Side ditches

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What is it?

A simple drainage feature comprising open V or U-shaped channels, dug into the ground alongside the path using either a spade or tracked excavator.

At Oatridge College, side ditches were installed either as earth or turf lined channels using a tracked excavator. Vegetation in the earth channels has re-established, covering the exposed substrate. The original vegetation in the turf-lined channels was removed when the channels were first dug and then re-instated.

turf lined ditch oatridge college

How does it work?

Side ditches are an essential element of any path drainage system whenever there is a problem of water running onto the path from surrounding areas.

The key function of any side ditch is to intercept and collect surface water runoff or groundwater before it reaches the path surface and direct it away. The collected water flows along the side ditch to pass through pipe culverts that carry the water under the path and then direct it away from the path to soak away in the surrounding area.

At Oatridge College, all side ditches collect surface water from the surrounding areas and direct the water to pipe culverts, which drain to a watercourse. Note that water draining into a watercourse from earth ditches may cause water pollution. If possible, disperse sediment-laden water to vegetated soak-away areas to allow sediment to settle.

Is it suitable?

A side ditch should be wide and deep enough to cope with the highest volume of water expected. It will quickly overflow if it is too shallow, causing flooding and path damage. If the channel is too narrow, fast flowing water will erode the bottom and undercut the sides. Depending on the amount of available space along side the path, a V-ditch should be your first option, because it is more open in cross section, making it more stable and less likely to erode. A V-ditch should be at least 300mm deep, with 45º sloping sides. The second option would be a U-ditch, if space along side of the path is limited. A U-ditch should be least 300mm deep and 450mm to 500mm wide at top, with the sides angled slightly back for stability. The vertical sides of a U-ditch may collapse under the weight of saturated ground above the ditch sides, or if water erodes and undercuts the sides. These ditch dimensions provide plenty of spade room for clearing out a ditch during routine maintenance.

Where gradients are shallow, a side ditch should have a smooth bed for unrestricted water flow. A side ditch must have sufficient fall to allow water to flow along the channel, but not so steep that water erodes the bottom and undercuts the sides. Gradients between 1:15 and 1:40 will work effectively. If steeper ditches are unavoidable, try to include bends and level areas to reduce the speed of the water flow. Short steep sections may be piped to avoid erosion.

How much will it cost?

Depending on whether the ditch will be dug by spade or machine, and is to be left as an earth or turf lined ditch, a ditch may cost between £2 - £10/ linear metre.

How do you install it?

A side ditch should:

  • Follow the lie of the ground by the path.

  • Be positioned a minimum distance of 300mm from the path edge, ideally 500mm if space allows.

  • Avoid steep gradients and straight lines.

  • Feature curves to blend in with the surroundings.

  • Slow down fast flowing water.

  • Avoid corners and sudden changes in direction that cause fast flowing water to erode the channel bottom or undercut the sides.

  • Maintain a fall throughout its length.

  • Avoid obstructions, such as bedrock.

The method of construction for any side ditch, whether it is dug by spade or machine is:

  • A path worker with a spade, or a machine operator with a tracked excavator fitted with V-ditching bucket, starts to dig the side ditch at the lowest end and works uphill excavating the channel, whilst maintaining a fall from the top end to the lowest end. The path worker or machine operator should keep the ditch deep and wide, angle the side walls to at least 45º, and make sure the excavated channel is smooth and even along its length to allow water to flow freely. The excavated ditch line should feature gradual curves, not long straight lines.

  • The path worker or machine operator should check that the base is graded and the side walls are angled back whilst digging the ditch line.

  • The path worker or machine operator should give the base of the ditch a final levelling for a smooth water flow and remove any obstructions, like protruding stones or tree roots, that will prevent the free flow of water or cause erosion.

  • Turfs and excavated spoil removed from the ditch can be used for path edge landscaping, if suitable, or infill for borrow pits. Turf can be used to line the ditch bottom and sides where erosion will be problem, such as in deep, soft peat or sandy soil. If no suitable use is found for the extra spoil, it should be hidden on site.

Turf lined ditches are a common drainage feature of upland paths, but can be used equally well for lowland paths where the visual impact from open ditches is an issue.

The method of construction for turf lined ditch is:

  • A machine operator with a tracked excavator cuts and lifts the existing vegetation as individual turfs, and carefully places them on the ground with the vegetation on top at an easily reachable distance for re-use.

  • The tracked excavator excavates the sub soil to form a shallow, semi-circular cross section ditch of maximum 2.0m wide and at least 0.5m deep.

  • With the tracked excavator bucket, the machine operator lifts the cut turfs and places them, roots down, onto the excavated ditch bottom and sides, tightly together without gaps or exposed roots showing.

  • If possible, turfs should not be placed directly on unconsolidated gravel / alluvial substrates because they can be easily washed out by flowing water. Use available soil to line the ditch before replanting turfs to help the turf to grow and survive. If necessary, for example where turfs are likely to be dislodged by flowing water, pin the turfs into place on the ditch sides using untreated wooden pegs. The pegs will biodegrade over time.

Detailed drawings - Open V or U ditch and turf lined ditch

You can download standard detail drawings and specification details here for:

icon Open V Ditch - Standard Detail Drawing & Specification Details

icon Open U Ditch - Standard Detail Drawing & Specification Details

icon Turf Lined Ditch - Standard Detail Drawing & Specification Details

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