A Hybrid Approach to Modeling Sediment for Total Maximum Daily Load Development

Abstract

Total maximum daily loads (TMDLs) have been developed for sediment in more than 50 watersheds throughout Virginia. The majority of these sediment TMDLs have been developed using the Generalized Watershed Loading Function (GWLF) model and a reference watershed approach. In this approach, the TMDL is set based on area-adjusted, GWLF-modeled sediment loads in a similar unimpaired watershed. While this approach has been used extensively, it possesses significant shortcomings. The approach is extremely sensitive to reference watershed selection, and modeled sediment loads are rarely calibrated or referenced to any measured sediment load. To avoid these limitations and accentuate the strengths of the GWLF model, researchers in Virginia developed a hybrid modeling approach for sediment TMDLs that combines GWLF modeling with load duration modeling. This hybrid modeling approach utilizes the strengths of each modeling tool while avoiding some of the limitations. This approach has been used successfully for sediment TMDLs developed in the Rivanna River, South River, and Spout Run, Virginia.