FIRST FLUSH CRITERIA FOR SOLIDS IN SMALL IMPERVIOUS WATERSHEDS

Rainfall-runoff events were examined to determine if a consistent interpretation of the first flush could be reached. It was determined that two definitions must be used to discuss the first flush including a concentration-based first flush, in which there is are high initial solids followed by a rapid reduction in concentration and a relatively low concentration for the remainder of the event, and a mass-based first flush, which is defined by a disproportionately high delivery of solids mass in relation to the hydrograph. A sampling methodology is recommended based on the chosen remediation goal as well as the definition of the first flush that is used. Finally common values for the water quality volume were investigated to determine the potential for mass and concentration bypass from a BMP designed to capture and treat only the WQV. Results indicate that without knowledge of site quantity-quality interactions with respect to a WQV based on site characteristics, common WQV criteria could result in significant bypass of solids and in the absence of site data suggest that the entire event requires capture and treatment.