PAC: Introduction

Environmental Services

working for clean rivers

PHONE: 503-823-7740

FAX: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

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Stormwater Presumptive Approach Calculator


The City developed a Presumptive Approach Calculator (PAC) that provides a standard methodology for sizing basins, planters, and sloped facilities to meet the stormwater hierarchy requirements of the Stormwater Management Manual. The PAC allows stormwater design professionals to size stormwater facilities using native soil infiltration rates and above- and below-grade storage volumes. The PAC uses the Santa Barbara Urban Hydrograph methodology.

The PAC is not intended to do any of the following:

  • Size multiple facilities at once or in sequence. (A separate calculation is required for each paired facility and catchment.)
  • Calculate conveyance.
  • Analyze the discharge point or emergency overflow pathway.
  • Size drywells.
  • Serve as an Operations and Maintenance Plan.

Professional judgment is still required when evaluating results and verifying the performance of any individual stormwater facility. Stormwater facility design will need to address site challenges and constraints in addition to all other relevant code, project or development requirements. If the project designer proposes to deviate from the allowable ranges, the City may require the project to be submitted or designed under the Performance Approach.

The PAC results are one element of a complete design or submittal package. See the Stormwater Management Manual for required submittals of the chosen design approach.

In order to use the PAC, users will need to have a account. Users are able to create, save, edit, import, export and delete PAC projects under their account name. Advanced technical information can be found in the PAC technical framework (see SWMM Appendix A.5).