Environmental Engineering Reference
In-Depth Information
as not to restrict oxygen flow, or be placed directly against the stems or trunks
of plants.
7. When working in areas with
steeper slopes
(up to 15%), it is critical first to
ensure that these BMPs are feasible. A geotechnical engineer should be consulted
to evaluate the suitability of installing a rain garden on or near a steep slope to
identify the risk of creating an unstable condition; underdrains may be required
for slope applications. When they do occur on slopes, rain gardens should be
terraced laterally along slope contours to minimize earthwork and provide level
areas for infiltration.
8.
Underdrains
are generally not needed unless in situ soils are expected to
cause ponding lasting longer than 48 hours (e.g., type C and D soils), for slopes
where soil instability is likely, and where there is insufficient space (
<
10 ft)
between the rain garden and a structure. Because infiltration is encouraged,
detailed discussion of underdrain design is not included in this topic but can
be reviewed in other guidance documents.
9.
Surface infiltration area
is the average area of a rain garden, defined as
area of rain garden at ponding depth
+
bottom area of rain garden
2
A
inf
=
=
infiltration area
(
average area
)
The size of the infiltration area necessary is determined by the volume of water
necessary to remove as determined by LID criteria, the depth of the ponded area
(not to exceed 1 ft), the infiltration rate of the soil, the loading ratio, and if
applicable, any subsurface storage in the amended soil or gravel.
Storage Volume
The storage volume of a rain garden is defined as the sum of
the surface and subsurface void volumes beneath the level of the discharge invert.
Intermedia void volumes may vary considerably based on design variations. Void
volumes should only be considered in soils when they have been amended to a
known depth with a known void ratio. If the rain garden is placed in existing
natural soils, the infiltration rate may be used to calculate discharge out of the
basin into the ground and the additional capacity available as a result.
The storage volume of a rain garden has three components:
(
ft
3
(
ft
2
1. Surface storage volume
)
=
average bed area
)
×
average design
water depth
2. Soil storage volume
(
ft
3
)
=
infiltration bed area
(
ft
2
)
×
depth of amended
void ratio of amended soil
3. Subsurface storage/infiltration bed volume
soil
(
ft
)
×
ft
3
(
)
=
infiltration area
×
void
ratio of storage material
Total rain garden volume
=
surface storage plus soil storage volume
+
subsurface storage
(
if applicable
)
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