A Simplified Approach for Sizing Green Stormwater Infrastructure in the City of Seattle

As part of the 2008 City of Seattle stormwater manual update, a simplified approach was developed for sizing parcel-scale facilities to meet the revised flow control requirements. The simplified, or "pre-sized", approach provides a relatively easy process for designers to select and size flow control facilities for smaller projects without the need for continuous flow modeling. In conjunction with a City-wide emphasis on sustainable designs, the approach prioritizes green stormwater infrastructure over traditional facilities, with an emphasis on retaining existing trees and implementing infiltration to the maximum extent feasible. Green stormwater infrastructure BMPs in the Pre-Sized Approach include retained trees, new trees, permeable pavement, green roofs, dispersion, bioretention facilities, cisterns, and bioretention planters with underdrains. The flow control benefits for each of these BMPs were quantified either as flow control credits (impervious surface reduction credits) or sizing factors (relating the facility size to the impervious area mitigated). Credits and sizing factors were developed for each of the City's flow control standards and typical facility design variations. A Pre-Sized Approach worksheet was developed to guide the designer through the process of selecting and sizing facilities. This worksheet provides guidance for flow control calculations and serves as documentation for project submittal.