Stormwater ponds remove pollutants and provide flood control, but many factors come into play when designing the right type.

By Gordon England

In the early 1980s, the State of Florida began to require stormwater treatment in new developments to reduce pollutants associated with rainfall runoff. In the ensuing years, the state designed and built thousands of stormwater ponds to fulfill this requirement. As many types of ponds were constructed and observed, the design criteria evolved to account for the different types of ponds and their use in Florida’s semitropical climate. This article explores the strengths and weaknesses of more commonly used ponds in Florida.

Typically, ponds provide 30-80% pollutant removal for various constituents, depending on the type of pond utilized and pollutant considered. Total suspended solids (TSS), total phosphorus (TP), total nitrogen (TN), floating trash, heavy metals, biological oxygen demand (BOD), bacteria, greases, oils, and turbidity are the more commonly targeted pollutants in stormwater runoff. Heavy metals, particulate phosphorus, and turbidity are generally associated with suspended solids. Therefore, settling of suspended solids in a pond usually decreases these pollutants. Biological processes in ponds provide for nutrient, BOD, and bacteria treatment, and properly designed ponds can effectively trap floating trash, greases, and oils. Although ponds have proven treatment capabilities, they are complex ecosystems, and many factors affect their removal efficiencies, including rainfall volumes, water temperature, turbulence, seasons, and biological activity.

The flood volume stored in the pond creates an additional positive benefit of ponds. This volume attenuates the additional flows produced by the impervious areas of existing development and reduces the impacts upon downstream stormwater systems. Ponds can reduce the need for municipal capital improvement projects required for flood control over time. Existing ponds that already provide flood control can easily be modified or retrofitted to provide water-quality benefits.

There are no standards for evaluating efficiencies at this time, but EPA is in the process of establishing such standards. Therefore, the efficiencies quoted are only approximate. The basis for removal-efficiency information provided for ponds comes from a report prepared by Harvey Harper, Ph.D., P.E., titled "Pollutant Removal Efficiencies for Typical Stormwater Management Systems in Florida" (1995), in which he performed a literature review of nearly 20 years of research on permitted systems in Florida. The design of these stormwater management systems came from rules promulgated by the Florida Department of Environmental Protection and regulations developed by Florida’s water management districts. Initially enacted in 1982, these regulations have been refined over the past 19 years.

To facilitate the use of the information discussed, it is important to define several key terms.

Offline. A type of pond design where the design treatment volume is diverted off-line to a treatment pond by a control structure. When the offline pond becomes full, the remaining runoff bypasses the pond. This approach allows for the segregation of the "first flush" of runoff, which may contain the majority of pollutants.

Online. A type of design where all of the runoff from a storm routes through a pond. This method may have a lower pollutant removal efficiency than offline ponds if the pond volume is not large enough to hold all the runoff from a significant rain event. This is because the most polluted volume is mixed with the remainder storm volume, and excess flows dilute the concentrations and leave the pond with the mixed polluted water. If the online pond is sized properly, it can treat the entire runoff volume.

Detention. The collection and temporary storage of stormwater, generally for a period of time ranging from 24 to 72 hours, to provide for treatment through primarily physical, biological, and–to a much lesser degree–chemical processes with subsequent gradual release of stormwater to downstream receiving waters.

Retention. Onsite storage of stormwater with subsequent disposal by infiltration into the ground or evaporation to prevent direct discharge of stormwater runoff into receiving waters.

Recovery Time. The length of time required for the design treatment volume in a pond to subside to the normal water level or bottom of the pond. This time should normally be between 24 and 72 hours. This recovery may be accomplished by either infiltration or controlled release through an outfall structure.

Design Considerations

Skimmer on pond control structure

When determining the type of pond to design, it is important to consider the seasonal high groundwater elevation. The groundwater surface level is not high enough to be a factor in many areas of the country, but it is a critical design component in certain areas where the groundwater surface often lies just a few feet below the ground surface. The groundwater elevation can affect the type of pond that can be constructed, which in turn controls the treatment volume available within the pond. A pond with a shallow storage depth requires a larger amount of land and will not be as cost effective as a pond that has a deep storage depth on a smaller amount of land. A high groundwater level also can prevent infiltration ponds and exfiltration trenches from functioning properly.

Soil type should also be considered when choosing a pond design. Ponds need soils with high percolation rates for any type of system that depends on infiltration of the water-quality volume into the ground.

