Building a Gravel Infiltration Trench

At the Kitsap Conservation District office, Poulsbo, WA

Gravel trenches are narrow, stone-filled channels situated over well-draining soil that utilize the empty spaces between stones to act as a temporary reservoir for stormwater as it soaks into the surrounding soil. When they detain and infiltrate water they reduce flooding and erosion and assist in the removal of harmful contaminants from the stormwater.

On an average, the Kitsap Peninsula gets more than 40 annual inches of rain. The Kitsap Conservation District, located in Poulsbo, Washington, captures much of the rain water that falls on land surrounding their office complex in seven 10,000 gallon cisterns. In order to avoid drainage problems and manage the excess water when the cisterns are filled, a gravel infiltration trench was built in May 2022.

Planning and Design

The Conservation District decided to build a gravel infiltration trench to divert runoff from a 3,000 square foot high tunnel to the infiltration trench, once the cisterns were full.

Runoff is captured in gutters along the sides of the high tunnel structure (similar to a greenhouse), then flows into a sedimentation basin. From there it flows through an underground pipe to the trench. In the trench it enters a perforated pipe that allows the water to flow into the gravel below to replenish the groundwater.

Runoff diverted from high tunnel

The high tunnel (a greenhouse-like structure) is 3,000 square feet. The gutters and sedimentation basin can be seen in the middle left of the photo.

Gutters alongside the high tunnel

The sedimentation basin, in the center of the photo, and one of the black cisterns in the distance. The trench is being constructed in the background.

Sedimentation basin

Runoff from the high tunnel roof flows into the gutter then into the sedimentation basin. From there it goes into an underground pipe to the infiltration trench (under construction in the distance).

Construction

Digging is just the first step in building a gravel infiltration trench. Construction was completed over two days and included the following.

Digging the trench

Starting to dig the 60′ long trench

Inspecting and leveling

Finished trench is 60 ft long, 5 ft across and 3 ft deep

Laying filter fabric

Filter fabric cut to length

Drape on 2 sides

Filling the trench

Trench is filled using a shoot.

Clean 3/4″ gravel is used (up to 3″ gravel is acceptable)

Checking the level

Laying the perforated pipe

Perforated pipe allows water to flow through. 4″ pipe was used

Laying the pipe the length of the trench

Connecting the inflow and overflow

Inflow on right, overflow on left

Measuring to fit pipes

Sawing pipe to fit

Covering the trench

Wrapping fabric over gravel and pipe to reduce sediment

Adding soil to cover

Gravel infiltration trench covered

The completed gravel infiltration trench

Maintenance

Maintaining a gravel infiltrations trench is pretty straightforward. The Kitsap Conservation District plans to:

• Remove debris when appropriate
• Weed as needed
• Mow vegetative filter strip and bag the grass clippings

Build your own

Learn more about gravel infiltration trenches on the Gravel Trench resources page.