WET WEATHER FLOW MODELING AND RISK ANALYSIS FACILITATE DEVELOPMENT OF EWA'S COST–EFFECTIVE PEAK FLOW MANAGEMENT PLAN

The Encina Wastewater Authority (EWA) is a joint-powers agency which provides wastewater treatment and disposal services for the 250,000 people who reside within the jurisdiction of EWA's six member agencies in North San Diego County. EWA recently developed a Peak Flow Management Plan, which evaluated effluent equalization storage as an alternative to a new ocean outfall. A unique aspect of the study was the application of a risk-based approach for projecting future flows and facility requirements. Central to implementing this approach was the development of a continuous simulation model of wet weather flows and storage/outfall facilities. The model represents the hourly variation of wastewater flow due to normal diurnal patterns and rainfall, considering how the volume and shape of infiltration/inflow hydrographs are affected by antecedent rainfall. Following calibration of the model to a year's worth of rainfall data, the model was run for a 50-year period of historical rainfall and ocean tides to generate estimates of peak flows and storage volume requirements over a range of return periods.This paper describes the development, calibration, and application of the continuous simulation model to evaluate storage and outfall alternatives. The model's ability to quantify both the costs and the risks involved in each alternative allowed EWA to adopt a cost-effective peak flow management plan. The innovative concepts and modeling methods used in this study should be of interest to agencies seeking solutions for managing peak wet weather flows within a collection system or at a wastewater treatment plant.