The drainage ditch is fundamental to the success of any drainage system. It is essential wherever there is a problem of water running onto the path from surrounding ground.
The key function of the ditch is to collect water and direct it away from the path. The ditch will intercept water above the path; direct it through path drainage features; direct it away from path drainage to soak away in the surrounding area. An invaluable by product of ditching is spoil and turf, which can be used for landscaping and site restoration.
Bill of Quantities (example)
Excavate drainage ditch 300mm deep and 300mm wide at base, chamfered to 500mm wide at top. Ensure that water drains freely along the ditch and away from the path.
POSITIONING OF DITCHING
Ditch positions are crucial to the success of the drainage system. The most efficient layout, with maximum effect must be the aim. Maintaining a low impact approach is essential, in landscape terms as well as any adverse effect on natural land drainage and associated ecology. The temptation to cover the hill with ditches must be avoided.
When assessing the site, the surrounding ground and options that exist for directing surface water away from the path must be considered. The water may need to be directed across the path with culverts or drains. Y-shaped ditching is often used in conjunction with these, creating two inflow arms above the path which join and flow through the drain and away through a single outflowing ditch. The use of a soakaway may be required if there is nowhere for water to drain to.
The presence of underground water is an important factor to consider. A traversing path often requires side ditch protection from seepage on the uphill side. To intercept as much water as possible, ditches should bepositioned as low as possible on the slope.
A ditch line should:
- follow the lie of the land
- avoid steep gradients and long straight lines
- incorporate curves to blend with surroundings and slow down the water flow
- avoid sharp corners or sudden changes in direction that cause flowing water to erode the ditch sides
- be effective and maintain a draining fall, or run, throughout its length
- avoid obstructions, such as boulders or bedrock
The dug ditch is a simple drainage feature comprising base and sides. It normally requires no stone or other materials.
The 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, or erode if water flow is too fast through a narrow channel.
Ditches should be:
- at least 300mm deep and 300mm wide at the base
- chamfered and widening to approximately 500mm at the top; vertical sides may collapse with the weight of saturated ground above the ditch sides, or if water erodes the base and undercuts the sides
- at least 500mm away from the path edge to avoid path collapse into the ditch
These dimensions also provide good spade room for clearing during maintenance.
Method of Construction
Start digging at the bottom end and work uphill - maintain a good draining run from top to bottom
- keep the ditch deep and wide enough - angle the side walls by chamfering
- keep the ditch base smooth and even - avoid creating hollows by digging too deeply
- incorporate gradual curves and avoid long straight ditches
Check that the base is graded and the side walls angled. If this is not achieved whilst digging the ditch line, do it immediately afterwards. Give the base of the ditch a final levelling, for a smooth water flow.
Use the turf and spoil removed from the ditch for landscaping and site restoration work, or in-fill for borrow pits. Excess turf may be used to turf the ditch sides. If no suitable use is found it should be hidden on site.
Key points to watch out for:
- avoid creating unnecessary ditches
- avoid sharp angles - put in curves to dissipate the rate of flow
- avoid steep gradients - fast water flows will result in ditch erosion
- ensure that water flows smoothly from top to bottom
- avoid collapse of path edge into side ditch - keep far enough away
Where bedrock has to be crossed the creation of side walls will be necessary. This is achieved by mounding with spoil and turfs.
Turfs can also be used to stabilise ditch sides where problems are encountered with weak edges such as in deep, soft peat. Turfed ditch sides will also reduce environmental impact.
A soakaway, also known as a catchpit or sump, may be required where there are no options available to ditch water away from the path. These are dug wide and deep enough to take the expected quantity of water. They may be filled with clean stone if available to allow the water to drain through. The sides should be built up with turfs to form a natural looking hollow.
Ditching easily becomes blocked and over-flows; or sides fall in from the pressure of water eroding the bottom of the ditch. The following maintenance tasks must be carried out on a regular basis.
- clear out debris and silt that may choke or block the ditch
- deepen and widen ditches that block quickly; regrade if water is not flowing
- re-chamfer side walls where collapse has occurred or is likely; stabilise with any available turf
- use removed silt or gravel to re-surface the path behind water features
- use removed turf on path edge or landscaping repairs; or hide tidily on site
- choose a ditch-line that is both effective and fits in with the surrounding landscape.
- leave the site looking as natural as possible; use or dispose of all excess spoil and turf properly
HEALTH AND SAFETY HAZARDS
- conditions underfoot may be wet, muddy and slippery, particularly when digging in the ditch.
- do not under estimate the amount of water expected at its wettest; carefully assess the direction of water flow in relation to the path.