Modeling first flush and particle destabilization: Implications for design and operation of stormwater BMPs

First flush, which describes a greater pollutant discharge rate in the earlier part of a storm event, is an important phenomenon to understand pollutant washoff behavior of stormwater runoff and to establish appropriate best management practices (BMPs). A number of partially successful efforts have been made to reveal the relationship between first flush and the factors involved in the physical processes, including the characteristics of the rainfall and the watershed.; In this study, a deterministic model was developed to predict the mass first flush (MFF) and to utilize it for better design of BMPs focusing on treating the first flush. The model used the kinematic wave equation to calculate flow. For the water quality calculation, the mass transport equation and erosion equation were used with the concept of two different sources of pollutant mass. The model parameters were calibrated by a parameter estimation procedure using three years' monitoring data from a highway runoff site. The MFF simulation results showed that there exists an optimum watershed size to maximize MFF and by maximizing MFF, BMPs can improve overall reduction of pollutant mass. Contours of watershed length, developed using MFF simulations for different conditions of rainfall and watershed geometry, can be used to design runoff collection system of a BMP for highways and parking lots.; Another aspect of BMP design is how to enhance pollutant removal efficiency. Although most stormwater BMPs use gravity settling to remove particulates, particles in highway runoff are generally too small to be removed effectively. Therefore, additional means to destabilize particles are required. Alum, ferric chloride and cationic polymers were evaluated to destabilize particle in the runoff. Alum or ferric chloride alone was not successful due to large dose requirement and sludge production. A small dose addition of polymer followed by a long period of gentle mixing was successful. The required dose for polymer and coagulant were proportional to the initial conductivity of the runoff and therefore, unattended operation in the field might be possible by controlling the dose based on the initial conductivity.