Modeling of CSO-Induced Pollutant Transport in Mt. Hope Bay

Abstract

Combined sewage overflow (CSO) discharges due to a storm event create strongly stratified pollutant plumes on the surface of Mt. Hope Bay, of Rhode Island and Massachusetts. To examine the CSO-induced pollutant impact, a three-dimensional numerical model was used to simulate the transport processes in the bay resulting from a dye release at CSO discharges during a storm event on September 22, 1990. The model used a vertical γ-coordinate system, with higher grid resolution near surface, to describe the constituent transport. Model predictions reasonably showed that the CSO plume remained concentrated near the surface and was transported down bay along the eastern shore. Agreement of predictions of sea surface elevation, currents, salinity, and dye concentrations with available observations was good. Time series of predicted dye concentrations were used to estimate the flushing time, which was approximately 2 days at midbay. Sensitivity studies were performed for several vertical-grid resolutions. The use of the γ-coordinate system was shown to be more efficient for describing the CSO-induced transport processes, with relatively less computational cost.