Sideslopes steeper than 4:1 cannot be mowed with riding mowers. Hand mowing increases maintenance costs.

The required retention volume is usually set with the goal of achieving an 80% reduction in pollutant loadings from the targeted pollutant. This volume sometimes increases for sensitive receiving water bodies based on project goals. This volume is typically the greater of 1.25 in. of runoff (as opposed to rainfall) over the impervious area within the drainage basin or 0.5—1 in. of runoff from the whole drainage basin. In addition, the functionality and effectiveness of every pond system are highly dependent on a strong maintenance and inspection program. Accumulated sediment must be removed, grass mowed, undesirable vegetation removed, and pond bottoms reestablished. An effective maintenance program will help ensure that the pond functions properly. Shallow, dry ponds may fill with sediment and require excavation after three to seven years. Shallow, dry ponds are usually not feasible when the inflow pipes or ditches are deep. Outfall structures should be designed to prevent trash buildup in order to function properly.

Skimmers (or hoods on top of the control structure) should be used on all ponds to prevent oils and floating trash from washing through the system. Skimmers over orifices and weirs prevent trash from plugging them. A 20-ft. flat access berm and easement should be provided around the top of any pond to allow access for maintenance equipment. Planting wetland vegetation in the shallow fringes of a pond may enhance nutrient uptake. Dry pond bottoms should be seeded, not sodded, for maximum percolation.

When used in new development, ponds generally require about 10% of the land area developed for subdivisions and 15-20% of the land area for commercial sites. These percentages include area for water-quality volume and peak flow attenuation. This might seem an onerous burden for developers–because to create ponds they must forfeit lots they could sell–but factors do mitigate their "loss." For example, the expense of pond construction is offset by lower storm-drain system costs because of the volume of water stored in the ponds. In addition, developers can use dirt excavated from pond sites for fill, also saving money. Finally, ponds often turn into amenities, offering hiking and biking trails, recreational opportunities, landscaping, and parklike settings.

Although there are many variations of pond designs, the following types are the more typical in Florida and will be discussed in detail:

1) Dry retention (online)
2) Offline retention/detention (dual pond system)
3) Wet detention
4) Wet detention with filtration
5) Dry detention

Online Dry Retention Pond

Online dry retention ponds receive all storm flows from the project watershed. Runoff volumes greater than the design retention treatment volume leave the pond through the properly designed outfall structure. The retention treatment volume is retained and percolates into the ground where the soil removes particulate pollutants. This is especially beneficial if groundwater recharge is desired. Properly designed and functioning dry retention ponds can have very high removal efficiencies because most of the stormwater runoff infiltrates into the ground and does not discharge to a receiving water source. Improperly designed ponds often become aesthetically unpleasing and tend to lose or never achieve their infiltration capacity, making them ineffective.

Dry ponds have been successfully integrated into multiple-use facilities such as parks and recreation areas, and with appropriate landscaping become an amenity. They also serve to recharge groundwater supplies. Table 1 shows treatment efficiencies for dry retention systems based on selected research studies in Florida.

Design Issues. The bottom of the pond should be kept at least 2 ft. above the seasonal high groundwater table or the pond might not function and might become infested with undesirable vegetation. If the seasonal high groundwater elevation is within 2 ft. of the pond bottom, a groundwater mounding analysis should be performed to ensure drawdown within the desired time.

Offline Retention/Detention (Dual Pond) Systems

Offline retention/detention systems consist of a two-pond system. The first is an offline dry retention pond that stores a pollution abatement volume that percolates into the ground. After the retention pond fills, a structure typically diverts the remaining flows to a dry flood detention pond for flood-control purposes. The detention volume of the detention pond is slowly released over a designated time period via an orifice or weir, typically in 36-72 hours.

The "first flush" stored in the retention pond is generally 0.5 in. of runoff from the drainage basin. The detention pond also provides for further treatment of remaining flows by settling and adsorption, even though most of the settable pollutants will be trapped in the retention pond. This type of system can have excellent pollutant removal efficiencies (Table 2).

Design Issues. The bottom of the pond must be kept at least 2 ft. above the seasonal high groundwater table or the pond might not function and might become infested with cattails or other similar vegetation. As with online dry retention ponds, if the seasonal high groundwater elevation is within 2 ft. of the pond bottom, a groundwater mounding analysis should be performed to ensure adequate drawdown within the desired time.

