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Detention / Retention

Dry Detention Basins > Dry Detention Vaults and Wet Vaults > Wet Retention Ponds >
Constructed Wetlands

Dry Detention Basins

Dry detention basins temporarily detain and store collected stormwater runoff for a period of time, releasing the stormwater slowly to reduce flooding and remove pollutants. They are referred to as "dry" because these devices dry out between rain events. Pollutants are removed by allowing particulates and solids to settle out from the water. Detention does not normally occur until the inflow rate exceeds the design outflow rate, therefore the runoff from smaller, more frequent events is not normally detained in conventional detention facilities. Overall pollutant removal in conventional dry detention devices is low to moderate. Important reasons for use of dry detention basins are reducing peak stormwater discharges, controlling floods and preventing downstream channel scouring.

Extended dry detention basins are a variation on the conventional design and are typically designed to detain the runoff from smaller, more frequent storms. Larger flows may be bypassed, depending on the design objectives. Extended detention facilities employ lower release rates than conventional detention facilities, resulting in a longer storage time for the detained water. In contrast to conventional dry detention basins, extended detention basins are designed to retain stored water for up to 72 hours following the design event. The longer detention time has two potential benefits:

  1. duration and frequency of bankfull flows can be reduced
  2. a significant reduction in particulate contaminants can be achieved through gravity settling during the time the water is contained in the basin (GVSDD, 1999).

Multi-purpose dry detention basins can be designed to meet several objectives in a single facility by designing the outlet structure to achieve both extended detention of smaller, more frequent events and conventional short-term detention of larger flows. This may require a multiple combination of orifices, weirs and spillways in the outlet structure. A permanent micropool may be included at the outlet to prevent re-suspension of sediments (GVSDD, 1999).


The design of dry detention basins includes proper site location, determination of the appropriate detention time, capability to treat the expected range in volumes of stormwater runoff and maintenance procedures and schedules. The storage and outlet structures should be sized according to design objectives for flood control, streambank erosion control or contaminant removal. The stormwater should be held for at least 24 hours for maximum pollutant removal. Soils should be permeable to allow the water to drain from these basins between storms and the water table should be more than 0.6 m below the bottom of the basin (to avoid a permanent pool of water in the basin during wet weather). A forebay is a section of the basin separated from the main part by a wall or dike which receives the incoming stormwater. Forebays help capture debris and sand deposits, which accumulate quickly, thereby easing routine cleaning.

Dry detention basins should be landscaped so that the facility integrates into the neighbourhood. The basin should be planted with native grasses or turf to enhance sediment entrapment and protect against erosion. This may require water tolerant species on the basin bottom. The design should also minimize thermal impacts including shading vegetation, north-south alignment and avoid excessive use of riprap and concrete (GVSDD, 1999).

For more details on general design criteria and considerations refer to the GVRD's Best Management Practices Guide for Stormwater. For more detailed design guidance see KC (1998), WEF (1998), FHWA (1996) and WSDOE (1992).


An appreciable body of knowledge has been accumulated on the design and maintenance of these structures. Conventional dry detention basins have proven effective at matching post-development peak flows to pre-development peak flows. Removal of contaminants from conventional basins is negligible; however, it is improved in extended designs. Horner et al. (1994) noted the following contaminant removal efficiencies:

  • suspended solids: 50% to 70%
  • total phosphorous and total nitrogen: 20% to 40%
  • lead: 70% to 90%
  • zinc: 30% to 60%
  • hydrocarbons: 50% to 70%
  • bacteria: 50% to 90%

Detention basins can serve small to rather large areas and are usually readily incorporated into the design of the overall development. Existing dry basins built to control stormwater peak flows can be modified to provide extended detention for stormwater.


