Retention Ponds: [Everything You Need to Know About]
Stormwater is a major concern because if not properly handled, it can lead to flooding, erosion, and pollution of neighboring water bodies.
Therefore, it is important to manage stormwater using the right tools and techniques.
Retention ponds are one of the most popular ways that help in managing stormwater.
They are man-made, shallow bodies of water that are built to hold stormwater runoff in order to avoid flooding.
A well-designed retention pond will reduce the negative environmental effects of stormwater runoff. In addition, it will provide additional benefits such as water storage and wildlife habitat.
In this post, we will cover everything you need to know about retention ponds, from basic design principles to available structures and costs.
What is a retention pond?
A retention pond is a man-made pond created by digging out or building an earthen dam to hold back stormwater runoff collected from a drainage area.
It is designed to prevent flooding, prevent erosion, and improve water quality.
A retention pond collects stormwater during a storm and slowly releases it back into the natural environment. Therefore, it should be built on a slope that directs water into the pond to achieve this goal.
A retention pond’s bottom is typically lined with an impermeable material that prevents water from seeping out. This allows the pond to retain and slowly release the water rather than letting it run off quickly.
A well-designed retention pond will offer a high level of stormwater treatment, which will improve the water quality before it is released back into the environment.
Because stormwater runoff can carry significant amounts of sediment and chemicals into these ponds during heavy storms.
Retention ponds are popular in residential areas because they can be constructed quickly and with little excavation.
They’re also very attractive to look at, and if well-designed, they can even become the property’s focal point.
Existing natural water bodies are not suitable as retention ponds. Because they will only add to the flooding problem in the surrounding area.
How does a retention pond work?
A retention pond works by storing stormwater and slowly releasing it back into the environment.
It is often used as a way to treat polluted runoff from urban areas, which helps improve overall water quality downstream.
In order for a retention pond to function properly, its surroundings must have enough slope to direct stormwater into its basin.
If the surrounding land is not properly sloped, stormwater may have difficulty entering the retention pond.
When water enters a retention pond, it flows over an outfall structure, which regulates how fast and in which direction it flows downstream.
During heavy rains, there may be an excess of runoff entering these ponds, causing them to overflow and release water more quickly than usual.
To avoid this, a well-designed spillway should be installed to help manage the flow of stormwater runoff downstream. When the pond’s water level reaches a certain level, it will spill over the designated path and flow downstream.
Retention ponds can be built in a variety of shapes and sizes depending on the landscape.
However, they should always be built with the local environment in mind to avoid harming wildlife habitat or water quality.
What are retention pond zones?
Although retention ponds appear to be a single structure, they are actually a collection of smaller zones.
Each zone serves a specific purpose and should be managed in accordance with the overall goals of the retention pond system.
All retention ponds should be built to handle the maximum amount of stormwater runoff from a given storm event.
The amount of water that a retention pond can store is determined by its storage capacity and infiltration rate. Therefore, it is important to build it with the appropriate size so that it will not overflow during heavy rainstorms.
Here is a list of different zones in a typical retention pond:
A sediment forebay is a settling basin at the start of a retention pond system. It works by slowing stormwater runoff and preventing sediment from entering the retention pond’s main body.
Sediment forebays should be large enough to allow for adequate settling time before the water enters other parts of the pond. This aids in the removal of debris and sediments from water, thereby reducing pollution.
The main basin of the retention pond is the main body of water that captures stormwater runoff.
This is where the majority of filtration processes occur which reduces pollutants in stormwater runoff.
Main basins can be designed to achieve a variety of water quality retention goals.
However, they should always have enough space to store stormwater runoff and slowly release it downstream while maintaining nearby water levels.
Infiltration basins are located at the end of retention ponds before the water flows downstream.
The purpose of this zone is to allow for additional filtration processes so that the water quality entering surrounding areas remains high.
It should have a large storage area that allows it to hold back stormwater runoff during heavy rainstorms while releasing at an acceptable rate until the next rainfall event occurs.
