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King County has embarked on an extensive program to assess infiltration and inflow (I/I) in each of 34 component agencies in the County's wastewater service area encompassing Seattle and nearby cities. I/I is the major driver in the need for 500 million dollars of new conveyance facilities over the next 30 years. King County is investing in the Regional I/I Control Program (I/I Program) hoping that reducing I/I will save tens or hundreds of millions of dollars by eliminating, reducing or delaying future capital facilities. A preliminary assessment in 1995 indicated that an expenditure of 100 million in sewer rehabilitation could eliminate the need for 300 million in conveyance facility improvements. The goal of this regional assessment is to develop detailed information about the location of I/I sources and potential cost-effective improvements to remove or reduce critical I/I sources.

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 with point rainfall measured at 72 rain gauges located throughout the 420-square-mile service area. Continuous simulation of rainfall-dependent inflow and infiltration, using the MOUSE modeling software from DHI, is being used to assess the I/I coming 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 approximately 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 lineal feet of mainline sewer. To ensure adequate capacity in the regional conveyance system and prevent sanitary sewer overflows (SSOs), King County is particularly interested in estimating peak 20-year recurrence interval flows throughout the service area. Therefore, the MOUSE RDII model was used with a 60-year record of local rainfall to generate a flow-frequency relationship for each model basin.

Preliminary model results indicate that model basin peak I/I flow rates and characteristics vary considerably. Peak hour 20-year I/I rates generated by the modeling effort have ranged from 700 gallons-per-acre-per-day (gpad) (or 5 gallons-per-day-per-linear-foot, gpdlf) to 22,000 gpad (100 gpdlf), with an average rate of 4,000 gpad (25 gpdlf) per model basin. It is estimated that 95 percent of the basins have peak I/I that exceed the acceptable service standard for the region, which was set in 1961 at 1,100 gpad.

The County's commitment to develop a comprehensive and equitable I/I assessment resulted in a data collection and modeling analysis effort that, in terms of detail and magnitude, is rarely undertaken in other I/I control programs. Modeling analysis that quantifies the I/I responses from each of the 150 model basins is expected to reveal valuable information regarding numerous aspects common to other I/I analyses. King County and consultant staffs have nearly completed the modeling work for the project, which is scheduled for completion in 2004. This paper presents the information gained through the modeling effort and describes the modeling information that will be used in King County's I/I Program.
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Document Type: Research Article

Publication date: 01 January 2004

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