DESIGN OF LID SYSTEMS - Low Impact Development and Sustainable Stormwater Management - page 144

Max. ponded water
depth (6” typ.)

Mulch with leaf compost
or shredded wood.
Avoid wood chips.

3:1 side slopes (typ.)

6”

12”/ domed riser
w/ domed grate
Wrap gravel (if used)
with nonwoven geotextile
6 lf of 10”/
perf HDPE pipe
12” min. sump

18”min modified
soil/planting
mix

12” stone bed
(optional)

Optional uniformly graded
course aggregate (AASHTO No. 3)
or clean washed sand

Figure 7-15 Rain garden schematic.

Rain garden vegetation serves to filter (improve water quality) and transpire
(reduce runoff quantity) stormwater. The plants absorb some pollutants, while
microbes associated with the plant roots and soil can also break down pol-
lutants. In addition to filtering pollutants, the soil medium allows storage and
(where feasible) infiltration of stormwater runoff, providing volume control. In
addition, engineered soil media may serve as a bonding surface for nutrients to
enhance removal of this pollutant source in runoff. Additional treatment capacity
is provided by a surface mulch layer, which traps sediments that can have great
pollutant loads. A botanically diverse rain garden, with a variety of plant species
and types, can help build a system that tolerates insect pests, diseases, pollution,
and climatic stresses [16].
Figure 7-15 illustrates a schematic diagram of a rain garden that is a more
technically engineered structure, designed to complete specific stormwater man-
agement goals. Pond depth, soil mixture, infiltration bed, perforated underdrains,
domed risers, and positive overflow structures may be designed in urban envi-
ronments according to the specific stormwater management functions that are
required.

Primary Components of a Rain Garden System
Pretreatment (may be necessary to help prevent clogging)

• Sediment removal through a vegetated buffer strip, cleanout, stabilized inlet,
water quality inlet, or sediment trap prior to runoff entry into the rain garden

Flow entrance

• Varies with site (e.g., parking island versus residential applications)
• Water may enter via an inlet (e.g., flared end section) or trench drain
• Sheet flow into the facility over grassed areas or level spreader