The outfall structure governs how water flows from the main basin into the retention pond system’s downstream sections.
It is designed to release stormwater runoff at a slow and controlled rate during rainstorms.
Outfall structures are made up of two parts: the control structure (weir) and the outlet pipe.
Depending on the design requirements for each retention pond system, the outflow structure should include both of these elements.
The spillway is a controlled release path that is used to manage the flow of stormwater runoff downstream during major rainstorms.
If too much water enters the retention pond system, it can spill over along this designated path and continue to flow downstream until the excess volume of water exits the system.
Retention ponds are an important part of the local community’s stormwater management solution.
They help to reduce the amount of pollution entering waterways by slowing down and filtering water before it enters downstream parts of the system.
Understanding each zone type can help you to better design a retention pond that will maintain optimal levels in surrounding areas while protecting wildlife habitats and improving water quality throughout your community.
Advantages of retention ponds
Retention ponds can benefit the environment and the surrounding community in a variety of ways.
Some of these benefits are immediate and can be seen right away, while others are long-term and will help to improve the surrounding environment over time.
Here are some of the primary benefits of retention ponds.
Retention ponds help reduce flooding by slowing runoff. This is critical during heavy rain events when stormwater runoff increases dramatically, causing waterways to overflow and potentially damaging surrounding areas.
However, in order to function properly, retention ponds must be appropriately sized for the catchment area and critical storm depth.
Retention ponds should be designed to reduce runoff for events as large as the drainage area’s 1 in a 30-year storm (possibly larger), with excess stormwater discharged within 24 to 72 hours of the event.
Improves Water Quality
Stormwater runoff has the potential to contaminate nearby waterways. Because water picks up harmful pollutants as it runs through lawns, streets, and parking lots.
Retention ponds filter the water runoff that enters them on a continuous basis.
This keeps pollutants and other contaminants out of downstream waterways, where they can degrade water quality over time.
These pollutants include oil, grease, and heavy metals such as lead and mercury. A retention pond can trap these contaminants in the water to some extent before they are released back into the environment.
Because contaminants accumulate in retention ponds, it is critical to monitor water quality by sampling the water at least once every three months.
This will allow us to identify any problems that may arise with our pond.
Retention ponds help to conserve biodiversity by providing habitat for animals native to the area.
Because the water in retention ponds provides a safe haven for fish, birds, insects, reptiles, and amphibians.
Frogs, turtles, salamanders, minnows, and crayfish are among the species that live in or near the pond, while others feed on insects found along its banks.
Retention ponds also help gradually recharge groundwater levels by recharging the groundwater aquifer with rainwater.
Erosion is a natural process in which the earth’s surface is worn away by water and wind. Erosion can be caused by fast-moving water or wind-carrying sediment, sand, and debris.
Retention ponds are intended to slow the speed of surface runoff so that it does not cause as much disturbance on land surfaces, thereby preventing erosion in surrounding areas.
This reduces soil loss during high-intensity rainfall events while also trapping sediment, debris, and other particles in the process.
Reduces peak temperature
Retention ponds’ large surface area allows them to absorb sunlight, which can help reduce the peak temperature of the surrounding environment during the summer.
High temperatures can be harmful to animals and plants, which cannot survive in such extreme conditions for long.
As a result, lowering the temperature helps to prevent overheating problems from developing over time.
Retention ponds provide both green space and open water. Depending on the quantity and distribution of vegetation, they can help to create cool islands in metropolitan areas (as a result of evapotranspiration, water supply, and shading).
Retention ponds prevent pollution by trapping sediment, debris, and other particles that are being carried along in the water before they can reach nearby waterways.
Although retention ponds cannot remove all pollutants from the water, they can reduce contaminants by trapping them.
Good design and adequate maintenance are likely to improve the ability to achieve high efficacy in pollutant removal.
A retention pond’s capacity to remove pollutants from runoff will improve with time if it receives adequate maintenance and care.
Because a pond is a living environment, it will eventually become more effective as plants and animals grow.
