REDUCING UNCERTAINTY IN PEAK WET WEATHER FLOW PROJECTIONS: THE DURHAM INFLUENT PUMP STATION DESIGN STUDY

Clean Water Services (CWS) provides wastewater collection and treatment services to the western suburbs of Portland, Oregon. To accommodate planned growth in a portion of their service area, CWS is upgrading the Durham Advanced Wastewater Treatment Plant and the associated Influent Pump Station (IPS). Two previous studies had arrived at very different estimates of future 5-year return period peak flows, one based on statistical analysis of observed flows and one based on single-event modeling. Which of these design flows were selected would have significant implications in terms of the initial cost of the pump station and its future performance. After reviewing the previous studies and performing an initial assessment of factors affecting rainfall-dependent I/I (RDI/I) in the Durham service area, the IPS design team and CWS decided to apply a continuous simulation model to reduce the level of uncertainty in the design flow estimates.A key feature of the model applied in this study is its ability to simulate the effects of antecedent rainfall on RDI/I. The model was calibrated to observed hourly flows over an eight-year period that included years having low, average, and high rainfall. Following a successful calibration, the model was run for 50 years of historical rainfall records to determine the 5-year return period peak flow. After modeling several future scenarios that varied in terms of development and assumed changes in RDI/I, the project team was able confidently proceed with the design of the Durham IPS.This paper describes the features of the continuous simulation model that was developed, emphasizing both the key advantages and limitations of the model. Calibration results are presented and discussed, as well as the final design flow estimates. The paper should be of interest to agencies and consultants who are looking for ways to reduce uncertainty in peak wet weather flow projections that provide the basis for the design of sewers, pump stations, and wastewater treatment facilities.