This fact sheet focuses on stormwater wet ponds. These are constructed ponds designed to capture stormwater runoff from impervious surfaces (e.g. streets, sidewalks, roofs). Wet ponds help mitigate flooding and improve water quality in urban areas.
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Urban areas contain roads, sidewalks, roofs and other hard surfaces that are impermeable to water. When rain falls or snow melts on these impervious surfaces, the water drains to storm sewers and eventually to lakes and rivers. Water that drains off impervious surfaces is called stormwater runoff.
Large storm events contribute large volumes of runoff moving at an increased rate, which raises the potential for erosion and flooding downstream. Stormwater runoff also contains pollutants that it picks up from the impervious surfaces, such as sediment, oil, salt, and nutrients associated with fertilizer and plant material such as grass and leaves. These pollutants can contaminate lakes and rivers. To reduce these negative impacts, stormwater professionals can implement several practices, such as rain gardens, permeable pavement, and green roofs. One of the most important practices is to retain stormwater runoff in a constructed basin or pond. Water is held in the pond for a period of time, allowing sediment to settle out and releasing the water over days instead of hours, which relieves flooding downstream. The most common type of pond is a wet pond, which has a permanent pool of water throughout the year. This pool of water allows for sediment and pollutants associated with that sediment to settle and remain in the wet pond, where they can later be removed and properly disposed.
The schematic at the right illustrates how stormwater wet ponds work. Water from impermeable surfaces drains to a wet pond. Most wet ponds have a forebay, a small pool of water that absorbs the energy of the incoming water and allows sediment to settle. Water flows from the forebay to a second pool, where additional settling occurs. The water level in this second pool, called the primary pool, is controlled by an outlet structure. If the water level exceeds the outlet elevation, such as after a heavy rainfall, the water is slowly released through the outlet structure. This diminishes the likelihood of downstream flooding, since the water is being slowly released downstream rather than accumulating all at once during the rainfall. In the case of extreme rain events, water may flow over the highest elevation (embankment), called the emergency spillway or overflow, in the pond.
Most ponds have some vegetation. Wetland vegetation occurs in places where the plants are always inundated with water. Plants growing in other locations within the pond must be tolerant of short periods of inundation, similar to plants grown in rain gardens.
Stormwater wet ponds may look like a wetland. They often have wetland vegetation and some wetland wildlife. They are not natural water bodies, however, and should not be treated as such. The main differences between natural water bodies and constructed ponds include the following.
Stormwater wet ponds are not intended and should not be used for recreational purposes. Although they can be aesthetically pleasing, they should not be managed for aesthetic purposes if this interferes with their primary function, which is to capture, retain, and treat stormwater runoff. For example, it is generally not appropriate to introduce fish to wet ponds as the fish may decrease the treatment effectiveness of the pond.
Wet ponds are typically constructed as part of a new urban development or redevelopment project. Often a construction stormwater permit is required for these projects. These permits have requirements for how stormwater must be managed once the project is completed. Developers typically bear the cost of constructing the wet pond. Developers contract with engineering firms that are qualified to design and construct wet ponds. Once the project is complete, ownership of the pond most often transfers to a homeowners association or to a local government unit.
The owner is responsible for maintaining the pond once it is constructed. The owner may conduct some of the routine maintenance, such as mowing and trash removal. More often the owner will complete an agreement with a maintenance team or landscaping contractor to conduct frequent inspection and maintenance items such as mowing, checking for clogs, and debris removal (link to example maintenance agreements).
The owner of the pond bears financial responsibility for maintaining the pond and implementing repairs if they are needed. This Minnesota Stormwater Manual provides some information on costs of maintaining ponds, but the cost information is highly variable. Estimated annual operation and maintenance costs for wet basins are 3 percent to 5 percent of construction costs. Mowing and sediment removal are the most costly activities. Responsible parties should establish a maintenance fund and assess annual fees on appropriate property/home owners. Local governments can establish stormwater utilities or charge inspection fees to carry out their maintenance responsibilities.
The effectiveness of a stormwater wet pond depends on several factors. Some general guidelines include the following.
Routine and long-term maintenance of stormwater wet ponds is essential to continue effectively removing pollutants. Maintenance includes the following.
Additional maintenance guidelines include the following.
Below are Internet links to additional information.