Retention ponds provide an excellent defense against landslides because they prevent the rapid flow of water over a large area.
When rainfall occurs rapidly, water can flow downhill to a nearby body of water which causes erosion. This reduces the stability of land surfaces and increases the risk of landslides occurring over time.
Retention ponds reduce this risk by slowing down surface runoff so it does not cause as much disturbance to slopes or hills surrounding them.
Absorb and/or retain CO2
Plants and trees help to remove CO from the atmosphere through photosynthesis, which absorbs C02.
Though ponds do not directly absorb or retain carbon dioxide, they can increase habitat for plants that are able to effectively capture this greenhouse gas.
Retention ponds provide a place where aquatic vegetation can grow while also providing an additional source of food for many organisms which increases biodiversity.
This can help to increase the ability of vegetation in and around a pond to absorb carbon dioxide from the atmosphere while also reducing greenhouse gas emissions over time.
Ponds can help to store water for later use. They assist the landscape’s ability to retain water in the event of a flood and, by slowing down runoff, make this water accessible for other uses (such as water to sustain aquatic and riparian ecology in and around the pond), though they are unlikely to be big enough to store water for significant human usage.
Natural biomass production
Natural vegetation and aquatic plants provide biomass production. Though retention ponds cannot produce a significant amount of biomass on their own, they can help to increase the total amount produced by an ecosystem over time as more organisms grow in and around them.
Produced biomass can attract wildlife and help with small-scale bioenergy production.
Simulate wetland treatment processes
A wetland is an ecosystem that contains both terrestrial and aquatic plants.
A retention pond can be designed to mimic some features of a natural wetland by including various types of wetlands vegetation as part of its design.
Wetlands vegetation helps to purify water by removing pollutants and trapping sediment, which improves the overall quality of runoff.
They can also improve biodiversity in an area while providing a habitat for many organisms that cannot survive in open areas without major changes.
Climate change adaptation and mitigation
Climate change is an important issue facing many countries around the world.
Freshwater ecosystems are at risk from many changes to the environment.
In particular, coastal wetlands that serve as natural buffers against flooding and storm surges could be lost due to rising sea levels or increased surface runoff in wetland areas leading to saltwater intrusion into freshwater systems.
Retention ponds contribute to adaptation to higher storm intensity storm occurrences projected as a result of climate change by helping to decrease urban runoff and flooding.
Furthermore, if additional vegetation, particularly woody vegetation, is introduced, it may boost carbon sequestration and help to moderate urban temperatures.
Provides Recreational Opportunities
Ponds are great places to go fishing, boating, kayaking, or canoeing in order to spend time outside and enjoy the beauty of nature.
They may also provide space for animals that require both land and water habitats.
They may even include pathways that allow people to walk or bike around them. This provides more opportunities for the community to engage with nature in a positive way.
Improves Property Value
Ponds can help to increase property value because they add an interesting outdoor feature that can be used for recreation and enjoyment.
They also add natural beauty to the surrounding landscape which may help increase a sense of peace or well-being in people who live nearby.
Therefore, if you are thinking of selling your property, building a retention pond could help attract more buyers that may be interested in the unique outdoor feature.
Disadvantages of retention ponds
Although retention ponds are useful for many reasons, they do have some disadvantages that you should consider before building one on your property or in the surrounding community.
Here are some things to think about:
Retention ponds need to be maintained regularly because the water tends to become stagnant over time.
The stagnation creates an environment that is more likely to support harmful bacteria and other pathogens rather than the desirable algae and vegetation that is present in healthy ponds.
This can be done by regularly removing any excess plant growth, adding additional aeration devices to improve circulation and oxygen levels, and removing any dead animals that may have accumulated around the pond.
Creates drowning risk
Retention ponds can create a drowning hazard, especially for young children and pets.
Therefore, it is important to take precautions when building a retention pond in an area where there are likely to be children or animals.
Creates Habitat for Mosquitoes
A retention pond can provide a habitat for mosquitoes if it is not properly maintained.
