GSI_Calc software

GSI-Calc, version 1

GSI-Calc was developed through a Washington State Department of Ecology Grant of Regional or Statewide Significance and administered by Kitsap County.

The Western Washington NPDES (National Pollutant Discharge Elimination System) Phase I and Phase II Municipal Stormwater Permits include several provisions related to low impact development (LID). GSI-Calc was developed to assist permittees in implementing these requirements by providing a regional sizing tool for LID stormwater Best Management Practice (BMP) design in western Washington lowlands. GSI-Calc allows sizing of LID BMPs, or “green stormwater infrastructure” (GSI), as a function of contributing impervious area, prevalent soil types in the region, representative site infiltration rates, and mean annual precipitation. This program is intended to assist developers and regulatory agency reviewers in sizing and designing LID BMPs without need for continuous simulation modeling, thereby reducing the barriers to the implementation of LID.

The scope of this initial version of GSI-Calc was developed in conjunction with project partners and additional interested NPDES Phase I and Phase II permittees. The BMP types, configurations, and other sizing tool variables were prioritized to maximize the benefits of GSI-Calc given finite grant funding. Future versions of the GSI-Calc tool could be developed to expand the options and applicability of the tool to meet ongoing regional needs in stormwater management and LID implementation.

Download the GSI_calc user's manual

Instructions on how to install the software

Download the software (.exe) file [NOTE: file is large, 237 MB. Download times may be long]