Reducing Stormwater Runoff Impacts

U-Tech: Underground Technology Cutting Edge Technical Information for Utility Construction and Rehabilitation
By Aaron Eder, P.E., Kennedy/Jenks Consultants | May 2010 Vol. 65 No. 5

All development and redevelopment projects are subject to the requirements of the Manual. The purpose of this manual is to provide stormwater management principles and techniques that help preserve or mimic the natural hydrologic cycle, minimize sewer system problems and improve water quality. The Manual provides developers and design professionals with specific requirements for reducing the impacts of stormwater from new development and redevelopment.

Strategies for meeting the requirements in the Manual depend on a number of site factors, including infiltration capacity, available infrastructure, proposed development plans, and the drainage basin the proposed development is in. The applicant’s ability to effectively use the design standards in the Manual depends on a demonstrated understanding of the development site’s ecology and of the upstream and downstream impacts resulting from stormwater management improvements. The standards addressed in the Manual are intended to make site-specific improvements to properties across the city and to comprehensively manage stormwater by watershed. Stormwater management is critical to maintaining and enhancing the city’s livability and improving watershed health. The Manual allows the city of Portland to protect both watershed resources and infrastructure investments with every land improvement. As each development and redevelopment project meets the requirements of the Manual, it will contribute to achieving these important citywide goals.

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The Manual presents three methodologies for sizing stormwater management facilities: Simplified Approach, Presumptive Approach and Performance Approach. The Simplified Approach may be used for projects with less than 10,000 square feet (.23 acre) of impervious area. This approach is most appropriate for private, small-scale residential development. It is not allowed for use on large, complex projects or on projects that have multiple catchments that, when combined, exceed 10,000 square feet of new impervious area.

The Presumptive Approach is available for medium-to-large-scale residential and commercial projects on either private or public property of any size. Its application is required for projects with a total impervious area that exceeds 10,000 square feet (.23 acre), where the Simplified Approach is no longer applicable. It can also be applied to size facilities on smaller projects where the more detailed hydrologic calculations will allow the applicant to size a facility more accurately by taking measured infiltration rates and other more specific design factors into account.