Wet Detention Ponds

Wet detention ponds are the most common and most researched ponds in Florida. These are inline ponds designed to maintain a permanent pool of water, which generally is at the seasonal high groundwater level. The control structure has a small weir or orifice set at the normal water level. The orifice is designed to slowly release the detention volume over a 24- to 72-hour period. A larger rectangular or V-notch weir is set at a higher level, which corresponds to the detention volume required. The weir length design allows postdevelopment flow rates out of the pond to be equal to or less than the predevelopment flow rates of the drainage basin for the required design event. This design flow rate determines the depth of water storage above the weir elevation. Larger storm events are typically released through an emergency overflow. Removal efficiencies in this type of pond are primarily a result of residence time. The residence time is equal to the detention volume divided by the outflow rate. Longer residence times provide higher removal efficiencies.

An additional benefit provided by wet ponds is the wildlife habitat created in and around the pond. Planting wetland plants in shallow littoral zones around the edges can further enhance the pond’s treatment and habitat values.

Many studies show that the ability of the system to remove TN is heavily dependent on the fraction of TN present as organic nitrogen. Organic nitrogen is not readily available through biological or chemical processes, and there are relatively few mechanisms for removal of this species in a wet detention system. In contrast, both NO₃ and ammonia are readily taken up in biological processes, which accounts for the relatively good removal efficiencies achieved for these species in wet ponds. In systems where organic nitrogen represents a dominant proportion of the TN in the incoming stormwater flow, removal of TN can be expected to be relatively poor. If inorganic species of NO₃ and ammonia represent the dominant nitrogen species found, then removal efficiencies for TN can be expected to increase. Table 3 shows treatment efficiencies for wet detention systems.

Design Issues. For safety purposes, sideslopes should have a maximum steepness of 4:1 if plans call for public access. The 4:1 sideslope should extend to 2 ft. below the normal water line and then can be steeper. Steeper slopes may be used if fencing restricts access, but mowing becomes more difficult. A minimum water depth of 5 ft. should be used to prohibit the growth of emergent vegetation. In areas of deep groundwater, a plastic pond liner can be used to keep the pond from drying out. Long flow paths should be used through the pond to reduce short-circuiting and maximize treatment times.

Wet Detention Ponds With Filtration Systems

Similar to wet detention ponds, this type has an underdrain pipe system to slowly release the retention volume instead of a weir or orifice. The underdrains are constructed around the perimeter of the pond with 18 in. of sand over the pipes to act as a filter. The underdrain level sets the normal water level in the pond.

These types of systems require a significant level of maintenance, which rarely takes place. Many of these systems fail after a few years. As shown in Table 4, these systems have poor removal efficiencies for nitrogen and phosphorus. The sand filters have significant effectiveness on suspended solids and constituents attached to the suspended solids but have little affect on dissolved pollutants. Therefore the use of these types of systems is not recommended. They are more expensive than wet detention ponds and have lower pollutant removal efficiencies. Agencies in the Florida have largely discontinued their use. Wet detention ponds with drawdown orifices are superior in design. Table 4 shows treatment efficiencies for wet detention systems with filtration based on selected research studies in Florida.

Design Issues. If the underdrain is below the seasonal high groundwater table, the amount of groundwater flowing into the pipe must be taken into account in sizing the pipe. It is more desirable to use a plastic liner around the underdrain bed to segregate it from groundwater infiltration. Cleanouts should be provided every 100 ft. of pipe. Gravel should be used for the upper 6 in. of filter bed to prevent vegetation from growing over the pipes, which greatly reduces percolation ability.

Dry Detention Ponds (Without Filtration)

Dry detention ponds are online ponds in which the retention and detention volumes are slowly released in two to three days via a control structure rather than by percolation. The principal pollutant removal mechanism is the settling action to the bottom of the pond. The same consideration of groundwater levels as for dry retention ponds must be observed. Pollutant removals are low so design criteria call for larger pond sizes to maximize settling times. Table 5 shows estimated treatment efficiencies for this type of pond.

Design Issues. The bottom of the pond must be kept at least 1 ft. above the seasonal high groundwater table. Other requirements include good vegetative cover on the pond bottom and low (nonerosive) velocities along the pond bottom during peak runoff events to prevent resuspending the solids that previously settled out on the pond bottom.

