An Overview of Wet and Dry Detention Facilities

A detention pond acts, in essence, as a temporary holding pond which decreases the impact of stormwater runoff in downstream drainage channels, pipes, etc.  Similar to how beaver dams act in nature, detention ponds serve to soften the impact of surges in storm runoff by holding a certain volume of storm runoff, and releasing the runoff at a decreased rate.

Depending on the municipality, detention ponds are required to hold different volumes of storm runoff. Often, detention ponds are required to hold up to a 100-year storm volume, and release at a 2-year or 10-year historic rate of flow.

Many detention ponds have “staged” outlet structures.  What this means is that the pond has been designed to release different flow rates at different stages of the pond’s operation.  A detention pond may be designed to release a very low flow rate in a 2-year storm event, but release much more in a 100-year storm. 

 Such a pond would have a “2-stage” outlet structure.  For the first stage, the structure would release up to a certain flow rate at up to the 2-year storm volume.  For the second stage, the structure would release up to a certain flow rate at up to the 100-year storm volume.

Whether a detention pond is “wet” or “dry” it performs the same function from a stormwater perspective.  In both cases the pond serves as a stormwater facility functioning to capture a specified volume of storm runoff and release stormwater flows at a reduced rate.  Detention, whether in the form of a wet or dry facility, serves to decrease the impact of stormwater runoff by decreasing the peak flow rate.

Wet ponds can add tremendous aesthetic value to a project. When we talk about a “wet” pond, we are typically referring to a detention pond that functions also as a holding pond.  Stormwater can be detained on top of the permanent storage volume.  The storage volume may be there for practical reasons such as holding a certain amount of irrigation water, or simply for aesthetic purposes.

Some considerations with wet detention ponds:

>Safety – Dry detention ponds only hold water for a limited amount of time; wet ponds pose a continual safety concern 
>Permitting Requirements – the State or other agencies will likely require a permitting process to be undergone for a wet pond 
>Groundwater levels and construction de-watering 
>Types of liner 
>Control of water level 
>Pond depth as it relates to maintaining water quality

Photos: A. Cvar