How to determine the
size of a rain garden
When full, the ponded area should have a maximum depth of
six inches. The planting soil usually has a minimum depth of
2.5 feet, but this depth may be constrained by the maximum
depth of the
underdrain at your site. The
underdrain must discharge down gradient from the
rain garden. Depending on your site
topography, the bottom of the rain garden may need to
be shallower than three feet below the ground surface.
When rain gardens are designed for new sites, their sizes
are matched to their drainage areas. When a site is being
retrofit, the size of the rain garden is often limited by
the availability of suitable space. Even in this case, it is
important to estimate how effectively your proposed rain
garden will capture, treat and infiltrate runoff from your
site. This can be accomplished using the
sizing tool.
