strategies, urban stormwater runoff remains the leading cause of nonpoint source pollution and flooding in the District, leaving watershed managers with significant ongoing stormwater challenges. Additionally, many of the metrics used to measure progress on these issues, such as reducing the number of impaired stream reaches, have proven difficult to achieve. Communities also face implementation barriers such as limited funding, inadequate operation and maintenance, and issues with property ownership and easements. These limitations may cause communities to focus on near-term actions (e.g., sink hole repair) that fall short of addressing the full extent of stormwater challenges while diverting resources from long-term strategies to improve watershed conditions.
Within this context, the District reframed its focus for stormwater and watershed planning in 2022 by developing a new Stormwater Forecast (Forecast). The Forecast is based on estimated urban runoff volumes at the HUC-12 basin-scale that have the potential to be managed by structural control measures (SCMs), such as detention basins. 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 most recent USGS land cover data) and calculates future runoff volume through 2040. The Forecast was developed using runoff management volume calculation methods presented in Volume 2, Section 3 of the Georgia Stormwater Management Manual. The approach applied standard watershed hydrology and site-scale SCM sizing methods for estimating total potential runoff management volume at the basin scale. The site-scale facility design methods were used to support simple conversion of Forecast results to basin-scale estimates of the total potential runoff management volume from development that may be managed by SCMs. Spatial calculations were conducted using Esri ArcGIS software and numerical calculations were performed using spreadsheet methods. A process diagram of the technical approach is presented in Figure 1.
For future planning scenarios (2030, 2040) in the Forecast, the District also developed localized future precipitation frequency estimates reflecting potential impacts from climate change for the District's 15-county area. Future precipitation estimates were developed using Localized Constructed Analogs (LOCA) methods to output a downscaled global climate models (GCM) with a daily timestep and with a spatial resolution of 1/16th of a degree. The Technical Memorandum (TM) that was developed along with the estimates provided a detailed description of the data sources used and types of analyses performed to develop future 24-hour duration design storm precipitation frequency events and the 85th percentile annual storm event for the District study area (CDM Smith, 2021). It should be noted that the scientific study and understanding of climate science is continuously advancing, so potential climate change impacts may need to be revisited as new information becomes available.
The objectives of the Forecast are to expand the District's focus beyond water quality for stormwater management solutions and to better connect water quantity management at both the site-scale and basin-scale. Reframing the District's focus on water quantity may also bring new stakeholders together (public and private; industrial and residential) around a metric that is more easily understood. Finally, as a future phase of the project, pairing the Forecast with an inventory of existing SCMs (capacity and performance) will provide a more comprehensive framework to understand long-term stormwater management needs. 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. Following the publication of the 2022 Plan, the District plans to partner with local jurisdictions to explore different ways the Forecast might inform new approaches to local stormwater challenges. Testing the Forecast under real-world scenarios will provide new insights on the functionality of the water quantity-based indicator to identify opportunities and constraints for stormwater management at the local level as well as inform potential improvements for the Forecast as part of the District's 2027 Plan update. In the near term, local jurisdictions may use the Forecast as a prioritization tool to evaluate their inventory gaps and to prioritize basins with high volumes of runoff relative to development trends or to evaluate basins with a higher proportion of unmanaged runoff.