Green Stormwater Infrastructure

Turns out, Seattle DPD has a publication all about GSI, Managing Rainwater (Dec 2010), as well as a website. Another website offers helpful resources, and being RainWise. What we're talking about is a "green roof," and a "rain garden." Projects with 2000 SF or more of new or replaced impervious surface (roof and paving) must implement a GSI. More than 750 SF of new or replaced pavement need drainage review from the Side Sewer desk (the existing driveway is 1,089 SF to the street).

The RainWise site says that this property is not eligible for any RainWise rebate programs.

Rain gardens provide bioretention, designed to get water to soak into the ground. Designed soil mix and native plants work better than a lawn. They also talk about conveyance furrows, kind of a directed French drain. There is a WSU Rain Garden Handbook. Recommended plants for the rain garden are found at Green Factor. The RainWise guide actually says don't do rain gardens, cisterns or rock-filled trenches around steep slopes or slide areas.

Rain water harvesting of 200 to 1000 gallons calls for a cistern. Smaller than 200 gallons is just a rain barrel. Rain water is better for plants than potable (chlorinated) water, but greywater (diverted from the sewer) has more code requirements.

Seattle DPD likes green roofs a lot. Typically less than 4" of "soil-like growing medium." There are extensive and intensive green roofs. Extensive is 10-50 psf, intensive 80-120 psf.

Geotechnical engineer Keith Cross suggested that I read:

• City of Seattle MC - grading = ch 22.170
• City of Seattle MC - ECA = ch 25.09 Directors Rules 4-2015; 17-2011; 18-2011; 3-2007
• City of Seattle MC - stormwater = ch 22.800 Directors Rules 15-2012; 15-2009; 16-2009; 17-2009(*); 18-2009