Dry basins are not very effective in removing soluble pollutants from stormwater. Also, many pollutants that settle out can be re-suspended in subsequent storm flows and discharged from the basin. Many dry basins end up with permanent pools of water because runoff from previous storms has not flowed out or infiltrated before another storm occurs. This may be caused by a high clay content in surrounding soils or high frequency of storms. The standing water can be a nuisance and an eyesore to residents, especially if floating and other debris accumulate. Because they take up large areas, dry detention basins are generally not best suited for high-density residential developments and should be located where they are not easily seen or where they can be concealed with landscaping. The maintenance costs associated with dry detention basins are higher than other stormwater treatment devices. If dry detention basins are not maintained, outlet structures may become clogged with trash and debris. Sites must allow easy access for equipment to maintain and clean the basin and remove sediment. Finally, longer detention times may limit the survival of bottom vegetation and produce boggy areas that can be difficult to clean and maintain. Vegetation may be difficult to establish and sustain in areas with extremely sandy soils, impermeable layers and/or bedrock outcrops.


Maintenance of dry detention basins is both essential and costly. General objectives of maintenance are to prevent clogging, standing water and the growth of weeds and wetland plants. This requires frequent unclogging of the outlet and mowing. Normal maintenance costs can range from 3% to 5% of construction costs on an annual basis (Schueler 1987). Cleaning out sediment with earth-moving equipment is expensive and will be necessary in 10 to 20 years.

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Dry Detention Vaults and Wet Vaults

Detention vaults and tanks are underground storage/treatment facilities constructed of reinforced concrete (vaults) or corrugated pipe (tanks). They may be used to handle general site runoff or be dedicated to runoff from impervious surfaces such as roofs and parking lots. Detention vaults may be designed to maintain a permanent water pool (wet vaults) (GVSDD, 1999).

Dry vaults are similar in nature to dry detention basins. Conventional dry detention vaults are on-line facilities designed to control the frequency of flooding downstream by limiting the peak runoff flow. Extended dry detention vaults are also on-line facilities, but they employ lower flow release rates to control the frequency of bankfull flows for streambank protection in addition to flood control. Unlike extended detention dry basins, extended detention dry vaults do not accomplish significant removal of contaminants (GVSDD, 1999).

Wet vaults are similar in nature to wet ponds except that, being underground, wet vaults lack at least some of the biological contaminant removal mechanisms that are present in wet ponds (e.g., uptake and conversion by algae to aquatic plants, filtering through root mats etc.). A live storage volume can be added above the permanent pool for flow control as well as water quality enhancement. If water quality improvement is the only objective, the wet vault is designed off-line, and larger flows are bypassed (GVSDD, 1999).

Contaminant removal for wet vaults is poorly documented and are assumed to remove particulate contaminants only because the potential for biological action is limited. For wet vaults, similar contaminant removal efficiencies to those from extended detention dry basins can be assumed. For dry vaults, assume negligible contaminant removal. For water quality improvement, dry vaults should include other upstream or downstream BMPs (GVSDD, 1999).

General design criteria for dry and wet vaults are provided in GVSDD (1999). For detailed design guidance see FHWA (1996), WSDOE (1992), KC (1998), WWC (1995) and Olympia (1994).

Both dry vaults and wet vaults can provide flood control, streambank erosion protection and help to prevent water level fluctuations. As mentioned above, contaminant removal is limited to particulates (sediments, particulate metals and organics) for wet vaults and is negligible for dry vaults. Wet vaults help prevent sedimentation and obstruction of the drainage system, and prevent sedimentation of fish spawning beds. Wet vaults may be more prone to odours than dry vaults. Dry vaults and wet vaults have a high capital cost compared to surface BMPs and are not normally cost effective unless space limitations preclude the use of equivalent surface BMPs (basins, ponds, wetlands). There are few aesthetic concerns since the vaults are normally underground (GVSDD, 1999).

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Wet Retention Ponds

Swales are natural depressions or wide, shallow channels used to convey and treat stormwater. Basic design variations include grassed swales and wet channels. Both are commonly referred to as biofiltration swales (GVSDD, 1999).

Grassed channels are gently sloped, open ditches lined with turf grass or native vegetation that can take the place of conventional stormwater conveyance and piping systems. Grassed channels are designed to convey stormwater during runoff events and are normally dry between storms. The vegetation helps to decrease stormwater flow velocities. The resulting increase in the time of concentration helps to reduce peak flow rates, which in turn helps to reduce flooding and streambank erosion. Some of the flow may also infiltrate into the ground, reducing overall runoff volume. In addition, removal of contaminants can be accomplished through filtration by plant stems, adsorption to soil particles and biological processes. Grassed channels are normally designed to convey the runoff from relatively large, infrequent storms (e.g., the 10-year event) while providing filtration and water quality treatment for smaller, more frequent events (e.g., the 6-month event).To treat smaller, more frequent storm, the flow depth does not normally exceed the height of the bottom vegetation (GVSDD, 1999).

