Page 25 - 7166 - Rain Garden Handbook

PLAN
L = 30 ft 1 19

Example

LENGTH x WIDTH = AREA IN SQUARE FEET (sq ft)

DRIVEWAY
15 ft x 30 ft = 450 sq ft

ROOF W = 15 ft
15 ft x 20 ft = 300 sq ft L = 40 ft
10 ft x 40 ft = 400 sq ft
TOTAL = 1,150 sq ft

The rain garden on page 26 is sized
to capture this total contributing
area. Refer to page 25 to see how L = 20 ft
this rain garden was sized using
the guidance in this handbook.

W = 15 ft
W = 10 ft

4. HOW MUCH WATER DO YOU WANT THE RAIN GARDEN TO HOLD?
A ponding depth of 6 inches or 12 inches is recommended. You may A. Choose which performance level you want to achieve—Good, Better,
wish to have a shallower or deeper ponding area in your landscape or Best. Find the performance levels in the sizing chart on page 21.
based on aesthetics. Keep in mind that a deeper ponding area
generally will take longer to drain and will need more water-tolerant Good performance will capture about 80 percent of the water from
plants in the bottom (except with sandy, well-draining soils). The depth the contributing areas that flows to your rain garden.
you choose can also influence how large the surface of your ponding Better performance will capture about 95 percent of the water from
area needs to be.
the contributing areas.
After you decide on a 6-inch or 12-inch ponding depth, it’s time to Best performance will capture most all the water from the
determine the size of the top surface of the rain garden ponding area. contributing areas.
Refer to the Rain Garden Sizing Chart on page 21. This chart shows
rainfall regions for western Washington, color coded to the maps on Under Best performance, a 12-inch ponding depth will require less
pages 22 and 23. The chart also shows soil drainage rates. Using the space than a 6-inch ponding depth. But under Good or Better
chart and rainfall region maps, follow steps A through D to determine performance, the rain garden will be generally the same size at either
the size of the top surface of your ponding area. 12 or 6 inches of depth.