DESIGN OF LID SYSTEMS - Low Impact Development and Sustainable Stormwater Management - page 146

Figure 7-16 Rain garden overflow structure.
Positive overflow
• Provides for the direct discharge of excess runoff during large storm events
when the subsurface and surface storage capacity is exceeded
• Examples of outlet controls include domed risers, inlet structures, weirs, and
similar devices are shown in Chapter 8
Domed riser. A domed riser may be installed in a rain garden to ensure
positive, controlled overflow from the system (see Figure 7-16). Once
water ponds to a specified depth, it will begin to flow into the riser
through a grate, which is typically domed to prevent the riser from
being clogged by debris.
Inlet structure. An inlet structure may also be installed in a rain garden
to ensure positive, controlled overflow from the system (Figure 7-16).
Once water ponds to a specified depth, it will begin to flow into the
inlet.
Generally, a rain garden system is a depression in the ground, with plants and
a surface mulch layer, which provides for the storage and infiltration of relatively
small volumes of stormwater runoff, often managing stormwater on a lot-by-lot
basis. This use of many small stormwater controls versus one large detention
area promotes the low-impact development goal of decentralized stormwater
treatment. If greater volumes of runoff must be managed or stored, a rain garden
system can be designed with an expanded subsurface infiltration bed, or each rain
garden can be increased in size. Typically, the ratio of impervious area draining to
the rain garden to the rain garden area should not exceed 5 : 1, and the total imper-
vious area draining to a single system should not be more than 1 acre [17, 18].