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

Fact Sheet - Detention Ponds

Extended detention ponds have been used for a number of years in urban applications, and are designed to mitigate highway runoff stormwater quality and/or quantity impacts. These systems function by storing the increased runoff volume that results from development, then slowly releasing it at predevelopment runoff rates. The controlled release rate is designed to maintain the existing hydraulic conditions in the downstream watercourse (ASCE, 1992). The most commonly built facilities are dry extended detention (ED) ponds and wet ponds with extended detention. Figure 8 illustrates a cross-sectional view of a standard ED pond system design.

Figure 8. Cross-section view of a standard extended detention pond system
(Schueler, 1992)

Water quality benefits are achieved by treating the "first flush" of runoff from impervious areas. The "first flush" of runoff often contains the most pollutants. When extended detention is the method used for water quality treatment, the required volume is released over a long period of time, allowing sufficient time for particulates to settle out. Nutrients, heavy metals, and other pollutants associated with these particulates can also be removed.

Applicability

In an ultra-urban application, detention ponds are generally applicable as an end-of-pipe treatment facility. The pond design will be site-specific and extremely dependent on the site soils, existing utility conflicts, property ownership, and drainage area to be routed through the pond. Additional space constraints may reduce the applicability of some pond enhancement features such as a forebay, micropool, and safety bench. For example, the additional area needed to provide a safety bench (0.3 m [1 ft] wide strip around facility) may not exist in an ultra-urban setting. A safety alternative such as a chain-link fence, although not as aesthetically pleasing, may be required.

Another problem that may occur in siting detention ponds in ultra-urban environments is finding an adequate 100-year storm overflow path. Unfortunately, in the ultra-urban environment, space is usually limited at the end of storm drain systems. Additional opportunities for siting extended detention facilities are in medians, interchanges, adjacent to ramps, and along rights-of-way adjacent to roads.

Effectiveness

Properly designed detention ponds can greatly reduce the stormwater runoff impacts of highway development. When coordinated with other BMPs in the watershed, they can effectively reduce stormwater peak flows. Dry detention ponds can also remove up to 90 percent of particulates (Kehoe, 1993). Dry detention ponds, however, are not as effective at removing soluble pollutants. Other design approaches such as wet ponds and wetlands may be used in conjunction with extended detention for more efficient water quality control. Additional data on pollutant removal effectiveness of detention ponds is shown in Table 10.

Siting and Design Considerations

The success of a stormwater management pond design is very dependent on site-specific conditions. The major components common to each system are the water storage area for quantity and/or quality control and some type of outlet structure. The outlet structure can be a concrete or corrugated metal pipe (CMP) riser with openings to release the stormwater at the predevelopment runoff rates for specific storm events. The calculations and routings may be accomplished with very simple techniques, such as the Rational and Storage-Indication methods, or more complex models, such as HEC-22 or the Storm Water Management Model (SWMM), may be used.

A number of physical conditions are critical to siting and designing a pond. The side slopes of the pond and embankment may be steep. To protect both pedestrians and passengers, sufficient barriers, such as fences, guardrails, and safety zones, must be incorporated into the design. The saturated soils found below a wet pond can affect the structural stability of adjacent road embankments. The rate and timing of the peak discharge of the pond may be critical to preventing or increasing downstream flooding.

Although ponds are classified into the major categories of detention and retention facilities, there are also hybrid facilities that contain features found in both systems. The most common of such facilities, which are described below, are extended detention dry ponds and wet ponds with extended detention. Additional design examples and information can be found in Urban Drainage Design Manual Hydraulic Engineering Circular 22 (Brown et al., 1996), Evaluation and Management of Highway Runoff Water Quality (Young et al., 1996), and Design and Construction of Urban Stormwater Management Systems (ASCE, 1992).

Extended Detention Dry Ponds

Extended detention dry ponds can be designed as two-stage, or water surface elevation, facilities. In these cases, the upper stage stores and reduces flood peaks and the lower stage is designed for water quality control. The lower stage volume may be able to treat a certain depth of water over the impervious area, such as 12.7 mm (0.5 in) or a design storm frequency, such as the 1-year 24-hour storm event. The water is drawn down over a period of time, normally between 24 and 48 hours, through an orifice in the riser of the principal spillway. This residence time may allow for as much as 90 percent removal of particulates through settling (Young et al., 1996). Residence times that are too long may allow the water to become heated, resulting in a potential thermal impact to receiving waters. Removal of soluble compounds is limited in dry ponds. A shallow marsh or wetland may be incorporated into the design to facilitate removal of nitrogen and phosphorus. The incorporation of a forebay, energy dissipator, or pretreatment facility before flow enters the pond from a channel or pipe is important to lessen the impact of sediment and grit on the pond and to facilitate pond maintenance.

Extended Detention Wet Ponds

Wet ponds use a permanent pool of water to aid in achieving water quality control. The pool may cover the entire pond bottom or may be located in only a portion of the pond. Sufficient drainage area, fairly impermeable soils, and an adequate base flow to the pond are important to maintain a permanent pool. Sizing of the wet pool should consider the "first flush" runoff volume.

Consideration must also be given to water depth and pond length for settling. The pond depth must be deep enough, usually 0.9 m (3 ft) or more, so that wind-generated disturbance of bottom sediments does not cause the resuspension of sediments. Also, the pond depth should be shallow enough, usually 2.4 m (8 ft) or less, so that mixing occurs and the pond does not become anoxic. Pond depths in excess of 2.4 m (8 ft) should be avoided to prevent thermal stratification (Schueler, 1987). Alternating areas of shallow and deep pools in wet ponds can also be used to increase the sediment trapping efficiency and habitat diversity. Forebays are usually included to reduce sediment deposition throughout the system and facilitate maintenance. Incorporation of wetland plants along the fringe of the pond helps reduce erosion on the banks, provides some habitat, and may provide opportunities for nutrient removal.

