MODELING WET WEATHER IMPACTS TO BOSTON'S CHARLES RIVER

This paper describes the development, calibration, and application of a two-dimensional hydrodynamic and water quality model of the Lower Charles River in Boston, Massachusetts. This stretch of the river supports extensive recreational activities and is a prominent feature in the city of Boston. The objective of the model was to assess the impacts of stormwater and combined sewer overflow (CSO) discharges during wet weather, and to evaluate the benefits of CSO reductions and stormwater best management practices (BMPs).A two-dimensional model of the Charles River was developed using the MIKE 21 software package and was applied to simulate fecal coliform and dissolved oxygen in the river during storm events. This paper concentrates on the fecal coliform simulations. Flows and loads from the discharges were determined through a combination of field measurements and collection system modeling using the Stormwater Management Model (SWMM) software package. An existing SWMM model of the combined sewage system was used to predict CSO discharges, and another SWMM model was developed by the United States Geological Survey (USGS) to predict stormwater discharges from separated areas. The river model was developed and calibrated using field measurements collected by the USGS, the United States Environmental Protection Agency (USEPA), the Massachusetts Water Resources Authority (MWRA), and the Charles River Watershed Association (CRWA).After calibration, the model was applied to assess stormwater and CSO impacts over a 4-month recreational season (June to September) as well as for the 3-month and 1-year design storms.Long term simulations were conducted for conditions corresponding to 1995 (before initiation of the CSO program), 2002, and the MWRA recommended CSO control plan combined with stormwater BMPs. The model indicates that the reduction of CSO activations and the elimination of illegal connections implemented since 1995 have resulted in significant improvement in terms of swimming criteria violation. The improvements were reflected both during the dry and wet weather conditions. The water quality improvements resulting from the recommended CSO control plan and stormwater BMPs is more significant during wet weather conditions.Design storm simulations using the 3-month and 1-year design storms were used to examine the effect of an 18% percent reduction in storm drain fecal coliform concentration that could be expected from extensive stormwater BMP implementation. The model showed that relatively small improvements would be obtained relative to the swimming and boating standards. Also, although the 1-year storm has greater precipitation and maximum rainfall intensity compared to the 3-month storm, the latter has more extensive violations because the additional stormwater flow in the 1-year storm provides additional dilution and, thus lower discharge concentrations.