A retention pond, often called a wet pond, is an engineered basin designed to manage rainwater runoff from developed areas. These basins serve two primary functions: controlling the volume of stormwater and improving its quality. Controlling volume prevents flood damage and soil erosion in downstream waterways. Improving quality ensures cleaner water is released back into the natural environment.
Managing Stormwater Volume
The design of a retention pond directly addresses the problem of rapid, high-volume runoff that occurs when rain falls on impervious surfaces like roads and rooftops. When a storm event occurs, a large volume of water quickly flows into the pond through a network of storm drains and inlets. This sudden influx causes the water level to rise temporarily above the permanent pool elevation, utilizing the pond’s temporary storage capacity.
This temporary storage prevents a surge of water from overwhelming downstream drainage systems and natural streams. The pond’s engineered outlet structure restricts the rate at which the stored water can leave the basin, a process known as flow attenuation. By slowing the release, the pond reduces the peak flow rate of runoff.
The slow, controlled release mimics the natural infiltration and flow rates that existed before the land was developed. This regulation prevents flash flooding and reduces the erosive power of water on stream banks and channels further along the watershed. Once the storm passes, the temporary volume of water is gradually discharged, often over a few days, until the water level returns to the permanent pool elevation.
Improving Water Quality
Retention ponds serve as large-scale natural filtration systems, providing a mechanism for cleaning the water through physical, chemical, and biological processes. The most significant mechanism is sedimentation, which is enabled by the presence of a permanent pool of water. When runoff enters the pond, the water velocity drastically decreases, allowing suspended particles to fall out of the water column.
These suspended particles include sediment, debris, and pollutants like heavy metals and phosphorus that often attach to soil particles. The time the water spends in the pond, known as the residence time, determines the effectiveness of this cleaning process. A longer residence time, typically several days, allows finer particles more time to settle at the pond’s bottom.
Biological uptake provides a secondary cleaning function, especially for nutrients like nitrogen and phosphorus. Aquatic plants and algae within the pond absorb these excess nutrients, removing them from the water before it is released downstream. Specific microbial processes, such as denitrification, further convert dissolved nitrogen compounds into harmless nitrogen gas, which is then released into the atmosphere.
Key Engineered Structures
The successful operation of a retention pond relies on several precisely engineered structures that manage the water’s entry, storage, and exit. A forebay is a small, deeper basin located right where the runoff enters the pond. Its purpose is to capture the heaviest and coarsest sediment load immediately, protecting the main body of the pond from rapid sediment accumulation.
The riser structure is built within the pond and contains the principal spillway. The principal spillway is the primary outlet, typically a pipe or orifice set at a specific elevation, which controls the normal, slow release of water from the temporary storage volume. The size and height of this opening ensure the controlled discharge rate necessary for flood attenuation.
An emergency spillway manages extreme weather events that exceed the pond’s normal capacity. This spillway is usually an armored, vegetated channel or a weir cut into the embankment at the highest designed water level. Its function is to safely pass the overflow from very large storms, such as a 100-year event, preventing water from flowing over and eroding the main embankment.