Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

3.3    Detention and Retention/Wetland Practices

3.3.1    Description and Purpose

Stormwater management ponds are designed to mitigate the hydrologic and water quality impacts of stormwater runoff by providing stormwater quantity management and/or quality control. The increased volume of runoff, due to increases in impervious area, is stored and then released at a controlled rate through an outfall structure. Controlling the rate at which flow is released from the facility can help to reduce downstream channel erosion and prevent downstream flood damage. Improvements in stormwater runoff quality can also be achieved through settling, infiltration, nutrient uptake, adsorption, and physical filtration. Ponds that incorporate permanent pools of water are designed to use the biological action of plants and organisms to trap and then treat pollutants.

The distinction between the various types of stormwater management ponds is based on the type of hydrologic control provided due to the design of the outlet control structure. Stormwater management ponds can be designed to provide for both control of increases in stormwater volumes and peak flow rates, as well as for treatment of stormwater pollution. In many cases, depending on the design of the pond, multiple functions are provided resulting in a hybrid system. The pond design features that determine its function, and thus provide the basis for categorization include: depth of permanent pool, storm event discharge volume/storage volume, storm event peak discharge rate, and detention time.

The most basic categorization of pond types is between wet and dry facilities. When designing for water quality treatment, this is one of the most important features to determine pollutant removal efficiency. For these facilities, detention time, generally a function of the travel distance or height of the outflow weir, is an important design feature for efficient pollutant removal. Generally, dry ponds are designed to provide stormwater hydrologic control. In most cases, this means providing enough volume to store the stormwater runoff from a site for a particular storm event, and release this volume at a predetermined rate (usually that of predevelopment conditions). The pond volume, coupled with the design of the outflow control structure, determines the extent of stormwater detention management provided. Outflow control structures can take various forms and shapes. Their most important features for ensuring adequate quantity control include height and area of outflow opening. Wet ponds generally are designed with a permanent pool to provide for greater treatment of stormwater pollution than by ponds.

A variety of forms of detention and retention practices exist. Detention ponds, extended detention ponds, and retention (or wet) ponds are common. Design of these facilities can take the form of a shallow marsh or constructed wetland when hydric conditions are available and facultative plant species are abundant, with outlet controls determining the extent of detention or retention provided. In addition, these practices can be designed as surface or underground facilities, depending on available funding, soils and groundwater conditions, and space limitations.

Where insufficient land area is available, as is the case in many ultra-urban environments, underground storage structures may be required. Underground storage areas are usually constructed of concrete vaults or corrugated metal pipe (CMP). Pretreatment for water quality can help reduce clogging.

3.3.2    Design Alternatives

Attenuation and treatment of stormwater runoff are the primary objectives for designing detention and retention/wetland practices. Design features can be added to provide wildlife habitat, aesthetics, recreation and educational opportunities and to improve property values (USEPA, 1995).

The attenuation and storage features of detention and retention practices reduce the volume and rates of discharge of stormwater runoff generated from a site. In an ultra-urban setting, design control volumes can vary depending on the available space for the facility. Following storage, volume reduction occurs through infiltration, evaporation, and evapotranspiration—key design features of infiltration basins, retention/wet ponds, and wetlands. Reduction in the rate of discharge of stormwater runoff occurs by constricting the outflow from these facilities, generally through an outlet control structure. In an ultra-urban setting, facilities should be designed to reduce the peak discharge rates, as well as their frequency of occurrence, to as close to predevelopment conditions as possible, while working within available space constraints.

Treatment of stormwater runoff is designed to reduce concentrations of suspended sediment, dissolved and particulate nutrients, trace metals, trash and debris, oil and grease, and toxins such as trace organics. An effective detention, retention, or wetland facility will reduce most or all of these pollutants to levels below predevelopment levels. Parameters of facility design that affect the pollutant removal performance of these facilities include the residence time (length-to-width ratio); the depth of the permanent pool; the total depth; the existence of a plunge pool; the presence, density, and type of vegetation; and the presence and length-to-width ratio of a forebay.

Table 9 provides a brief description of the alternative detention and retention facilities and their respective design goals, as well as important design features.

Generally, stormwater management ponds can be categorized into five basic types (Figure 7). Each type is described briefly below.

Figure 7. Design Variations

Stormwater Detention Ponds

Stormwater detention ponds are usually dry ponds that provide hydrologic controls for increased runoff discharge flowrates. However, detention control volumes can be provided in wet ponds, above the retention volume. These ponds temporarily detain stormwater, releasing it at a predetermined design flow rate, generally that of predevelopment conditions. They are intended to remain dry between storm events. Unless significant infiltration occurs, the post development increases in total stormwater runoff volume are not significantly changed by detention ponds (Schueler, 1987).

Stormwater Retention (Wet) Ponds

Stormwater retention ponds are often referred to as "wet" ponds because they are designed to have a permanent pool of water. This permanent pool enhances particulate settling by increasing residence time, and also provides conditions for growth of aquatic vegetation, thereby enhancing filtration, and metals and nutrient uptake (transpiration). The permanent pool volume is often defined as the volume equivalent to three times the water quality volume or 12.7 mm (0.5 in) of runoff from the contributing drainage area (Yu and Kaighn, 1992). Pollutant removal efficiency is a function of pond depth, residence time, drainage area-to-pool volume ratio, and existence of aquatic vegetation. The post development increases in total stormwater runoff volume may not be significantly changed by retention ponds.

Stormwater Extended Detention Ponds

Stormwater extended detention ponds are designed to temporarily detain stormwater runoff for an extended period of time, generally 12 to 24 hours. Longer detention times have been found to provide optimal pollutant removal (Schueler, 1987). The detention time is a function of the size of the outflow opening with respect to the storm event runoff volume. These facilities are usually designed for the purposes of providing water quality treatment for the first flush of stormwater runoff, and may also provide quantity control for small storm events (1-2 year) necessary to minimize downstream bank erosion. Pollutant removal of particulates is primarily accomplished by gravitational settling (Schueler et al., 1992).

Extended Detention Dry Ponds: Dry extended detention ponds are normally "dry" between storm events, and therefore, do not have a permanent pool of water (Schueler et al., 1992).

Extended Detention Wet Ponds: Wet extended detention ponds improve the water quality treatment efficiency of their dry counterparts by providing additional settling and particulate removal. The extended detention volume is computed as the volume above the normal (permanent) pool elevation.

Wetlands/Shallow Marsh Systems

Stormwater wetlands are typically hybrids of either detention, retention or extended detention ponds, that temporarily store stormwater runoff in shallow pools throughout the facility. Design conditions are such that emergent and riparian wetland plants thrive within these facilities, adding to their pollutant removal and wildlife habitat benefits. These facilities require adequate baseflow conditions to maintain their permanent pool to support vegetation.

References

Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual For Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments, Washington, DC.

Schueler, T.R., P.A. Kumble, and M.A. Heraty. 1992. A Current Assessment of Urban Best Management Practices - Techniques for Reducing Non-Point Source Pollution in the Coastal Zone. Metropolitan Washington Council of Governments, Department of Environmental Programs, Anacostia Restoration Team, Washington, DC.

USEPA. 1995. Economic Benefits of Runoff Controls. EPA 841-S-95-002. U.S. Environmental Protection Agency (USEPA), Office of Wetlands, Oceans and Watersheds.

Yu, S.L., and R.J. Kaighn. 1992. VDOT Manual of Practice for Planning Stormwater Management. Federal Highway Administration. FHWA/VA-92-R13, Virginia Department of Transportation, Virginia Transportation Research Council, Charlottesville, VA.