Page 23 - 7166 - Rain Garden Handbook

PLAN 1 17

Colleen Miko

REPEAT TEST THREE TIMES DURING THE DRY SEASON
If it’s the wet season (December through April), do this
soil drainage test once. If you must test during the dry
season, do the test three times (with each test performed
immediately after completion of the last). Use the third test
as your drainage rate (measured in inches per hour). Testing
three times during the dry season provides a better estimate
Erica Guttman of wetter conditions present in the winter when the rain
garden is doing the most work. Testing in the dry season is
not recommended, but sometimes may be unavoidable.

The soil drainage test is important for determining the size
Local and State Requirements Alert of the rain garden, but the test also helps you make the
following important decisions:

• If the drainage rate is less than 0.25 inches per hour,
The Washington State Department of Ecology calls for more stringent soil testing for but more than 0.1 inches per hour, the location may
new development and redevelopment projects required to meet Minimum Requirements be OK for the rain garden. However, keep in mind that
1-5. Ecology’s soil testing procedure is discussed in Appendix C with full details in standing water may be present for extended periods
the 2012 Stormwater Management Manual for Western Washington. Consult with your during the wettest months (December through April).
local municipality for specific soil drainage testing procedures they may require.
• If the drainage rate is less than 0.1 inches per hour,
consider a different location for your rain garden.