Stormwater management - Southwest Washington


A two-pool wetpond.





A wetpond, or wetpool, is an open basin that has a pool of water year-round. The volume of the wetpond allows sediment to settle out as stormwater runs in. Wetland vegetation is typically planted to provide additional treatment by removing nutrients such as phosphorus and nitrogen.

Typically, there are two pools. Stormwater flows into the first pool, where sediment settles before moving into the second pool. The water is then discharged to streams and groundwater.

Wetponds also have additional temporary storage above the permanent water level to detain and slowly release stormwater. They often are fenced for safety. 

Signs that maintenance is needed and a checklist for maintaining the facility including how to fix problems.

Additional elements to wetponds

Field inletField inlet
A field inlet is a concrete structure that collects stormwater and routes it through underground pipes. Fitted with a slanted, slotted grate, it often traps sediment and debris. Regular maintenance is important. Keep the opening clear of obstructions. A field inlet is usually cleaned by a truck with a vacuum hose but sometimes can be cleaned with hand tools.

Flow controlFlow control structure/flow restrictor
Underground flow control structures and flow restrictors direct or restrict flow in or out of facilities. They slowly release stormwater at a specific rate. It is important to make sure they do not become plugged or damaged because the facility could release water too quickly or too slowly and overflow. They are usually found in manholes.

Debris and access barrierDebris and access barrier
A debris or access barrier is a bar grate over the open end of a pipe larger than 18 inches in diameter that prevents large material, people or animals from entering. They typically are located on the outlet pipe from a detention pond to the control structure.

For safety purposes, only qualified personnel should remove debris from the barrier when water is flowing through the pipe.