Detention ponds (dry ponds) and retention ponds (wet ponds) are two different types of water management ponds. In summary, both ponds aid in flood control and stormwater runoff treatment. They collect excess water and hold it for a period of time to help control erosion and water quality.
Sediment and bacteria, metals, nutrients, and other suspended solids settle out of stormwater in the ponds. These ponds can restrict and prevent the transport of pollutants to lakes or streams during storms.
Table of Contents
The Difference Between Detention and Retention Ponds
The most significant distinction between a retention basin and a detention basin is whether a permanent body of water or pond is present.
- These ponds typically hold water all year round.
- Always contain a riser to allow for excess water to flow out during rain events.
- Always have an emergency overflow area where the water can rush out without eroding to pond edge in the case of an extreme storm event.
- Water levels fluctuates due to water runoff and precipitation from the surrounding areas.
- Keeping a pool inhibits resuspension and maintains accumulated sediments on the floor of the holding area.
Wet retention ponds are a form of stormwater management that collects and treats polluted stormwater runoff. They regulate the quality and quantity of stormwater through the retention and storage of stormwater runoff. By holding the water, the pond’s natural mechanisms then remove pollutants.
Properly graded pitches and a system of underground pipes links storm drains and divert the water to a wet retention pond. These structures direct significant quantities of water to the pond, while the outlet releases lesser amounts of water as required to keep the water level at the optimum level.
Standing water is still a source of concern from a health perspective. Standing water can be a potential drowning risk, especially for children. Mosquitoes can breed in the stagnant areas of any pond, which can lead to mosquito-borne diseases. When designing retention ponds, developers should always plan for safety ledges, circulation, and maintenance plans to avoid these pitfalls.
It is important to remember that each new retention pond creates a new habitat and haven for animals. If appropriately designed, they will improve water quality, reduce flood concerns and erosion, and raise surrounding property values.
Too often, wet retention ponds are installed with the right intention but then never maintained or maybe over-maintained, leading to an unhealthy environment.
The retention ponds that exist in more wild settings that are allowed to grow beneficial aquatic and surrounding plants do an excellent job of filtering and cleaning water.
When the retention ponds are located near subdivisions, they are all too often mismanaged, which leaves them full of fertilizer runoff, void of beneficial plants, and chemically treated in an attempt to eliminate the symptoms caused by the mismanagement.
- These ponds are dry for the majority of the year.
- Always have outflow pipes and chutes to allow for storm water to flow through.
- Will always have protected emergency spillways to avoid erosion and failure during extreme weather event.
- Water will slow and stand in them during storm events.
- They are used more to control and slow stormwater runoff than to retain it.
In dry areas, detention ponds (or dry ponds) are more prevalent and function as significant flood control structures. They are normally dry, save for times when snow is melting or rain is falling.
Their main objective is to slow down the water flow and hold it for a limited time—at least 24 hours. This allows enough time for soil particles and related contaminants to settle out. These systems are used in urban environments to minimize peak runoff levels caused by storms which will help to alleviate flooding.
Dry ponds may be developed to accommodate a wide range of storm occurrences and uses. Designers will consider slopes, watershed areas, plant material, etc., when specifying the layout and size of a new detention pond. In addition, an emergency spillway is typically needed to ensure protection during floods.
The basins are essential for storing and controlling stormwater runoff velocity from surrounding areas, especially in places where asphalt or concrete construction occurs. Stormwater runs much more quickly from these surfaces than from the naturally vegetated ground, so it must be diverted and slowed down to prevent excess sediment transportation and erosion.
The amount of water that can be cleaned and treated is dependent on the size of the basin. Only flood flows are regulated by dry basins (detention basins). By eliminating contaminants and sediments, a retention pond can help to improve water quality.
Dry detention ponds are most effective in locations with ten acres or more of land. The large expanses of land will collect more runoff and make a detention pond more of a necessity.
