Numerical Modeling of CSO Storage to Avoid Backups and Flooding and to Minimize Overflows

The King County, Department of Natural Resources, Wastewater Treatment Division has constructed a 7 million gallon CSO storage and conveyance tunnel, is beginning construction of a 8 million gallon tunnel and has completed design of a 4 million gallon tunnel. These projects are all located within the City of Seattle in Washington State.The function of each of these tunnels is to store combined sewage flow, and in the case of two of the tunnels, provide treatment by settling and screening. A separate CSO treatment plant serves the other project.The storage & treatment projects are designed to reduce untreated overflows to a frequency equal to or less than 1 event per year on average. Long term numerical modeling is used to find and verify design parameters, which include gate and pump operation rule sets to operate the storage systems during normal conditions. Additional modeling has been performed to also find operation strategies for extreme events such as combinations of large storms, high tides, or unusual flow diversions. The additional modeling to refine the control strategies of tunnel operation is the focus of this paper.The conditions of concern may not always have a high probability of occurrence, but an operation rule set can include a strategy for dealing with them, often with minimal additional programming or sensor additions. The long term numerical modeling verifies the suitability of rule sets for conditions which often can not be anticipated by modeling using a set of design storms.The process used to find solutions in the form of rule sets to manage each of these CSO storage facilities during unusual events will be reviewed along with the similarities and the unique elements of the design goals and operational conditions.