This happens when the water becomes stagnant and the mosquitoes find it as a suitable breeding ground.
In order to prevent this from becoming an issue, you should take measures such as adding additional aeration devices and removing any excess plant growth so that the water is not allowed to become stagnant over time.
If you plan on having a retention pond in your yard, it is important to take measures like working with a pest control service to prevent mosquitoes from breeding in the area.
Kills animals if not well-maintained
Although a well-maintained retention pond can provide a habitat for fish and other aquatic life, stagnant water is not ideal for the health of the organisms.
This is especially a bigger concern for smaller ponds that are not capable of sustaining a certain type of water quality and nutrient balance that is necessary for the survival of these species.
In order to keep your pond healthy and prevent fish from dying, you should regularly add aeration devices that will improve circulation and oxygen levels in the water.
You can also remove dead animals or plants on a regular basis if they begin to accumulate around the area.
Requires Drainage System
Retention ponds are built to retain water, but they also need access to a drain if they are to be effective.
This is why it’s important for the pond to have a drainage system in place that will help prevent standing water or overflow.
A drainage system that is poorly designed and maintained can lead to the pond overflowing and causing flooding in nearby areas, which can be both messy and dangerous.
Creates Environmental Impact
A retention pond can make a significant impact on the environment especially if it is in an undeveloped area.
Construction of retention ponds almost always requires the clearing of natural vegetation and/or generating pollution, losing healthy soil, and disturbing the natural ecosystem.
If you are building a retention pond in an area that was previously undeveloped, it is important to take these factors into consideration before starting construction on one.
How to maintain a retention pond?
A well-maintained retention pond is an attractive asset that contributes greatly to the environment.
To keep your pond healthy and looking great, here are some tips on how to maintain it:
Remove Visible Trash
All ponds accumulate trash, whether it’s from natural causes or through human interference.
In order to keep your pond clean, you should make sure to regularly inspect it for visible trash and remove it as soon as possible.
Add Aeration Devices
As mentioned earlier, retention ponds can be low-oxygen environments that are not conducive to the survival of fish and other aquatic life.
To keep your pond healthy, you should install aeration devices in order to improve circulation and oxygen levels throughout the water column.
Regularly Check the Drainage System
One of the most important factors in maintaining a healthy pond is its drainage system.
If it’s not properly designed and maintained, there can be standing water or overflow which will cause significant problems for surrounding areas.
You should regularly inspect this area to make sure that everything is working as intended and fix any issues immediately.
Keep Drainage System in Good Repair
If your drainage system is damaged or poorly maintained, then water can accumulate and become stagnant instead of draining properly.
This lets mosquitoes breed in the area which causes a public health concern for everyone living nearby.
To make sure your drainage system is in good working order, have a professional inspect it regularly.
Regularly Monitor The Water Quality
The water quality of your pond should be tested regularly to make sure it does not have high levels of nitrogen or phosphates, which are toxic to fish and other aquatic life.
However, there are some clear signs that should let you know if your pond is not doing well simply by looking at it.
If your pond is green, brown, or has a blue-green tint to the water then you should have it tested right away to make sure it’s safe for your aquatic life.
If you can see algae, scum, or foam on the surface of your pond then it is most likely due to excess nutrients, and you should have it tested right away.
Provide the Right Habitat
Although retention ponds provide a habitat for aquatic life, it may not be possible to achieve it immediately after the pond is created.
To accelerate the process, you should plan ahead and provide the right habitat for your aquatic life.
This includes creating a healthy ecosystem with plants that will absorb excess nutrients in the water, provide food and shelter for fish, and keep the water clean.
Fish will find their way to the pond after a while, but you can speed up this process by bringing them in yourself and introducing them to the new habitat.
However, this should not be done until the water quality is tested and found to be healthy for aquatic life.
Even if the pond’s water is tested and found to be healthy, you should look for getting help from a professional who has the knowledge and expertise to provide the best habitat for your aquatic life.