Dry retention ponds typically have a very minimal slope to divert water. The inlet must be no more than fifteen percent higher than the outlet to ensure the proper amount of water flows into and through the system.
This facility functions by providing a large space for water collection. The water eventually flows out the bottom of the structure through the outlet or soaks into the ground. Concrete and other objects may be used as buffers to slow the flow of water and collect debris.
Dry detention basins are advantageous as stormwater control devices for many reasons. They are less expensive to install than a wet retention pond since they occupy less space than other solutions, and they are fairly straightforward when it comes to design. When properly positioned and built, they can efficiently minimize peak rate and volume to pre-development levels.
It is important that the plants in and around the detention basins can tolerate both dry weather and standing water for a while so that they can survive the environmental extremes that these basins encounter.
The downside of detention basins would be that they take up a considerable amount of real estate that can’t really be used for any other purpose other than maybe nature walks. The built structures may make the land they are on less desirable for sale and limit the potential for development. Many people will see this as a downfall, not understanding the valuable service they provide, ensuring the long-term stability of the area.
While different in function, people often prefer a retention pond that holds water and provides a valuable water body.
General Maintenance and Problem-fixing Tips
One of the most important things for both basins is to ensure that the overflow and outflow devices do not get clogged or blocked. This is one of the most critical maintenance tasks since the ponds and basins will work effectively only if the pipes are kept free of debris. Ensuring regular upkeep for detention and retention ponds will save money in the long run.
The type of vegetation in the basin’s surroundings will significantly affect the level of maintenance required. Too often, surrounding homeowners who don’t understand vegetation’s role in water filtration will mow their lawns right to the water’s edge. This encourages more debris and pollution entering the pond and promotes fertilizer runoff into the pond.
Some Maintenance Tips for Retention Ponds
- Retain and encourage a band of tall un-mowed vegetation surrounding the pond to aid in the filtering and cleaning of water.
- Remove any trees or saplings that may obstruct outflow structures.
- Never fertilize the lawns in the ponds’ watershed. Chemical fertilizers are certainly not natural to pondwater and will upset the pond’s balance, always resulting in an algae-laden pond.
- Aquatic plants and terrestrial plants are essential for water cleaning, but the introduction of invasive plants such as cattails can lead to increased long-term maintenance as some of these plants will grow out of control and end up filling in the pond over time.
- Be sure that native or easy-to-maintain species are planted in slopes, banks, and other vegetation-friendly areas.
- Maintenance must always include repairing any areas that have experience soil erosion. Look for gullies and other problems on the bank a few times a year and after severe storms.
- Reseeding areas with exposed soil, particularly on slopes around the site, will help to prevent erosion.
- In areas where erosion seems to re-occur, rip rap and sediment barriers should be installed to prevent future erosion events.
- Always remove debris and sediment from pipes to maintain their satisfactory operation.
- Remove the debris or prevent debris from entering the pond to help alleviate outflow clogging problems.
- In the case that excess sediment has changed the ability and function of overflow structures to function correctly, it must be physically removed.
- Fertilizers, pesticides, or herbicides should not be used in or around the facility. Chemicals pollute the water and may encourage the growth of undesirable plants.
A Note on Algae
As temperatures rise and the nutrients in the water increase, algal blooms become more frequent. Nutrient levels rise when manure, pollution, and runoff enters the water.
There are potential toxic algae blooms, but these are very rare. For the most part, excess algae is simply a symptom of an unbalanced pond.
Humans tend to believe that the algae needs to be killed when it actually needs to be controlled, not killed.
Algae is a natural part of the pond ecosystem that can easily be controlled naturally, using proper maintenance as listed above and potentially increasing water flow and oxygenation using aerators.
Mosquitoes prefer to lay eggs in and near standing water; Aedes aegypti (a mosquito that spreads the Zika virus) is regarded as a “container-breeding mosquito.”
To minimize the mosquito breeding areas, the introduction of aeration and water circulation is suggested.
According to studies, female mosquitoes tend to lay eggs in collected water or water in human-made containers.