Bacteria loads from a combined sewer system were simulated as part of the development of seven Total Maximum Daily Loads in the Middle James River watershed in Lynchburg, Virginia. While a Storm Water Management Model (SWMM) model had been developed to simulate the hydraulic response
of the City's combined sewer, this model was not available for this study. The lack of an available sewer system model, limited data describing overflows, and a rigid project schedule made it necessary to simulate these loads using a simplified but laborious approach. Linear regressions
of available data were performed to characterize system responses and fill gaps in the data. These regressions were used to estimate overflow responses to simulated precipitation and synchronize the responses with the model. This approach adequately simulated the bacteria loads from the combined
sewer for the purpose of developing the TMDLs, with no false spikes and an estimated annual bacteria load of the same magnitude as the bacteria load estimated by the City's system model. While the described approach provided a good result for the purpose of developing the TMDLs with no
waste load allocation given to the combined sewer with the data conditions encountered, use of combined sewer models is preferred when available and necessary if a waste load allocation is to be assigned to the combined sewer.
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