Where you can build a retention pond?
Retention ponds can be built in almost any location, but there are a few things to think about before making a decision.
The first and most important consideration is whether or not there is enough room for your pond at your chosen location, which you may verify by taking measurements.
Retention ponds may be built on an artificial surface, depending on site stability.
If the water quality in the pond is threatened by soil contamination, the lining may be necessary to prevent the leaching or leaking of contaminants into the pond.
To prevent soil and water contamination, the lining should be installed on a compacted clay base covered with at least two inches of granular drainage material.
Retention ponds can be used in agricultural settings to collect runoff from poor permeability surfaces (such as tracks and farmyards) or as part of the agricultural landscape.
In agricultural areas, the pond should be designed to prevent livestock from falling in and drowning.
The banks should be set back a minimum of three feet to provide adequate safety, or fencing may need to be installed around the pond’s edge.
Forests and Semi-Natural Areas
Retention ponds can be built in forests and semi-natural areas to prevent pollution of water supplies, but it is important to be aware of potential impacts on the local ecosystem.
The pond should not alter natural drainage patterns or interfere with tree root systems.
It must also leave enough space for native plants to grow around its perimeter.
The vegetation surrounding the retention pond provides habitat for wildlife, so it’s important to maintain this vegetation during the construction process.
Retention ponds cannot be constructed in wetlands because the water table is generally already too close to the surface.
By installing a pond in close proximity to the wetland you could actually cause flooding, soil erosion, and sedimentation of the wetland.
What is the required storage area to support a retention pond?
The drainage area required to support a retention pond can be as tiny as 0.03-0.1 km2 (Environment Agency, 2012), or even smaller if the retention pond is connected to another source of water, such as a spring.
There are no particular restrictions on the maximum drainage area for retention ponds, though 3-7% of the upstream catchment area is normally required for the pond (CIRIA, 2007).
Larger retention ponds (volumes more than 25,000 m3) necessitate extensive impoundment and may be subject to additional inspection and structural restrictions.
What are the considerations for designing a retention pond?
The primary consideration in designing a retention pond is the site’s ability to support and sustain the structure.
The stability of the soil needs to be checked, as additional precautions may need to be taken if it cannot support an adequate weight load for both construction and maintenance purposes.
The other primary consideration in designing a retention pond is its function will be as below:
A retention pond should have sufficient storage and detention volume to meet the desired level of service.
This volume generally depends on the required time to provide full capacity, which is dependent on the drainage area and rainfall intensity.
The retention pond’s depth affects how much water it can hold before overflowing. So this should also be taken into consideration when determining storage volume requirements.
A small retention pond can dry out during the summer months or in the case of drought.
It is also difficult to maintain the water quality in small ponds due to the limited retention time. A larger pond will have a higher water quality because changes in water chemistry will be slower.
The pond’s flow path length-to-width ratio should be between 3:1 and 5:1. Inlets and outlets should be set to maximize the length of the flow path through the pond.
Ponds should be wedge-shaped in order for the water to flow into the pond and spread out gradually, improving sedimentation and potentially enhancing the water quality.
The permanent pool’s depth should be between 1.2 and 2 meters. Stratification and anoxic conditions are more likely with deeper pools.
During the summer months, shallower pools are more likely to experience algal blooms and high biological activity.
To protect public safety and maintenance access, slope angles should not exceed 1:3.
To allow for the biological treatment of dissolved pollutants, the permanent pond residence time should be at least 20 days.
Depending on the severity and duration of the storm, as well as pond design criteria, additional storage space was emptied in 24-72 hours following the rainfall.
To allow for several incidents, the outfall design should ensure that at least half of the full storage volume is discharged within 24 hours.
Retention ponds should have variable pond depths (rather than a flatbed) and some areas of the pond should be shaded.
The amount of space needed to support the construction of a retention pond is determined by the size of the rainfall regime and the amount of storage required to achieve the desired level of runoff.