Alum Treatment Systems

Alum treatment of stormwater introduces active treatment concepts rather than traditional passive detention systems. It is the first step toward the type of chemical treatment used in wastewater treatment plants or water-supply reservoirs. It consists of injecting alum liquid into storm sewer lines before they discharge into a lake. The alum binds with suspended solids, heavy metals, and phosphorus, causing them to precipitate out to the bottom of the lake in a stable, inactive state. The use of alum treatment provides a 90% reduction in TP and suspended solids; 40-60% reduction in TN; 50-90% reduction in heavy metals; and greater than 99% reduction in fecal coliforms. It also increases water clarity significantly throughout the whole lake.

The use of alum in existing lakes results in immediate and substantial improvements in water clarity and quality, including long-term improvements to benthic communities of the lake. The increased benthic activity has kept floc accumulation depths to less than 1 cm/yr. Long-term monitoring of benthic organisms shows increases in total numbers of macroinvertebrates as well as a shift toward more desirable species. The safety of alum is proven in its use for treating potable water since Roman times. The first alum treatment system was designed at Lake Ella in Tallahassee, FL, in 1986 by Environmental Research and Design Inc. (ERD). Since then, ERD has designed more than 30 alum treatment systems throughout Florida. The information comes from the report "Alum Treatment of Stormwater - The First Ten Years: What Have We Learned and Where Do We Go From Here?"(1997) by Harvey Harper, Ph.D., P.E.; Jeff Herr, P.E.; and Eric Livingston.

The system is generally used with existing pipes discharging into existing lakes but may also be designed in conjunction with creating a new holding pond or with offline floc settling ponds and automatic floc disposal systems. When used with an existing lake, there is no cost of land acquisition and pond construction. The cost of an alum treatment system is around $250,000, regardless of drainage basin size. Therefore, these systems are not cost-effective for watershed areas smaller than 50 ac.

Design Issues. Injection points in the pipes should be 100 ft. upstream of the discharge points. Because alum concentrations should vary with the drainage basin’s pollutant loading characteristics, jar testing must be performed to determine alum dosing rate and the need for pH control. These are mechanical systems with high levels of maintenance of pump equipment, power, chemical replacement, routine inspections, and equipment replacement. Maintenance often runs $25,000-$50,000/yr., although it may be lower. Regulatory agencies require continued monitoring of water quality for alum systems, which further increases maintenance costs.

Because alum systems are usually used for whole lakes, normal removal efficiency evaluation is not appropriate. Rather, long-term water quality of the lakes is monitored. In the three lakes shown in Table 6, water-quality records for one year to 11 years before construction were compared to two to three years of monitoring after construction. In addition to reporting standard pollutant constituents, more holistic parameters are used, such as Secchi disc depth and Trophic State Index.

Summary

Ponds are generally the most common BMP used because of their ability to treat the highest number of pollutant parameters and their ability to reduce downstream flooding. When designing a pond, there are several factors to consider, including groundwater elevation, soil type, recreational uses, maintenance, and desired pollutant removals. There is no single best pond type. Table 7 compares design considerations and removal efficiencies for the various pond types.

In Florida, the wet detention pond is the most common type of pond used. Wet ponds offer very good pollutant removals, aesthetic amenities, and wildlife habitat, but it is not always desirable or feasible to construct a wet pond. Wet ponds with filtration have high-maintenance issues, and their use is not recommended.

Online dry retention ponds offer excellent pollutant removal and aquifer recharge. They can also have multiple uses in park settings, although the habitat values are minimal. Offline dry retention ponds also provide maximum pollutant removal, but they are not used as often because of additional land requirements.

Offline dry detention ponds provide poor pollutant removals for most parameters except TSS. They also require larger detention volumes, and this loss of additional land discourages their use.

Alum treatment systems are state-of-the-art treatment facilities giving significantly higher pollutant removals than wet detention ponds. Alum treatment also works on large lakes, reducing or eliminating the need for treatment systems upstream of the lake. This can be very attractive in lakes with highly developed watersheds. They are also very expensive, and a commitment to high maintenance should be recognized.

Gordon England, P.E., is a project manager with Creech Engineers Inc. in Melbourne, FL. This article is adapted from his book, Guide for Best Management Practice (BMP) Selection in Urban Developed Areas, available from ASCE.

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