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420 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
periodically is inundated with water during storms; this area should be vegetated and rela-
tively flat to promote settling and filtering of sediments and vegetative uptake of nutrients.
The highest level will be inundated only during extremely heavy rainfall; this area also
should be vegetated to prevent soil erosion. At least 30% of the surface area of a wet pond
should be a vegetated zone. Typically, this vegetation is concentrated at the outlet as a final
“polishing” biofilter. The sizing of wet ponds is similar to that of dry ponds in that a num-
ber of different “sizing rules” provide varying levels of pollution control. Generally, these
rules specify the volume of runoff to be detained in the wet pond during a storm. For exam-
ple, the Maryland Water Resources Authority specifies that the permanent pool of a wet
pond should be large enough to contain 0.5 in. (13 mm) of runoff distributed over the im-
pervious portion of the contributing watershed. In Florida, storage volume for 1 in. (25.4 mm)
of runoff above the normal pool elevation is recommended. This volume must be released
at a slow rate; no more than half should be discharged within 60 hours after the event, and
all the volume must be released after 120 hours. A hydraulic retention time of 14 days for
the permanent pool volume is recommended.
The design of water quality wet ponds must also take into consideration the possibil-
ity of large storms. Emergency spillways should be included in the design to prevent flood-
ing difficulties. In addition, the pond’s inlet and outlet structures should be separated and
constructed at either end of the pond to maximize full mixing when large flows occur and
avoid short circuiting. By separating the inlet from the outlet, the detention time of the
pond can also be increased.

Maintenance Requirements. Like many other BMPs, wet ponds require routine mainte-
nance to be effective. Wet ponds are designed to allow for settling of suspended solids;
therefore, periodic removal of the accumulated sediment must be performed (perhaps
every 10 to 20 years). In addition, the pond slopes should be regularly mowed to make the
sediment removal process easier and to enhance the aesthetic qualities of the area. Inlet
and outlet structures should be inspected periodically for damage and accumulated litter,
and the pond bottom should be inspected for potential erosion. Erosion of the pond bottom
from high-velocity flows can result in increased sediment transport and overall reduction
in the pollutant removal capabilities of the pond.
Limitations on Use. Water quality wet ponds have large land requirements and usually
are more suited to new development projects where they can be designed into the site. In
addition, wet ponds are not suitable for use in areas with porous soils or low groundwa-
ter levels because a pool of water in the bottom is key to their design. Wet ponds should
be built into the groundwater with their control elevation set above the level of seasonal
high-water tables. Synthetic impermeable materials or clay can be used to prevent seep-
age. Wet ponds also have physical limitations related to the site topography; since locat-
ing wet ponds in areas with extreme slopes is often difficult, relatively flat locations are
preferable.

12.5.3.3 Constructed Wetlands Constructed wetlands are effective in removing many
urban stormwater pollutants. Two prevalent types of systems are shallow-constructed
wetlands (Fig. 12.8) and wet detention systems (Fig. 12.9). The wet detention system is a
wet pond with extensive shoreline shallow wetland areas. Wetland systems combine the
pollutant removal capabilities of structural stormwater controls with the flood attenuation
provided by natural wetlands. Proper design of constructed wetlands—including their
configuration, proper use of pretreatment techniques to remove sediments and petroleum
products, and choice of vegetation—is crucial to the functioning of the system.