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Prioritizing Low Impact Development (LID) in Project Design

Design engineers, planners, developers, and others interested in applications to Permit Sonoma

From: Reg Cullen and Nathan Quarles, County Permit Sonoma

Date: 23 December 2009

This guidance letter notifies local engineers, land planners, and developers that effective January 1st, 2010 the County of Sonoma will be prioritizing Low Impact Development (LID) Best Management Practices for all redevelopment and new development projects requiring post-construction storm water treatment. These requirements result from the North Coast Regional Water Quality Control Board’s (RB1) adoption on 1 Oct 2009 of the NPDES storm water permit (Order No. R1-2009-0050). The NPDES permit applies only to sites that drain to a county owned/maintained storm drain system with the Mark West Creek and Laguna de Santa Rosa watersheds. All other projects are subject to the County storm water regulations.

This new storm water permit regulates both storm water and non-storm water discharges into the County of Sonoma municipal separate storm drain system. The basic intent of the permit is to reduce storm water pollution, to protect water quality in local creeks, and to promote groundwater recharge through the use of storm water Best Management Practices (BMPs).

LID is a way to maintain or reproduce the way storm water infiltrates and/or runs off a site after it is developed. “LID is a storm water management and land development strategy that emphasizes conservation and the use of on-site natural features integrated with engineered, small-scale hydrologic controls to more closely reflect predevelopment hydrologic function” (permit, pg. 40 Sec. E(5)(2)(a)(1)). LID principles control storm water runoff by using landscape based features that are distributed throughout the site. Project design following LID principals emphasizes infiltration, evapotranspiration, and/or capture and reuse of storm water.

The new LID prioritization requirement will apply to all redevelopment and new development (RAND) projects that have not received discretionary approval by January 1, 2010 (see permit pg. 40 Sec. E(5)(2)(a)(2)). Such projects that propose to use non-LIDBMPs, such as structural separator units or traditional detention only, will be required to document the infeasibility of using LIDBMPs based on site specific circumstances. For further details on the prioritized categories of LIDBMPs and additional information on the new storm water permit please visit the Permit Sonom awebsite.

Other important permit changes include projects that have not received tentative map approval (or equivalent) by June 1, 2010 will fall under the new sizing threshold requirements. That is, after June 1, 2010 projects that create or replace 10,000-SF or more of impervious area will require post-construction storm water BMPs and fall under the SUSMP requirements.

Revisions to the existing SUSMP Manual will begin in January 2010. A revised SUSMP/LID Technical Manual, which incorporates all of the new permit requirements and LIDBMP design details, is anticipated to be released by April 1, 2011. Until the SUSMP/LID Technical Manual is released, designers should use the existing SUSMP manual or other adopted LID manuals to design LIDBMPs that achieve the water quality objectives of LID. Go to the City of Santa Rosa site for updates on the implementation schedule for the storm water programs.