STRUCTURAL MEASURES: CONSTRUCTION, OPERATION, AND MAINTENANCE - Low Impact Development and Sustainable Stormwater Management - page 176

with preventive nonstructural BMPs. Design and construction procedures may
vary from very simple for “backyard” rain gardens to highly engineered biore-
mediation cells in ultraurban areas.
It is important to note that rain gardens should not be confused with constructed
wetlands or wet ponds that impond water permanently. Rain gardens are best
suited for areas with at least moderate infiltration rates (more than 0.25 in./hr).
In situations where permeability is less than 0.25 in./hr, special variations may
apply, such as using amended soils or underdrains.
Rain gardens are often suitable for retrofit projects, through integration with
previously developed lots and sites. An important concern for all rain garden
applications is their long-term protection and maintenance, especially if under-
taken in multiple (adjacent) residential lots where individual homeowners provide
maintenance. In such situations it is important to provide management guarantees
that ensure their long-term functionality, such as deed restrictions, covenants, and
maintenance agreements.
1. Sizing criteria
a. Surface area depends on storage volume requirements and permeability
of the underlying native soil, but should generally not exceed a maxi-
mum loading ratio of 5 : 1 impervious drainage area to rain garden area.
However, for design purposes, the total volume of water generated from
the contributing drainage area must be used, not just the impervious
portion.
The rain garden surface area required is determined by taking the vol-
ume of runoff to be controlled according to LID criteria, maintaining the
maximum ponding depth, the loading rate, and the emptying time. Infil-
tration and evapotranspiration are increased by increasing the surface
area of the rain garden. The total surface area needed may be divided
into multiple cells. This configuration, for example, may be useful to
collect runoff from both the front and back of a building.
b. Surface side slopes should be gradual. For most areas, maximum 3:1
side slopes are recommended; however, where space is limited, 2:1 side
slopes may be acceptable.
c. Surface ponding depth should not exceed 12 to 18 in. in most cases and
should empty within 24 to 48 hours.
d. Ponding area should provide sufficient surface area to meet required
storage volume without exceeding the design ponding depth. The sub-
surface infiltration bed is used to supplement surface storage where
appropriate.
e. Planting soil depth should generally be between 24 and 36 in. where
only herbaceous plant species will be utilized. If trees and woody shrubs
will be used, soil media depth may need to be increased, depending on
plant species (especially in poorly drained sites). Provided that they
meet drainage criteria, native soils can be used as planting soil or can
be modified on many sites.