What Are Soakage Trenches?
Also called infiltration trenches, soakage trenches are underground, gravel-filled trenches with a long, horizontal, perforated pipe that runs along the top of the gravel the length of the trench. Since they are buried, soakage trenches are a great solution for property owners who would like to manage stormwater onsite but do not want to sacrifice surface landscaping in order to do so.
How Soakage Trenches Work
Runoff from roofs, parking lots, and other hard surfaces flows into the perforated pipe along the trench. Slowly, the water drains through the holes and seeps through the gravel into the surrounding soil.
Summary of Soakage Trench Requirements
These site and design requirements can help you decide if a soakage trench might be appropriate for your project. Consult the Stormwater Management Manual for complete requirements.
- Underlying soils should have an infiltration rate of at least 2 inches per hour. The bottom of the trench must be at least 5 feet above permanent or seasonal groundwater and 5 feet above a rock layer, hardpan, or other impervious underground layer.
- The slope must be 20 percent or less.
- Soakage trenches should have at least 1 foot of soil cover over the top of the drain rock.
- Backfill with soil to be even with the surrounding soil level and parallel to the site contour.
- The surface must be lower than nearby foundations.
- Soakage trenches are not allowed under impervious surfaces. The surface of the soakage trench once filled, must be left pervious.
- To stay safe and avoid damaging buildings or other structures, the edge of the soakage trench must be at least 10 feet from foundations or basements, 5 feet from any property lines, and at least 5 feet away from all utility lines.
- Consult the Stormwater Management Manual if your property has a drainfield, grade changes, slopes or a private drinking water well for additional setback requirements.
Soakage trenches are not appropriate for all sites. Your property may not be suitable for a soakage trench if it:
- Has slopes greater than 20 percent.
- Is within 50 feet of a wetland or waterbody or in a floodplain.
- Has high groundwater, seeps, or springs.
If any of those conditions apply to your property, contact Environmental Services Private Property Drainage Inquiries if you are considering a soakage trench for your property.
When to Call a Professional
Consult with an engineer if:
- The project’s impervious area exceeds 10,000 square feet.
- You cannot meet the setbacks outlined in the Stormwater Management Manual.
- You plan to use a manufactured stormwater technology along with a soakage trench.
Costs and Permits
Once you’ve located the site for your soakage trench, you will need to apply for a city plumbing permit before you start digging. Environmental Services must approve soakage trench siting and sizing. New or altered plumbing connections require a plumbing permit from the Bureau of Development Services.
Soakage trenches on residential properties do not have to go through the Oregon Department of Environmental Quality (DEQ). In all other cases, installing a soakage trench could require registration with the DEQ’s Underground Injection Control (UIC) Program. Owners or operators of new and existing UICs that are designed to infiltrate underground should check with the Oregon DEQ before installation.
In addition, a stamped and signed geotechnical report addressing slope stability is required for soakage trenches sited on slopes greater than 20 percent.
If there is a silt trap or inlet, inspect it periodically and after major storm events to ensure proper operation (at least twice a year). Remove sediment and debris as needed.
Stormwater Management Manual
Consult the City’s Stormwater Management Manual for the complete set of requirements on how to safely site, build, and maintain soakage trenches or other stormwater management solution on your property.
Portland's Clean River Rewards Program
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.