Application of EPA SWMM5 to Integrated Stormwater and Sanitary System Modeling – Seattle’s Broadview Case Study

Sam Gould, Dave Jacobs, David Hartley and Patty Dillon


This paper will document development of an integrated storm drainage and sanitary sewer model of Seattle’s Broadview neighborhood, an older residential area with substantial sanitary inflow and infiltration as well as a complex and ill-defined surface drainage system. Stormwater runoff from Broadview outfalls to Pipers Creek, an ecologically sensitive, salmon-bearing stream in a forested park. The model is designed to fulfill multiple objectives including:

  • Identification of locations and severity of existing sewer back-ups and surface flooding problems
  • Quantification of benefits of I&I reduction alternatives on sewer system conveyance capacity and frequency of sewer backups into basin homes
  • Analysis of impacts of I&I alternatives on neighborhood surface flooding and stream flow regime
  • Planning and conceptual design of Green Stormwater Infrastructure (GSI) projects and other stormwater man agement improvements necessary to mitigate impacts of I&I control

In order to meet these objectives, the model must:

  • Replicate observed sewer back-up and surface flooding problems;
  • Represent the basin at a sufficiently fine spatial scale to be used as a planning and design tool
  • Realistically quantify and distribute both impervious runoff and subsurface flow to each collection system for both existing and proposed future conditions.
  • Account for surface transfers between systems, e.g. street sewage back-ups entering the storm system, street flow or surface ponding entering sanitary system maintenance holes.

Our talk will describe the challenges and techniques employed in the application of EPA SWMM5 to meet these project objectives.

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