Have you seen a sign for a “detention pond” or “retention pond” in your town?
Curious what these are?
Well, these ponds are created because stormwater runoff can pose issues for cities around the globe.
This runoff is typically generated from rain and snowmelt that flows over impervious surfaces (i.e., paved streets, parking lots, building rooftops, etc.), which means it doesn’t soak into the ground.
When this occurs, the runoff can overwhelm the city sewer system and clog local waterways, making it dangerous to travel.
For this reason, both detention and retention ponds are used to collect water and prevent both flooding and erosion.
But…what’s the difference?
Here are the key differences you should know between the two.
1. What is a detention pond?
A detention pond is a depression that collects and slowly releases stormwater from large drainage areas.
The detention basin is not permanently covered with standing water.
2. What is a retention pond?
A retention pond is a large, constructed depression in an urban landscape that receives and stores the stormwater runoff from big drainage areas.
They normally hold a permanent pond of water.
3. What’s the difference between a detention pond and a retention pond?
A detention pond temporarily stores stormwater runoff.
It’s designed to manage stormwater runoff by both storing and releasing the water gradually until it’s entirely drained.
You may hear detention ponds referred to as “dry detention basins” because they only control flood flows.
A retention pond, on the other hand, is designed to permanently hold water.
These basins are used to permanently hold water, which makes them distinct from the temporary status of detention ponds.
A retention pond doesn’t have an outlet structure.
To exit the pond the water collected will either infiltrate the ground or evaporate.
You may hear retention ponds referred to as “wet retention basins” because they allow for a large amount of water to enter the pond.
If you’re still a bit confused, here are the four key differences between the two types of basins.
You’ll often hear detention ponds referred to as “dry” and retention ponds referred to as “wet.”
This is because the biggest difference between the ponds is how long they hold water.
Detention ponds are designed for different reasons than retention ponds.
Detention ponds divert water to another outlet, and they’re often used for construction sites that have a slope or an emergency spillway in the event of a flood.
Conversely, retention ponds retain water either permanently or temporarily (as their name would suggest).
The level of water in these basins will fluctuate depending on the amount of precipitation and the runoff in the surrounding areas.
These basins rely on precipitation, and thus, the location and climate of a place will determine which are necessary.
For instance, if a location is really dry, then detention ponds are more often used to collect rain, snow, etc.
The basins will also vary in size depending on their function to settle stormwater particles and reduce peak flows.
The ponds are constructed differently depending on if they’re retention or detention.
For example, a retention pond has an orifice (also called a riser) level at the bottom of the basin.
This allows the water to eventually drain out after a storm.
A retention basin has a riser at a higher point, so it retains a permanent pool of water.
The basin slows and stores runoff from impervious surfaces, such as rooftops and pavements.
This process helps to reduce the number of pollutants that go into our natural sources of water like streams and lakes.
Orifices can become clogged due to erosion or sediment build-up.
They thus need to be repaired immediately so that flooding doesn’t occur.
Detention ponds also have orifices.
These orifices are located low in the basin, which means you must repair them immediately.
Otherwise, you may see a reduced performance.
Both types of basins are intended to assist with flood management.
Retention and detention ponds interrupt the transportation of water to streams and lakes during storms.
If the water is polluted, the ponds will help to reduce these pollutants.
Detention ponds normally hold storm water long enough to settle sands and larger silt particles.
However, fine silts and clays don’t have a chance to settle and will continue down the water cycle.
Retention ponds hold stormwater for longer periods of time and allow even the fine sediments to settle to the bottom of the pond.
To avoid clogged orifices, pests, odors, algae blooms, etc., these ponds should be maintained regularly.
This will allow them to stay in proper working order and optimally manage floods.
4. Who is responsible for these basins?
Everyone is responsible for protecting the quality of Earth’s water.
However, if you’re the property owner of a parcel of land, then you are responsible for maintaining the retention or detention ponds on that land.
In some cases, homeowners’ associations, businesses, or even local governments will take over the care and maintenance of certain basins.
5. How do you care for a detention pond?
To care for a detention pond, you must keep the pond clear and clean by removing leaves, plant debris, and trash.
Detention ponds will have a dry space ready for water until there’s a large influx each year.
Once water is in the pond, the basin should be regularly checked and adjusted.
You’ll want to prevent algal growth in the pond as this can lead to a smelly or unhealthy environment.
Additionally, you’ll want to check water levels, pump out sediment buildup, and prevent any clogging for best results.
Cleaning a detention pond includes trimming and mowing the area around the detention pond.
You may also need to stabilize the slopes and clean any debris in the pond.
If there’s any excess sediment in the basin, then you’ll need to remove it to prevent clogging.
Depending on the mechanical construction of your pond, you’ll need to review and repair any elements that have degraded or broken over time.
Remember, detention ponds operate best when they are created as an amenity in addition to a utility.
Utilities can become eyesores for people in the community.
However, when properly designed, your detention pond can add to the space.
Your pond design should include areas where people can walk around it in safety.
During this design process, the architect will determine how much water can safely enter and exit the pond to eliminate flooding concerns.
6. How do you care for a retention pond?
Retention ponds can increase pollutant discharge downstream if they’re not properly maintained.
