First Flush Concepts for Suspended and Dissolved Solids in Small Impervious Watersheds

Abstract

Eight rainfall-runoff events were examined from each of two small paved urban transportation land use watersheds (A=544 m² and 300 m²) in an attempt to distill multiple definitions of the first flush phenomenon into a consistent framework and examine common volumetric capture requirements. Results indicated that two separate criteria must be employed to describe the delivery of suspended sediment concentration (SSC) and total dissolved solids (TDS) as aggregate indices of entrained particulate and dissolved matter. The concentration-based first flush criterion is defined by high initial SSC or TDS concentration in the early portion of a rainfall-runoff event with a subsequent rapid concentration decline. In contrast, the mass-based first flush (MBFF) has several published forms, shown to be equivalent herein. The MBFF is defined generally as a disproportionately high mass delivery in relation to corresponding flow volume. For mass-limited events, mass delivery was skewed towards the initial portion of the event while the mass delivery in flow limited events tended to follow the hydrograph. This study also investigated published estimates of the water quality volume (WQV); assuming that an in-situ Control Strategy or Best Management Practice (BMP) captures and treats only this WQV, while flows in excess of this volume bypass the BMP. For the two watersheds, results indicate that a relatively large runoff volume must be captured to effect meaningful reductions in mass and concentrations (as event mean concentrations) despite a disproportionately high mass delivery early in the event. Results suggest the potential for misinterpretation of overall BMP effectiveness may be significant based on use of a number of these common published estimates based on a WQV.