Drainage Devices

If your farm structures rest on a slope then intercepting surface water before it invades key areas like buildings, livestock confinement areas, waste storage or compost bins is necessary. You may want to intercept and divert surface water with a:

  • Perimeter or Gravel or French Drain
  • A rocked trench
  • A berm
  • A swale

These structures should be used on slopes of 5% or less. These descriptions are not to be considered a complete design or instruction. Consult a Snohomish Conservation District engineer for steeper slopes or for more detailed instructions on drainage structures.

 Trench Drain

Perforated pipes surrounded by gravel or drain rock are often referred to as French drains. These drains intercept surface water and water in the upper profile of the soil before it reaches a building, heavy use paddock, compost bin or farm road.

Be aware that it is not legal to outlet a drain onto a neighboring property. Outlet water at least 50 feet from a stream or property line.  It may be necessary to slow the velocity of the water from the outlet with heavy rock or a dispersion device such as a level spreader or earthen catch basin. 


An earthen hole about five feet deep filled with drain rock. The idea is that the water is captured by the drywell and water is allowed to percolate slowly into the surrounding soil. The surrounding soil must be sandy or gravelly.

Consult with an SCD engineer on appropriate sizing for your region.


A rise on the land surface along a continuous line. The berm can be as little as a few inches high, simply enough to redirect the water around whatever structure you are trying to protect. The angle of the berm should be gradual toward the downside of the slope and allow passage of the water in the desired direction. The berm should be vegetated to keep the soil in place.

 Grass Swales

Broad shallow ditches with gentle slopes vegetated with grass. They are used on land with gradual slopes of 5% or less. The grass filters the water while also directing the water. During the dry season, they can be grazed and mowed. The swale bottom should be at least two feet wide or wider. Consult with an SCD engineer on sizing. 

 Water Bars

Small ditches or ridges that run diagonally across a road or trail at about a two to three percent angle and divert surface water to a vegetated area. This prevents water from gathering speed and eroding gravel or soil from the road. The steeper the slope of the road or trail, the closer the water bars should be spaced. Ridges are compacted material with a base of about six inches.

In place of a ridge, you can securely attach a flexible conveyor belt to a 4 x 6 beam whose length is the width of the road. The conveyor belt should be at least 12 inches wide. Bury the beam in the road at the desired angle leaving three to four inches of conveyor belt exposed. Compact the ground over it. If vehicle or bicycle traffic will be using the road the angle of the water bar should be a 3:1 ratio or flatter.  

 Level Spreaders

Devices used to take a concentrated flow of water and disperse it over a wider area as it exits a pipe or drainage structure. First the water is slowed by a shallow linear trench perpendicular to the drainage channel. Water fills the trench and then exits through a series of notches along a beam or seeping out of gravel in the trench dispersing the water over a wider area. Contact SCD engineers for dimensions.

 Rainwater Catchment

from roofs can be done with rain barrels or channeled into livestock watering troughs. An outlet from the rain barrels or watering trough needs to direct excess water out of the area. See the “Rain water-Use It or Lose It” brochure for more information. If using roof water for livestock watering, research the type of material on your roof and see if any toxins from the roof material would make it inappropriate for livestock use.