A New Method for Sizing Flow-Based Treatment Systems to Meet Volume-Based Standards

As BMP sizing standards have evolved towards a focus on volume-based approaches (e.g., capture of the first 1 inch of runoff), sizing methods are typically absent or lacking for flow-based BMPs. Yet, flow-based BMPs that are certified with high pollutant removal performance can serve an important role in stormwater management in space constrained areas or as part of treatment trains in combination with volume-based systems. In response to these factors, Modular Wetland Systems, Inc. commissioned the development of a new sizing methodology for forty-five regions within fifteen states on the eastern seaboard of the United States for the flow-based MWS Linear treatment system. This methodology is based on providing equivalent long term capture efficiency of stormwater runoff volumes as compared to volume-based regulatory standards. The sizing methodology developed first involves establishing a target capture efficiency for each region, obtained directly from state guidance, or developed via continuous simulation modeling using long-term, high-resolution precipitation data. Facility flow rates needed to achieve the target long term capture efficiency for different drainage area conditions were then calculated via continuous simulation modeling. Findings were normalized and used to develop sizing tables to allow for application to flow-based facility sizing for user-specific drainage area types.