Introduction

The City developed a Presumptive Approach Calculator (PAC) that provides a standard sizing methodology for vegetated stormwater facilities designed under the presumptive design approach 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 is based on the Santa Barbara Urban Hydrograph methodology.

The PAC is not intended to:

• Size multiple facilities at once or in sequence. A separate calculation is required for each paired facility and catchment.
• Calculate conveyance.
• Analyze discharge point or emergency overflow pathway.
• Size drywells or other underground injection devices.
• 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 submission package. See the Stormwater Management Manual for required submittals of the chosen design approach (see SWMM Chapter 2).

The City has transitioned the PAC from Excel to a web application. In order to use the PAC, users will need to have a PortlandOregon.gov 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.4).