The Narragansett Bay Commission (NBC) owns and operates sewer collection systems and wastewater treatment facilities (WWTF) in the Narraganset Bay watershed. In the upper bay, a number of the sewer collection systems are part of combined sewer/stormwater infrastructure which is susceptible to combined sewer overflows (CSOs). These CSOs introduce untreated sewerage in to the bay during larger precipitation events due to the large stormwater volumes. The sewerage results in elevated fecal coliform (FC) concentrations within the receiving waters which poses public health risk through both contact of recreational water users and potential toxic effects from shellfish consumption if the shellfish is exposed to these bacteria, and many shellfishing areas have been closed to prevent harvesting of affected shellfish. NBC is evaluating various design alternatives to reduce CSO frequency and severity.
RPS performed numerical modeling and analysis in support of evaluating the alternatives with respect to water quality benefits. The model predicted spatially and temporally varying FC concentrations within Narragansett Bay. The model results were post processed to determine the acre-days above critical thresholds for shell fishing closure and recreational contact.
Evaluation of Water Quality Benefits from CSO Alternatives for Narragansett Bay
Narragansett Bay Commission through MWH Global (now part of Stantec)
NBC wanted to evaluate water quality benefits from various design alternatives for various precipitation events. While a first order approximation of benefit can be made by evaluating the total loading of various scenarios, NBC wanted to investigate whether the alternatives would reduce the frequency and duration of FC concentrations in shellfishing areas. Since the desire was to evaluate multiple loading scenarios that are theoretical in nature, the evaluation could not be performed using observations, which are sparse and only reflect present day loading. Therefore, NBC needed a model that could be used to simulate various theoretical scenarios and further needed analysis to interpret the trends observed from sparse observations.
RPS developed a model of the receiving waters that included tidal, riverine, and meteorological boundary forcing as well as integrated FC loading and flows from the WWTFs and CSOs. In support of the model application development, in-situ observations were gathered and analyzed for defining boundary conditions or used for model verification. The model application was first verified through a simulation of a time period with available in-situ observations that experienced overflows. The model was used to simulate the theoretical 3-month and 12-month recurrence precipitation events and corresponding loading for the various design alternatives.
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