Combined Sewer System Capacity Risk Assessment and Mitigation

Abstract

The City of Portland, Oregon, Bureau of Environmental Services (BES) recently completed a Wastewater Sewer System Plan to address capacity and structural deficiencies in the combined and separated sewer systems. BES developed this plan by applying Asset Management principles for business risk (in dollars). Risk is defined as the product of consequence of failure (in dollars) and likelihood of failure (probability). Projects were developed to mitigate the deficiencies and meet BES levels of service (LoS). BES considered impacts of structural and capacity failures, including sinkholes from collapsed sewers, basement sewer backups, surface flooding, and emergency repairs. This paper focuses on the assessment and mitigation of capacity risk by describing: 1) the modeling system used to characterize and develop alternatives for mitigating the capacity LoS deficiencies and; 2) how asset management, risk-based methods were used to select preferred alternatives and prioritize the implementation of recommended projects in the Capital Improvement Program (CIP). These projects will eliminate or significantly reduce street flooding risk and basement sewer backup risk (BSBR) for the 25-year design storm, which is a BES technical LoS related to the protection of public health. A DESCRIPTION OF THE BES EXPLICIT MODELING SYSTEM To support the capacity analyses of combined sewer systems within the City of Portland, Bureau of Environmental Services (BES) has developed a highly detailed simulation technique called Explicit Modeling. The technique is explicit in that it models public and private facilities (manholes, pipes, green streets, onsite vegetated facilities, roof downspout disconnection, etc.) and impervious surfaces at the tax lot (parcel) level. Explicit modeling enables BES to more clearly define the sources of basement sewer backup risk (BSBR) and capacity problems throughout the basins, to efficiently calibrate to flow monitoring data with more certainty than traditional