The North Carolina Department of Transportation (NCDOT) needs an efficient method to assess potential adverse effects of highway runoff on receiving waters to optimize stormwater-treatment decisions. The NCDOT, in cooperation with the U.S. Geological Survey (USGS), developed a decision-support tool based on the North Carolina version of the USGS Stochastic Empirical Loading and Dilution Model (SELDM). This tool is designed to identify potential adverse effects of highway runoff by using a criterion based on a measurable change in water quality from a surrogate pollutant, such as suspended sediment. Design engineers can use this decision-support tool to establish stormwater treatment goals for the project without having to learn SELDM or interpret its statistical output. The decision-support tool is an excel workbook that requires input values for various upstream basin properties (drainage area, ecoregion, etc.) and highway preliminary design specifications (length, width, etc.). The tool then generates a report that provides a suitable stormwater treatment goal, one of: direct discharge, basic BMP treatment, or advanced BMP treatment. NCDOT’s policy is for the minimum treatment goal to include basic BMP treatments whenever possible, but the direct discharge is included to demonstrate when untreated highway runoff is expected to have minimal impact to a receiving water body. The NC SELDM decision-support tool represents practice-ready methods that can be modified by other DOTs and municipal permittees to streamline project planning and preliminary design while minimizing impacts to water quality.