However, typically 3 – 7% of the upstream catchment area will be necessary as the baseline.
Ponds are frequently positioned in a low location in the watershed where gravity can assist drain the water. A large site may necessitate several ponds divided into topographic sub-catchments.
The position chosen should allow for safe channeling of flows over the pond’s design event, with the consequences of any pond embankment failure taken into account.
Ponds should be positioned outside the floodplain of any watercourse that could cause the pond to be inundated during the design event, such as outside the floodplain of the watercourse to which the pond would drain.
Ponds should be positioned in non-intensively managed settings where native flora is already developed and/or will thrive, whenever possible.
The site and slope stability
It is difficult to build a pond on steeply sloping terrain. Therefore, ponds should be built on solid ground.
Ponds should not be developed on a waste fill, uncontrolled fill, or non-engineered fill. Instead, soil stability and groundwater levels should be carefully considered.
Soils and groundwater
The soil beneath the pond should be sufficiently impermeable to prevent it from drying out.
For permeable soils, a liner or an impermeable material, such as puddle clay, may be used.
In places with contaminated soils or groundwater, the pond should be completely sealed to prevent water from transferring from the pond to the soil/aquifer.
Where groundwater levels are close to the surface, the design must ensure that the outflow volume from the temporary pond volume is more than the maximum projected groundwater level.
If the underlying soils are permeable, a liner or other impermeable material, such as puddled clay, will be necessary to prevent the pond from drying out or if the soils are particularly prone to contamination.
Retention ponds should be paired with upstream sustainable drainage components such as smaller detention basins and swales that give primary treatment and sediment management.
What are the costs associated with retention ponds?
|Cost Type||Cost Range|
|Land Acquisition||Highly subjective, influenced by location value|
|Investigations & Studies||$3,000 – $15,000|
|Capital Costs||$25 – $50 per cubic meter of storage volume provided|
|Maintenance Costs||$2 – $7 per square meter of retention pond area, may increase with added features such as plantings or landscaping|
Retention ponds are large-scale water reclamation projects implemented in metropolises.
The initial cost is thus the expense of land acquisition, as well as the potential cost of not developing that land for another purpose.
Land acquisition is a highly subjective criterion that is influenced by the value of the location in question and is hard to quantify.
Construction of these basins in existing built areas is frequently more expensive than creating one in an undeveloped zone because of higher land prices.
Investigations & Studies
Geotechnical studies are important before construction to verify the land’s stability and underlying soil/geological conditions.
These studies will typically cost $3000 to $15000 depending on the scale and depth of investigation.
The capital costs of a retention pond are typically between $25 and $50 per cubic meter of storage volume provided.
However, pond lining requirements, as well as building on steeper slopes or less solid soil, may raise these figures to preserve the pond’s integrity.
The annual maintenance costs will be typically between $2 to $7 per square meter of the retention pond area.
However, the inclusion of plantings, landscaping, or other features may increase these costs.
What are the management and safety considerations associated with retention ponds?
Designing & Building
A retention pond should be designed to reduce the risk of adverse consequences in the event of an embankment failure.
The design must include measures that will allow for safe channeling of flows over the pond’s design event, with the consequences of any pond embankment failure taken into account.
It is also important to consider the location of the outfall and to design it so that it is clear of structures, trees, or other obstacles.
Retention pond construction is typically supervised by a geotechnical engineer or civil engineer.
The design should also consider the safety of those who may be working on-site, as well as any nearby residents and property owners.
Maintaining & Monitoring
Retention ponds require regular maintenance to prevent blockages from sedimentation and other hazards such as weeds, algae, and ice.
Monitoring is also necessary to ensure that the pond’s design requirements are met.
Health implications of open water places must also be considered, particularly in warmer locations where ponds may provide a favorable ecology for mosquitoes, which can be linked to increased disease transmission.
Effective signage and barriers are essential to ensure that ponds are kept secure and the public is not exposed to any dangers.
Planned signages should be placed throughout the pond to help employees and site visitors understand safe practices.