What is a Drywell and How Does It Work?
A drywell is an underground cylindrical shaped container with holes in it. It is buried underground and surrounded by drain rock. A pipe connected to a downspout or other rain drain or downspout directs rainwater from a roof to the drywell. Rainwater collects in the drywell and then slowly leaks out through the holes and into the ground. A drywell may be made of plastic or concrete and may be large or small depending on the size of the area that drains to it.
Some drywell systems are more complex and involve a silt trap (also known as a catch basin). Silt traps filter out physical sediment to prevent sediment buildup in the drywell.
Drywells help recharge groundwater and are for rainwater only.
Summary of Drywell Design Requirements
Drywells require optimal conditions for onsite infiltration. They cannot be installed where soil or topographic conditions are not considered ideal. Below is a brief summary of drywell design requirements, but you should consult the Stormwater Management Manual for complete information if you are considering installing a drywell.
Drywells vary in size. They must be sized to accommodate the area that drains to it and runoff from large storms without overflowing. The smallest drywells, called mini-drywells, are 2 feet in diameter and can handle runoff from a 500-square-foot area. Only one or two mini-drywells can be installed at a single location.
Other drywell requirements to consider:
- For some commercial and industrial sites located within the City's Wellhead Protection Area, drywells may not be allowed. Learn more about the Groundwater Protection Program.
- The top of the drywell must be lower than nearby foundations (including basement foundations).
- There must be at least 5 feet of vertical space between the bottom of the drywell and permanent or seasonal groundwater.
- A drywell must be 5 feet above a rock layer, hardpan, or other impervious underground layer.
- To stay safe and avoid damaging buildings or other structures, drywells must be placed a safe distance from foundations, basements, and property lines. If your property has a cesspool, a drainfield, slopes, or grade changes, then there are additional requirements regarding placement. Consult the Stormwater Management Manual for setbacks and details.
- Silt traps are recommended, but they are optional if the water is coming from a residential roof or pedestrian sidewalk.
- Usually, installing a drywell requires digging a hole deep enough so the drywell is surrounded by at least 1 foot of gravel on all sides and at least 1 foot of soil on top.
- Property owners should not plant large shrubs or trees over a drywell in order to maintain access to it in case of needed maintenance.
When to Call a Professional
Get professional help installing a drywell if you do not meet, or are unsure of, the design and setback requirements in the stormwater manual. You will also need a professional to help you with:
- Any drywell greater than 4 feet in diameter.
- Drywells under buildings.
- Soil infiltration rates less than 2 inches per hour.
- A runoff area greater than 5,000 square feet.
- If the drywell will be collecting runoff from a public street.
Costs and Permits
Drywells and mini-drywells are commonly available at construction supply companies and are relatively inexpensive to install and maintain.
A City permit is not required for drywells on single-family residential properties. However, a plumbing permit from the Bureau of Development Services may be required to connect the drywell to downspouts when running pipe underground.
In all other cases, installing a drywell could require registration with the Oregon Department of Environmental Quality’s Underground Injection Control (UIC) Program. Owners or operators of new and existing UICs that are designed to infiltrate belowground should check with the Oregon DEQ before installation.
With proper maintenance, a drywell can last up to 30 years.
Conduct occasional maintenance to ensure the drywell is working as intended. This maintenance includes:
- Inspecting the drywell for structural stability.
- Removing debris from the silt trap or drywell.
- Cleaning and repairing inlets.
- Refurbishing or replacing clogged drywells.
- Regularly cleaning gutters and downspouts to keep debris out of drywells.
Stormwater Management Manual
Consult the City’s Stormwater Management Manual for the complete set of requirements on how to safely site, build, and maintain a drywell or other stormwater management solution on your property.
Clean River Rewards Eligibility
Residential and commercial property owners who install qualified stormwater management solutions may be eligible for discounts on the stormwater charges of their sewer, stormwater, and water bill through Clean River Rewards.
Drywells that meet the safety and space requirements of the stormwater manual are eligible for Clean River Rewards. Visit Clean River Rewards to learn more.