The extended detention volume for a wet pond occurs above the water quality volume and below the crest of the pond. The water is released through openings in the outlet structure. An emergency spillway should be required to allow water to discharge safely in the event of a large-scale storm event.

Maintenance Considerations

Many detention facilities are embankment ponds. Regular inspections are required to check for seepage through the embankment, burrowing animals, deep-rooted vegetation, and erosion along the embankment and sides of the pond. Other routine maintenance includes reseeding of the pond banks and bottom and removal of debris from the spillway. Over time, sediment accumulation may significantly reduce the capacity of the pond. Studies have shown that every year up to 1 percent of the storage of the 2-year 24-hour storm event can be lost to sediment deposition (siltation) (Yousef et al., 1986). Sediment can reduce the quantity storage in a pond up to 20 percent over a 10-year period. Dredging of the material may be required every 5 to 10 years to restore the capacity of the pond. The sediment should be tested to determine if it is a hazardous material. Other considerations critical to the efficiency of the pond include maintenance of outlet structures, flow splitters, and clean-out gates (Koon, 1995).

Cost Considerations

Cost factors for stormwater management ponds are extremely sensitive to site conditions. Availability of in situ materials for embankment construction, outlet protection, cost of excavation, liner materials, and land costs are significant factors. Maintenance and inspection costs for mowing and periodic dredging are postdevelopment factors. Other technologies such as infiltration trenches may be more cost-effective in smaller drainage areas due to construction and long-term maintenance costs (Young et al., 1996). Studies have suggested that preliminary costs can be estimated by the following equation (adapted from Wiegand et al., 1986):

C = 168.39 x V0.69

C = construction cost estimate (1995 dollars) and
V = volume of storage of the pond (cubic meters) up to the crest of the emergency spillway.
This cost should be increased by 25 percent for construction contingencies.

References

ASCE. 1992. Design and Construction of Urban Stormwater Management Systems. The Urban Water Resources Research Council of the American Society of Civil Engineers (ASCE) and the Water Environment Federation. American Society of Civil Engineers, New York, NY.

Brown, S.A., S.M. Stein, and J.C. Warner. 1996. Urban Drainage Design Manual Hydraulic Engineering Circular 22. FHWA-SA-96-078. Federal Highway Administration, Office of Technology Application.

City of Austin. 1990. Removal Efficiencies of Stormwater Control Structures. Environmental Resources Management Division, Environmental and Conservation Services Department, City of Austin, Austin, TX.

City of Austin. 1995 (draft). Characterization of Stormwater Pollution for the Austin, Texas Area. Environmental Resources Management Division, Environmental and Conservation Services Department, City of Austin, Austin, TX.

Gain, S.W. 1996. The Effects of Flow-Path Modifications on Urban Water-Quality Constituent Retention in Urban Stormwater Detention Pond and Wetland System, Orlando, Florida. Florida Department of Transportation, Orlando, FL.

Harper, H.H., and J.L. Herr. 1993. Treatment Efficiencies of Detention with Filtration Systems. Environmental Research and Design, Inc, Orlando, FL.

Kehoe, M.J. 1993. Water Quality Survey of Twenty-Four Stormwater Wet-Detention Ponds. Southwest Florida Water Management District, Brooksville, FL.

Koon, J. 1995. Evaluation of Water Quality Ponds and Swales in the Issaquah/East Lake Sammamish Basins. King County Surface Water Management Division, Seattle, WA.

Martin, E.H., and J.L. Smoot. 1986. Constituent-Load Changes in Urban Stormwater Runoff Routed Through a Detention Pond-Wetlands System in Central Florida. U.S. Geological Survey Water Resources Investigations Report 85-4310. Tallahassee, FL.

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.

Wiegand, C., T. Schueler, W. Chittenden, and D. Jellick. 1986. Cost of Urban Runoff Quality Controls. In Urban Runoff Quality - Impact and Quality Enhancement Technology, ed. B. Urbonas and L.A. Roesner, p.366-382. American Society of Civil Engineers, New York, NY.

Young, G.K., S. Stein, P. Cole, T. Kammer, F. Graziano, and F. Bank. 1996. Evaluation and Management of Highway Runoff Water Quality. FHWA-PD-96-032. Federal Highway Administration, Office of Environment and Planning.

Yousef, Y.A., M.P. Wanielista, and H.H. Harper. 1986. Best Management Practices - Effectiveness of Retention/Detention Ponds for Control of Contaminants in Highway Runoff. Florida Department of Transportation, Gainesville, FL.

Yu, S.L., M. Barnes, R.J. Kaighn, and S.L. Liao. 1994. Field Test of Stormwater Best Management Practices in Watershed Wastewater Treatment. In Proceedings of the 1994 National Conference on Environmental Engineering. American Society of Civil Engineers, New York, NY.

Yu, S.L., S.L. Barnes, and V.W. Gerde. 1993. Testing of Best Management Practices for Controlling Highway Runoff. Virginia Transportation Research Council, Charlottesville, VA.

Yu, S.L., and D.E. Benelmouffok. 1988. Field Testing of Selected Urban BMPs in Critical Water Issues and Computer Applications. In Proceedings of the 15th Annual Water Resources Conference. American Society of Civil Engineers, New York, NY.