New Guidance and Tools for Evaluating Water Quality Performance, Volume Reductions, and Life-Cycle Costs for Highway Stormwater Controls
Two new national-scale guidance documents and associated BMP evaluation tools have recently been completed as products of multi-year projects under the National Cooperative Highway Research Program (NCHRP). These projects will be summarized as a two-part presentation that will describe the resulting guidance, summarize the related analysis tools, and identify how these tools can be utilized and/or customized by local jurisdictions including non-DOT entities.
The first project, NCHRP 25-40: “Long-Term Performance and Life-Cycle Costs of Stormwater Best Management Practices”, resulted in the development of detailed guidance on the performance, maintenance, and costs associated with BMP implementation in the highway environment. BMP Evaluation Tools were also developed in a spreadsheet format that provides average annual performance and whole life costs for treatment BMPs. Concentration reduction performance information is based on influent-effluent statistical regression of BMP performance studies contained in the International Stormwater BMP Database. Highway runoff water quality was based on the FHWA Highway Runoff Database and the National Stormwater Quality Database; however, the user is free to overwrite or modify these values. The tools support analysis of vegetated swales, vegetated strips, dry detention basins, wet ponds, bioretention, permeable friction course overlays, and sand filters. The tools are designed to allow simple site-specific inputs, conduct necessary intermediate calculations, and provide output on pollutant effluent concentrations, load reductions and whole life cost for each BMP. Cost per unit weight or measure of a pollutant is also provided to assist in optimizing BMP selection for specific pollutants of concern.
The second project, NCHRP 25-41: “Guidance for Achieving Volume Reduction of Highway Runoff in Urban Areas”, resulted in the development of guidance on the selection, performance, and design of volume reduction approaches (VRAs) that considers physical site constraints and competition with other design objectives when trying to retain and infiltrate stormwater runoff near or within roadway rights-of-way. The guidance include practical recommendations for assessing site conditions, a comprehensive menu and conceptual designs of highway-specific VRAs, and a structured prioritization method for evaluation and selection of control approaches. A spreadsheet tool has also been developed that can be used to quickly compare VRAs (or series of VRAs) given site conditions and constraints. The types of VRAs included in the tool include: vegetated conveyance, dispersion, media filter drains, bioretention, permeable pavement shoulders, infiltration basins, infiltration galleries, and infiltration trenches.
A significant value of the tools developed for these two projects is the ability to estimate the volume captured and volume reduced by stormwater control BMPs. Most BMPs, whether designed for infiltration or not, have associated volume loss from infiltration and to a lesser extent, evapotranspiration. Volume captured and reduced is estimated by the Tools by simulating a range of conditions (physical BMP dimensions, soil type, volume input) using the USEPA’s Storm Water Management Model (SWMM). A large number of continuous simulations were run to develop hydrologic performance curves for 340 long-term rain gages across the country that are incorporated into the tools to account for volume captured and reduced via infiltration and evapotranspiration. These tools are expected to be a resource for users in the transportation environment as well as other project types where rapid BMP assessments are needed to assist with stormwater planning efforts.