A drywell is an excavated pit or structural chamber filled with gravel or stone that provides temporary storage of stormwater runoff from rooftops.
The storage area may be constructed as a shallow trench or a deep well. Rooftop runoff is directed to these storage areas and infiltrates into the surrounding soils prior to the next storm event. The pollutant removal capability of drywells is directly proportional to the amount of runoff that is stored and allowed to infiltrate.
Drywells can be used in both residential and commercial sites and are best suited for treating runoff from small drainage areas such as a single rooftop or downspout. Drywells are not appropriate for treating runoff from large impervious areas such as a parking lot. Successful application is dependent upon soil type and groundwater elevation.