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424 Chapter 12 Urban Runoff and Combined Sewer Overflow Management

Concrete level spreader
Inflow
Grass channel
Emergency
Stilling basin spillway

Flat basin floor with
grass turf
Outfall
Backup underdrain

Riser/
barrel

Plan view

Inflow Stilling basin Embankment
Riser Emergency
Extreme flood control
spillway
Overbank flood control
Channel protection
Infiltration storage Cleanouts Valve
Stable
outfall

Barrel
Backup underdrain pipe in case
of standing water problems Anti-seep collar or
filter diaphragm
Profile
Figure 12.11 Infiltration Basin
(Source: New York Department of Environmental Conservation)

Maintenance Requirements. Infiltration basins require moderate to high levels of peri-
odic maintenance. Most are designed with vegetated bottoms to provide stabilization and
promote some vegetative uptake of nutrients. Periodically, the bottom of the basin must be
mowed and accumulated sediments must be removed to maintain desired infiltration rates.

Limitations on Use. Infiltration basins often have relatively large land requirements and
are better suited for location in developing areas than in already developed areas.
Infiltration basins also require suitable soil to be effective. Accumulating runoff must
be able to infiltrate the soil in the bottom of the basin. Typically, sand and loam, with
infiltration rates greater than or equal to 0.27 in./h (6.9 mm/h), are the preferred soils
for infiltration systems. The use of infiltration basins can be restricted by high ground-
water elevations. For infiltration to occur, groundwater levels should be located at least 2
to 4 ft (0.60 to 1.2 m) below the bottom of the basin.

12.5.4.2 Infiltration Trenches/Dry Wells Subsurface infiltration practices, such as infil-
tration trenches or dry wells, force runoff into the soil to recharge groundwater and remove
pollutants. These infiltration structures are located below ground and usually must be built