Page 24 - 7166 - Rain Garden Handbook
P. 24
1 Determine the Size
18 PLAN
of Your Rain Garden
Once you've tested the drainage rate of the soil, you're ready to
determine the size of the rain garden. Five questions are important:
1. How well does your soil drain?
2. How much area will drain to the rain garden?
3. How much rainfall does your location receive? David Hymel
4. How much water do you want the rain garden to hold?
5. How much room do you have for your rain garden?
1. HOW WELL DOES YOUR SOIL DRAIN?
In the last section (Test Your Soil) you calculated
the soil drainage rate. Refer to this rate to size
your rain garden using the sizing chart on page 21.
2. HOW MUCH AREA WILL DRAIN TO THE
RAIN GARDEN?
David Hymel David Hymel Michael Carey To determine the drainage area, measure the
driveway, rooftop, landscape areas, and other
surfaces that will drain to your rain garden, and
note the area in square feet. To find the area of
a rooftop, measure the length and width of the
Local and State building, including the roof overhang area. Don’t
Requirements Alert worry about the slope of the roof since it doesn’t
influence the drainage area.
The Washington State Department of Ecology has a minimum sizing
criterion for new development and redevelopment projects subject to 3. HOW MUCH RAINFALL DOES YOUR
Minimum Requirements 1-5. Ecology’s sizing methodology is discussed LOCATION RECEIVE?
in Appendix C with full details in the 2012 Stormwater Management
Manual for Western Washington. Also, be sure to check with your local Look on the maps on pages 22 and 23 to find
municipality to see if they have specific sizing requirements. your location and how much rain that location
receives. Now go to page 21 to determine the
size of your rain garden.
