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Sewer system evaluation surveys, flow monitoring, and flow modeling can all be used to examine the conditions of the sewer system, identify areas of high flows during wet weather, and help to identify likely sources of I/I.

Sewer system evaluation surveys (SSES)

  • CCTV cameras can be robotically sent down sanitary sewer lines and along each side sewer to record sewer conditions. CCTV inspections can identify breaks, root intrusions, leaking water and deteriorating conditions.

  • Smoke testing involves pumping smoke through sewers from manholes in streets and observing where smoke exits. The exiting smoke can indicate a broken pipe or identify where roof or foundation drains are improperly connected to the sewer system.

  • Dye testing involves pouring non-toxic fluorescent colored dye down roof drains or catch basins to see if that dye makes its way into the sewer. This provides verification that the storm drainage being tested is directly connected to the sewer.

Flow and rainfall monitoring

  • Flow monitoring establishes a direct correlation between each local agency's wastewater flows and the effect of rainfall on I/I entering the system. The effects are measured and compared downstream.

  • Rainfall monitoring measures and reports rainfall quantities to provide a credible basis for the local agency flow data.

Flow modeling

King County and the local agencies use flow monitoring data and other data to model existing flows and to project future flows in the system.

For detailed information about the County's flow modeling methods:
Executive's Recommended Regional Infiltration/Inflow Control Program, Chapter 3, Assessing I/I Levels and Reduction Technologies (2005)

For more information, please contact:

Steve Tolzman

Contact your local sewer service provider for specific information about the sewer system in your neighborhood.

Steve Tolzman
Capital Project Manager