Calculating Overflows from Combined Sewer Systems

Determining annual overflows from combined sewers is essential for evaluating overflow impact on receiving waters. A statistical methodology is developed based on acceptable hydrologic principles and correlating data with a neighboring combined sewer district. Calculation begins with estimating the average annual set of rainfall events. These are converted into the average annual set of rainfall excesses by applying an infiltration index to pervious areas and a depression storage value to impervious areas. For this calculation, rainfall excess can go to system storage, flow to the treatment plant, or overflow to natural water courses. The sum of overflows resulting from calculations on each of the 152 annual occurrences is the average annual value of overflow. The calculated values, correlated with the neighboring district, are within 2% of those observed when comparing average annual rainfall excess, flow to treatment plant, and overflows.