King County, Washington (encompassing Seattle and nearby cities) has embarked on an extensive program to assess infiltration and inflow (I/I) in each of 34 component agencies in the County's service area. Computer modeling of I/I into the sewer system is a core effort in the I/I Program.
Over 775 flow monitors were placed throughout the service area during two wet seasons. High spatial resolution of rainfall was achieved by combining radar imaging (CALAMAR) with point rainfall measured at 72 rain gauges located throughout the 420-square-mile service area. The MOUSE modeling
software from Danish Hydraulic Institute (DHI) is used to provide a continuous simulation of rainfall dependent inflow and infiltration and to assess the I/I flowing into the local and King County sewer system. Using measured rainfall data as input, MOUSE RDII hydrologic models were calibrated
to observed sewer flow response in 150 model basins that define the existing separate sewer service area. On average, each model basin encompasses an area of 1,000 acres and contains 100,000 linear feet of mainline sewer. Based on the locations of the flow monitors, the model basins were further
divided into approximately 750 smaller mini-basins. Results from the model basins are used to quantify flows within the King County conveyance system, which will assist with the assessment of I/I impacts and determination of future capacity needs. In contrast, mini-basins, with average 22,000
linear feet of tributary sewer, will be used as the basis for targeting future I/I control improvements and estimating the probable system rehabilitation costs. To assist in targeting potential future I/I control improvements at the mini-basin level, the composition of I/I needs to be defined
as accurately as possible in each of the mini-basins. Initially, it was believed that apportioning the model basin I/I back into the composite mini-basins would be adequate to define the I/I composition in each mini-basin. Extensive research has been completed to identify methods that can
complete this I/I apportionment effort efficiently. Examples of methods tried are: 1.) Using weighted proportioning to distribute I/I from each model basin back to its constituent mini-basins, and 2.) Modifying storm water unit hydrograph methods to estimate I/I components from each mini-basin
which in turn forms the I/I response contributed by each respective model basin. Research results show that these methods could not accurately account for the varying I/I responses of mini-basin resulting from different rainfall events and antecedent moisture conditions. As a result, these
methods failed to generate acceptable I/I component estimates. It was concluded that modeling each of the mini-basins hydrologically is the most defendable way for identifying the composition of I/I in the mini-basins and apportioning model basin's I/I back into their mini-basins. However,
this means developing and calibrating models for 750 mini-basins within the resources and time constraints of the King County Regional I/I Control Program (I/I Program).
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