This will increase the instability of downstream channels as well as raise the risk of downstream flooding and prompt additional aesthetic issues.
If you poorly maintain your retention pond, it’s doomed to fail.
Whoever owns the land that the pond resides on is responsible for maintaining it.
This could be the local government, the local association, or the homeowner.
Following heavy bouts of precipitation, perform regular inspections to identify and repair areas of erosion, gullies, and other damage.
This gives you time to remove sediment and debris from the pond before it reaches the outlets.
Ponds that aren’t in consistent use can often be overlooked.
We recommend beautifying your retention ponds to make them fixtures in the community.
You can add grass, shrubs, and other vegetation to make the pond a central piece of the landscape.
This type of beautification often requires upkeep like mowing, garbage and litter cleanup, modest landscaping, and sediment removal.
These steps can help you keep a pond up to code and avoid fines that accompany a failed pond.
7. What should you look for during a pond maintenance check?
If you’re inspecting a detention or retention pond, here’s a checklist of what you should be looking for.
Trash, dirt, and excessive sediment clogging or obstructing outlets
Erosion on the slopes or at the top of the head wall
Excessive vegetation around and surrounding the pond
Pilot channels should be clear and open
Mechanical device functionality (pumps, flood gates, etc.)
Inlet and outlet pipes in good condition
Anything interfering with the pond’s intended functioning
What are the best maintenance steps if the above is discovered?
Grass mowing, pruning, and vegetation control
Maintaining mechanical elements in the pond
Removing excess sediment from the basin’s outflow and inflow pipelines
8. How deep are detention and retention ponds?
Most detention ponds will have a depth of 3 to 12 feet.
Most retention ponds will have a depth of 3 to 6 feet.
9. What are the advantages and disadvantages of a detention pond?
Here are some reasons you may or may not consider a detention pond:
- A detention pond is surrounded by a vegetative buffer that can withstand both wet and dry conditions
- A detention pond can cost less to implement than a retention pond because they are generally smaller (even if the range for depth is larger)
- A detention pond still requires a large amount of space
- A detention pond does not improve water quality
- A detention pond can become a breeding ground for mosquitoes
- A detention pond can detract from property value while retention ponds can add value
10. What are the advantages and disadvantages of a retention pond?
Here’s why you may or may not consider a retention pond:
- A retention basin is a simple addition if space is provided
- A retention basin offers a higher removal of pollutants
- A retention basin naturally processes water without additional equipment
- A retention basin improves stormwater collection and flood control
- A retention basin can also serve as an aesthetic or recreational amenity
- A retention basin may serve as a habitat for some wildlife, which will become a self-sustaining ecosystem
- A retention basin can be a drowning hazard
- A retention basin requires a large area of land
- A retention basin poses negative impacts on water quality if it’s not properly designed
11. How do you handle erosion created by stormwater runoff (even with detention or retention ponds)?
Both detention and retention ponds are intended to help with flooding and erosion.
However, neither solve this challenge seamlessly.
Some outlets for these ponds can face severe erosion.
How can this happen?
This article discusses a retail store’s stormwater detention pond outlet being impacted by substantial erosion.
The outlet is pictured after it was updated in a wood clearing.
Before these updates, however, overgrown trees allowed roots to entangle themselves with the outlet pipe.
Over time, the roots grew, cracked the pipe, and caused a washout.
This ultimately resulted in the collapse of the pipe, which allowed water to flow freely over unprotected soil.
Heavy rain events also prompted water to swirl in front of the outlet.
This series of events lead to massive erosion.
To repair the damage, the entire slope needed to be reconstructed to maximize stabilization.
A team removed the source of the problem — overgrown vegetation.
While vegetation is often beneficial to stormwater systems, the vegetation wasn’t properly maintained, and the roots were left unchecked.
After the area was clear of all vegetation and roots, the pond had to be drained.
After it was drained, the sediment and debris were removed from the eroded area.
Dry and compatible soil was also saved and used in the reconstruction of the pond later on.
This helped to minimize the cost of repair because existing materials were reused.
The land needed to be reshaped and graded so the outlet area would be stabilized against erosion.
It was reinforced with material like geo-textile liner and riprap.
The final steps were fencing and landscaping.
For example, the area was seeded with love grass and juniper for erosion control as their root systems are too thin to pose a risk to the newly installed pipe.
Overall, the location has been entirely revamped against erosion.
It can now be properly maintained, so it won’t ever face severe erosion again.
If you’re starting to see signs of erosion near your detention or retention pond, all is not lost.
In fact, catching erosion early is the key to ensuring that your land doesn’t require a dramatic overhaul.
Here are some steps you can take.
Check for gullies or other disturbances on the bank after major storms
Keep pipe clear of debris and remove sediment to ensure proper function
Remove debris around and in ponds before it reaches the outlets to prevent problems
Maintain vegetation according to what you have surrounding your basin (i.e., weekly grassing mowing or bi-annual hedge trimming)
While detention ponds and retention ponds sound similar (if not the same), the terms cannot be used interchangeably.
Both of these basins help to reduce stormwater runoff on a site, so the risk of flooding is decreased.
However, detention ponds slowly release the water while retention basins allow water to infiltrate the soil or evaporate.
Consider these basins for your land if you’re having challenges with stormwater runoff!
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Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.