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
Gutters alongside the high tunnel
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
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.