Retention ponds, or Stormwater Wet ponds, are permanent bodies of water that provide opportunities for treatment and sedimentation of stormwater or agricultural runoff. This has major benefits for groundwater quality and can protect the watershed from toxic pollutants like pesticides, nutrients, or other chemicals. Time, sunlight, gravity and chemical-hungry microorganisms are all natural ways that retention ponds treat polluted water. Sediment traps, or simply allowing time for larger particles to settle via gravity, as well as decreasing the flow rate, lowers the chances that chemicals trapped in dirt or solids may travel further downstream. Decreasing the number of particles suspended in the water also reduces the water’s turbidity, allowing more sunlight to permeate the water, improving the health of fish and plant life alike.
Eventually, through outlets and/or spillways, water in the pond is released out into local streams or rivers to return into the watershed. Unlike detention ponds, which are only meant to hold water for a short period and then quickly drain away; retention ponds always maintain a certain water level. This allows for both the aesthetic appeal of a body of water and can serve as a thriving ecosystem for all kinds of inhabitants, both desirable and pesky. This permanent body of water also requires additional maintenance, as algae growth or sediment build-up will quickly and drastically reduce the pond’s ability to treat and filter the water.
Anatomy of a Retention Pond
Inlets
These are where water enters the retention system, and can come in many different forms
- Retention ponds are typically installed at the lowest point of a particular area to accumulate as much runoff as possible. Some ponds are installed in chains to capture any runoff or excess water from previous basins for extra flood protection.
- French drains and gutter downspouts or open street-drains and large culverts at the lowest points of an area
- Surface runoff that accumulates from paved surfaces or surrounding open fields and slopes that drains into ditches or open channels
The Basin
The reservoir itself, the body of water, the storage container.
Treatment Pool
- This is the permanent pool, known as a wet pond, that always maintains at least a certain level of water
- Utilizes sunlight and biochemical processes (like algae and nitrification), to cleanse the pond of bacteria and nutrients
- Water is held here long enough for gravity and other sediment traps to remove larger solids and debris from the water as they settle and/or are caught
- Treatment design features help enhance the ability of a stormwater treatment practice to remove pollutants. Several features can enhance the ability of wet ponds to remove pollutants from stormwater runoff. The purpose of most of these features is to increase the amount of time that stormwater remains in the pond.
- This can be done by increasing the volume of the wet pool, thereby also increasing the total retention time of stored water.
- Other features can assist in this pursuit, like creating underwater berms or other designs to create longer flow paths within the reservoir. Some reservoirs are built in stages or series, where water is allowed to flow through multiple ponds to maximize treatment.
Flood/Overflow Embankments
This is a temporary storage volume for flood attenuation, created via landscaped banks to the permanent pool that gives room for major storms and the resulting overflow.
Forebay
This is a sediment forebay or another form of upstream pre-treatment system where larger solids and sediments are given time to settle
- Dredging frequency will depend on the total sedimentation allowed to be eroded from the pond itself or brought into the pond from runoff and drainage
- Plant decay and the amount of debris that is allowed to flow into the pond will also require consideration when choosing how often to schedule dredging and/or maintenance
- A retention pond that’s at least 50% filled with sediment/debris will no longer effectively treat the water inside, and must be dredged
Outlets/Emergency Overflow
This helps maintain your pond’s maximum permanent level, allowing excess water to escape via weirs, pipes, or spillways in case of a flood
Debris Trap/Trash Rack
This trash filter will help catch larger solids and particles that aren’t detained within the sediment trap.
- Dirt from lawns or uncovered soil
- Litter and yard waste
- Salt or sand during winter months
- Fallen leaves and branches
- Shingles or other house-damage after a storm
- Heavy metals from homes and vehicles
- Construction debris
Aquatic Bench
EPA regulations require that most retention ponds include an aquatic bench or littoral shelf: a shallow shelf that allows sunlight to permeate the water. This encourages plant growth and provides habitation and food for fish. This reduces the incoming pollutants and stabilizes the bank’s soil; decreasing erosion.
Vegetation
Installing native plants, that will perform well in environments with high moisture, can be an excellent first step. Grass is always an ideal choice when looking to add in some greenery near these ponds. Not only will adding in a splash of green add in some physical benefits to the area, but it will also create a more aesthetically pleasing area to look at.
The best time to try to add plants around the retention pond is during a dry time of the year, and even though it’s an obvious point to make, be sure that these new plants do not block the access points to the retention pond’s maintenance entrances.
Access
Access to the pond may involve widened pathways for equipment, ladders and railings for operator safety, or other ergonomic considerations when it comes to maintenance or repair.
