Page 477 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch12_398-456.qxd 8/4/10 9:37 PM Page 435

12.5 Urban Runoff Control Practices 435

12-in. PVC outlet 12-in. PVC inlet
24-in.-diameter manhole
cover (typical)

6 ft

14 ft

24-in.-diameter manhole
Grade with riser (typical)

3-in. gap
4-in. precast
1 ft 5 in.
concrete
baffle wall
(3) 12-ft-diameter
holes 4 ft 6 in.
6 ft 9 in.
5 ft 7 in.
4 ft 6 in.
2 ft 6 in. 2 ft 6 in.

6˝

3
12-in. compacted 4 -in. stone
Figure 12.20 Conceptual Water Quality Inlet. Conversion factors: 1 in. 25.4 mm;
1 ft 0.3048 m
(Source: U.S. Environmental Protection Agency)

Limitations on Use. There are few physical site limitations on the use of water quality in-
lets. The inlets are generally designed as belowground structures and do not require large
amounts of land. Given their small size, however, large watersheds cannot be drained into
a water quality inlet. Removal efficiencies depend on the detention time in the water qual-
ity inlets. Their use is usually restricted to small watersheds of less than 2 acres (0.81 ha).
Another restriction on the use of water quality inlets is dry-weather base flow. If dry-
weather base flow cannot easily be removed from a drainage system, a large water quality
inlet and more frequent maintenance are needed to accommodate this flow as well as the
flow resulting from a rainfall event.