COMPARISON OF WET WEATHER FLOW ESTIMATION TECHNIQUES

Many collection systems and wastewater treatment facilities are designed and analyzed using a variety of wet weather flow prediction methods. The accuracy of a method can be highly variable depending on the implicit assumptions used and the way in which the method is applied. Inaccuracies resulting from inappropriate assumptions may result in unnecessary costs, ineffective permits or system failure.This paper compares the advantages and disadvantages of wet weather flow estimation techniques. A comparison of several techniques including the rational method, regression method, synthetic unit hydrograph method, and the non-linear reservoir method are included. The non-linear reservoir method, using a modified version of SWMM RUNOFF, is applied to predict SSOs, urban stormwater, and rural watershed flows. This method can be used to accurately predict wet weather flow within +/− 5 percent of measured flows.Examples are included to illustrate the limitations of several flow estimation techniques as they are applied to predicting wet weather flows in a sanitary sewer system. A modified version of SWMM RUNOFF is also applied to predict urban stormwater flows and rural wet weather flows discharged from the Napa River watershed. A watershed approach was applied using the nonlinear reservoir method to predict different components of wet weather flows for an SSO elimination program in Northern California.