1.3.2 Using LID to Meet Core Elements

The 2004 Ecology SWMMEW identifies eight Core Elements for stormwater management applicable to development and redevelopment sites and provides guidance for designing water quality treatment and flow control BMPs to meet Core Elements #5 (Runoff Treatment) and #6 (Flow Control). The goal of Core Element #5 is to treat approximately 90 percent of the annual runoff generated by the pollutant-generating surfaces at a project site. Core Element #6 is intended to mitigate to the maximum extent practicable the impacts of increased storm runoff volumes and flow rates on streams in eastern Washington. See Chapter 2 of the 2004 SWMMEW for detailed discussion of the applicability of the Core Elements and the associated guidelines.

The 2004 SWMMEW recommends infiltration as the preferred method for flow control and treatment where feasible. The infiltration-based BMPs provided in that manual (e.g., infiltration ponds, infiltration trenches, infiltration swales, and bio-infiltration swales) are similar in some cases, but are not identical to the infiltration-based BMPs described herein (e.g., bioretention, amended soils, and permeable pavement).

The focus of this Manual is to describe LID planning principles and LID BMP design techniques that can assist in meeting applicable Core Element requirements and water resource goals. While the use of LID may greatly assist in meeting or, in some cases, completely satisfying Core Elements #5 and #6, the designer is responsible for demonstrating the design’s compliance with Core Element requirements.

Appendix C: Sizing of LID Facilities provides a demonstration of how LID BMPs may be sized to meet flow control and treatment requirements. The demonstration sizing is intended to illustrate a process for using hydrologic modeling to route stormwater flows through various LID BMPs and evaluate their effectiveness at mitigating peak runoff discharge rates and volumes. The demonstration sizing is not intended to replace or supersede local jurisdiction requirements for sizing BMPs to meet Core Element requirements.

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