Drywells are one of the most common methods of stormwater disposal. Classified by the Oregon Department of Environmental Quality (DEQ) as a stormwater underground injection control system, or “UIC”, a drywell can be a relatively cost-effective approach to stormwater management. However, it is important to recognize that drywells are not always the best choice or appropriate for all development sites.
Drywells infiltrate stormwater runoff into the ground therefore it is important to know if your project site’s soils are suitable for stormwater disposal. The Bureau of Environmental Services will typically require infiltration testing prior to approval of your building permit. An allowable infiltration rate for a Single Family Residence is 2in/hour or better (added safety factor may be required based on site conditions and/or the size of the development). The site’s slope/topography and available open space are also important to consider as infiltrated stormwater may cause detrimental impacts such as flooding and landslides. A poor drywell installation may cause damage to your development and to neighboring properties as well. Drywells also must be installed in compliance with building code setbacks. A summary of these setbacks are detailed in the table below:
The 2014 Stormwater Management Manual summarizes the minimum design and installation requirements for drywells and other acceptable infiltration systems. For technical assistance on stormwater facility construction, contact the Bureau of Environmental Services at 503-823-2059.