Wet swales are similar to grassed channels except that the channel contains standing water between storms. Standing water may be due to a high ground water table or high baseflow, in which case the swale will be wet for extended periods. Alternatively, check dams, with or without low flow openings may be added to store the smaller storm volumes in shallow ponding areas within the swale. Check dams help reduce flow velocity, promote infiltration, enhance settling of particulates and can result in increased infiltration and evapotranspiration for smaller storms. Enhanced contaminant removal occurs in standing water pools within the swale through mechanisms such as gravity settling of particulates, filtration of solids by root mats and soils, adsorption to soil particles, chemical transformations and uptake or conversion to less harmful forms by plants and bacteria. Sections of the swale that will contain standing water for extended periods are planted with water tolerant or wetland vegetation, while the slopes are normally planted with turf (GVSDD, 1999).

The primary function of swales is water quality enhancement; however, they do provide some attenuation of peak flows for flood control and streambank erosion protection. Some ground water recharge is possible. They can be used as pretreatment for other BMPs and are particularly suited alongside roadways and parking lots. Swales are also suitable for new developments and may be retro-fitted to existing developments and re-developing areas or to existing ditches if space is available (GVSDD, 1999).

Grassed channels and wet swales are relatively effective for capturing suspended solids, oils and particulate metals but are less effective for removing soluble metals and nutrients. Contaminant removal is a function of length and the following efficiencies were reported in FHWA (1996):

  • total suspended solids: 70%
  • total phosphorous: 30%
  • total nitrogen: 25%
  • heavy metals: 50% to 90%
  • oxygen demand: 25%
  • oil and grease: 75%

General design criteria and considerations for grassed channels and wet swales are provided in the GVRD's Best Management Practices Guide for Stormwater (1999). Detailed design criteria are provided in Horner et al. (1994), MMS (1992), WEF (1998), Claytor and Schueler (1996), KC (1998), Horner (1988) and WSDOE (1992). Swales should consist of a sandy loam topsoil layer with organic content of 10% to 20% and no more than 20% clay. Manure mulching and high fertilizer hydroseeding should not be used for establishing groundcover because it may cause nutrient export. Compaction during construction should be minimized and runoff should be diverted during establishment of vegetation (GVSDD, 1999).

Swales should be inspected routinely (monthly) and especially after large storms (WEF, 1998). Erosion problems should be corrected as necessary. Routine mowing will help keep the grass in an active growing phase and maintain dense cover; clippings should be removed to prevent clogging of outlets and prevent nutrient release. Inlet flow spreaders should be kept free from debris, and debris and trash should be removed for aesthetic purposes. If sediments accumulate to the point where they cover vegetation or the capacity of the swale is reduced, they can be removed by hand using a flat shovel. Damaged areas should be re-seeded immediately (GVSDD, 1999).

Grassed swales and wet channels are relatively inexpensive (more costly where the swale channel is deep), technically simple and can significantly reduce the the sediment and contaminant load on downstream facilities. They are more appropriate for removing low concentrations of oil and grease (<10 mg/L) than coalescing plate separators. They can be aesthetically pleasing if properly maintained, however insects may become a nuisance in wet swales (GVSDD, 1999). Swales can reduce development costs by combining conveyance and treatment in one system; they have a lower construction cost than conventional conveyance systems that include curbing, inlets and pipes (DDNREC, 1997).

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Constructed Wetlands

Constructed wetlands are not typically intended to replace all of the functions of natural wetlands, but to serve, as do other water quality BMPs, to minimize point source and non-point source pollution prior to its entry into receiving waters. Constructed wetlands can provide many of the water quality improvement functions of natural wetlands with the advantage of control over location, design and management to optimize those functions. While costs can vary significantly, constructed wetlands have successfully provided these functions at lower cost than conventional wastewater treatment options (USEPA, 1988).

