WATERSHEDS - Water Quality Engineering in Natural Systems - page 222

efficiencies of these systems can seldom be pre-
dicted with certainty. In snowbelt zones of North
America and Europe, some detention ponds
receive flows with extremely high concentrations
of salt. high salinity increases the solubility of
metals in sediments and water, and the sediment
accumulated in ponds can actually become a
source of metals rather than a sink. hence, the
removal efficiencies in winter are much less than
in a nonwinter period.

weir, and Figure 6.16c shows a wide view of the
dry detention basin, with the inflow structures on
the left and the outflow structure in the back-
ground. A safety overflow spillway is part of the
basin and is used for conveyance when the storage
capacity of the basin is exhausted. The outlet of
dry detention basins used for flood control is typi-
cally sized for large storms, and smaller but pollut-
ing runoff events will pass through such basins
mostly without appreciable attenuation of the
pollution load. Such dry detention basins are inef-
fective for urban runoff quality control. By com-
bining the dry detention basin with an infiltration
system located at the bottom of the basin or
lengthening the flowway between inlet and outlet,
the pollution control capability is enhanced and
can be effective for pollution removal. Such basins
are called modified dry detention basins. Modified
dry detention basins, also known as extended dry
detention basins, are effective pollution control
devices.

Dry Detention Pond. A dry detention pond or dry
detention basin is a stormwater detention facility
that is normally dry and is designed to store
stormwater temporarily during high-peak-flow
runoff events. An example of a dry detention
basin is shown in Figure 6.16, where Figure 6.16a
shows the inflow structure that directs surface
runoff from the parking lot into the dry detention
basin, Figure 6.16b shows the outflow structure
consisting of a circular orifice below a rectangular

(a) (b)

(c)

Figure 6.16. Dry detention. (a) Inflow structure. (b) Outflow structure. (c) Wide view.