Constructed wetlands vary widely in their pollutant removal capabilities, but can effectively remove a number of contaminants (Bastian and Hammer, 1993; Bingham, 1994; Brix, 1993; Corbitt and Bowen, 1994; USEPA, 1993). Schueler (1997) reported the following contaminant removal efficiencies:

total suspended solids: 78% organic carbon: 28%
total phosphorous 51% soluble phosphorous: 39%
total nitrogen: 21% nitrate nitrogen: 67%
lead: 63% copper: 39%
zinc: 54% cadmium: 69%
hydrocarbons: 90% bacteria: 77%

Among the most important removal processes are the purely physical processes of sedimentation via reduced velocities and filtration by hydrophytic vegetation. These processes account for the high removal rates for suspended solids, the particulate fraction of organic matter (particulate BOD) and sediment-attached nutrients and metals. Oils and greases are effectively removed through impoundment, photo-degradation and microbial action. Similarly, pathogens show good removal rates in constructed wetlands via sedimentation and filtration, natural die-off and UV degradation. Dissolved constituents such as soluble organic matter, ammonia and ortho-phosphorus tend to have lower removal rates. Soluble organic matter is largely degraded aerobically by bacteria in the water column, plant-attached algal and bacterial associations and microbes at the sediment surface. Ammonia is removed largely through microbial nitrification (aerobic) - denitrification (anaerobic), plant uptake and volatilization. Nitrate is removed largely through denitrification and plant uptake. In both cases, denitrification is typically the primary removal mechanism.

Microbial degradation processes are relatively slow, particularly those that are anaerobic, and require longer residence times, which contributes to the more variable performance of constructed wetlands systems for dissolved constituents. Phosphorus is removed mainly through soil sorption processes, which are slow and vary based on soil composition. Phosphorus is also removed through plant assimilation and subsequent burial in the litter compartment. Consequently, phosphorus removal rates are variable and typically are less than those for nitrogen. Metals are removed largely through adsorption and complexation with organic matter. Removal rates for metals are variable, but are consistently high for lead, which is often associated with particulate matter.

In golf course design, constructed wetlands can be a key factor in removing pollutants and moderating peak flows. They can be incorporated into the rough or built along the periphery but should not disrupt the functioning of existing natural wetlands or drainage patterns. These wetlands filter, settle and take up nutrients and other pollutants and can tolerate short period water level fluctuations of up to a meter. The main vegetative elements are emergent grasses, sedges, rushes, reeds and cattails as well as floating plants.


General design criteria are provided in GVSDD (1999). For detailed design guidance refer to CWP (1997), WSDOE (1992), KC (1998), Horner et al. (1994), CDM et al. (1993), OMEE (1994), WEF (1998) and WWC (1995).

The use of constructed wetlands for stormwater treatment is still an emerging technology, so there are no widely accepted design criteria. Certain general design considerations exist. It is important first to reduce stormwater inflow velocities and provide an opportunity for initial sediment deposition. Facilities should be periodically maintained to avoid the likelihood of re-suspending deposited sediment in subsequent inflows. It is important to maximize the nominal hydraulic residence time and maximize the distribution of inflows over the treatment area, and avoid designs which may allow for hydraulic short-circuiting. Emergent macrophyte vegetation plays a key role and is intimately linked with the sediment biota. These plants provide attachment sites for periphyton; physically filter flows; are a major storage site for carbon and nutrients; are an energy source for sediment microbial metabolism; and are a gas exchange vector between sediments and air. It is therefore important to design for a substantial native emergent vegetative component. Anaerobic sediment conditions should be ensured to allow for long-term burial of organic matter and phosphorus. A controlled rate of discharge is the last major physical design feature. While an adjustable outfall may seem desirable for fine-tuning system performance, regulatory agencies often require a fixed design to preclude subsequent inappropriate modifications to this key feature. The outfall should be fitted with some form of skimmer or other means to retain oil and grease. Plants must be chosen to withstand the pollutant loading and the frequent fluctuation in water depth associated with the design treatment volume. It is advisable to consult a wetlands botanist to choose the proper vegetation.


Constructed wetlands can be used for peak flow control and streambank erosion protection, water quality enhancement and community enhancement (i.e., recreational and aesthetic values). They are suitable for new developments, but not for existing developments unless they can be retrofitted to existing parks and greenways. Constructed wetlands are suitable for residential areas, municipal office complexes, municipal repair or maintenance yards, small on-site facilities or large regional facilities. They are not practical for highly developed urban areas (GVSDD, 1999).

The use of constructed wetlands has expanded recently to the treatment of solid waste landfill leachate. Experimental work has shown promise for this application as a low-cost alternative to collection and transport to wastewater facilities. Leachate from solid waste landfills can vary widely in composition, but is often sufficiently high in BOD, ammonium, iron, and manganese, and sufficiently reduced as to be toxic to plant and animal life. Dornbush (1989) described an interception trench with predominantly open water habitat which successfully intercepted and improved a leachate ground water plume from a municipal solid waste landfill. In addition to dilution effects, the leachate quality was apparently improved by processes resulting from aeration of the anoxic ground water. Hydrogen sulphide, methane, and carbon dioxide gases were believed to be oxidized, while the high carbonate content provided for chemical precipitation of metals in the aerobic environment. Surface et al. (1993) obtained significant, low-cost improvement of leachate using subsurface flow wetland systems. They found that substrate mixtures of sand and gravel achieved significant removals of BOD, ammonium, iron, manganese, potassium, and phosphorus, and provided better treatment than pure coarse or pea gravel media. All media types showed seasonal performance patterns.


The location of constructed wetlands in the landscape can be an important factor in their effectiveness. Mitsch (1993) observed, in a comparison of experimental systems using phosphorus as an example, that retention as a function of nutrient loading will generally be less efficient in downstream wetlands than in smaller upstream wetlands. He also cautioned that the downstream wetlands could retain a larger mass of nutrients and that a placement tradeoff might be optimum. Mitsch observed that creation of in-stream wetlands is a reasonable alternative only in lower-order streams, and that such wetlands are susceptible to reintroduction of accumulated pollutants in large flow events as well as being unpredictable in terms of stability. Such systems would likely require higher maintenance and management costs.


Properly constructed and maintained wetlands can provide very high removal of pollutants from stormwater. Constructed wetlands can be used to reduce stormwater runoff peak discharges, to provide water quality benefits, and to provide a pleasing natural area. Wetlands are highly valued by residents; therefore they can be given high visibility, they can serve as attractive centerpieces to developments and recreation areas, and they typically increase property values (Schueler, 1987; Shaver, 1992). Constructed wetland systems can provide ground water recharge in the area, thus lessening the impact of impervious surfaces. This recharge can also provide a ground water subsidy to the surficial aquifer, which can benefit local vegetation and decrease irrigation needs.

In the Herrings Marsh Run watershed of North Carolina, an 3.2 ha wetland was constructed by stabilizing the walls of an old dam. This produced a wetland with a maximum depth of 2 m but a mean depth under 0.5 m. Of this area 40% was covered by aquatic plants and 40% was treed. The total area of the wetland was less than 1% of the watershed draining through it but it reduced incoming water of 7 ppm nitrate to outgoing water of less than 1 ppm nitrate in warm months. The nitrogen load to the stream was lowered by 40% (ENN, 1998).


Constructed wetlands may contribute to thermal pollution and cause downstream warming. This may preclude their use in areas where sensitive aquatic species live. They are not a competitive option compared to other treatment methods where space is a major constraint. Cost can also be a factor as they are more complex to construct than most other surface BMPs. The efficiency of constructed wetlands may be delayed until aquatic plants are well established (GVSDD, 1999). There may be some public concern with respect to nuisances (mosquitoes, odours) and the ponded water may be a safety hazard to children.


Constructed wetlands have an establishment period during which they require regular inspection to monitor hydrologic conditions and ensure vegetative establishment. Maintenance of structures, monitoring of vegetation and periodic removal of accumulated sediments must be provided to ensure continued function (Wetzel, 1993; Bingham, 1994). Maintenance costs vary depending on the degree to which the wetlands are intended to serve as popular amenities. Frequent initial maintenance to remove opportunistic species is typically required if a particular diverse, hydrophytic regime is desired. Operators of wetlands may need to control nuisance insects, odours and algae.

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