BTCV Handbooks Online

Book:	Waterways & Wetlands
Chapter:	12 Ditches and pipes
Section:	Ditches
Metadata:	Details	Buy this book

Ditch design

The design of a ditch is determined by the following factors:

Depth

The depth should be appropriate to the function. A simple ditch to intercept runoff may only need to be 150mm (6") deep, although it may need annual clearance to prevent it silting up. Most interceptors should be 375-600mm (15-24") deep, while ditches serving as carriers should be at least 1m (3') deep.

Batter and bottom width

Some soils are more stable than others, allowing a steeper batter or slope to the ditch sides. The batter should never exceed 1 horizontal to 2 vertical (1:2) or 63°. Very deep ditches may require a batter of 2:1 (27°) to prevent collapse, even in stable soils.

The diagram indicates angles of repose, which are the steepest angles to which various types of soil will slide if unsupported. They can be taken as guidelines for the maximum batter of ditches which lack timbering or revetments. Note that temporary shuttering is necessary for safety when digging in unstable soils (see pipes).

For general purposes, ditches should be 300mm (1') wide at the bottom if dug by hand. This may need to be modified to prevent erosion or siltation under conditions of great or low flow (see below).

Factors influencing erosion

Ditches may function even if the fall is very slight, but if the flow is too slow to hold materials in suspension the ditch will silt up. On the other hand, too rapid a flow will erode the bank. The table below shows approximately the velocity and slope at which erosion starts to occur in various soil types. The figures for maximum slope are based on a mean hydraulic depth of 300mm (1').

Soil type Maximum velocity Maximum slope

Sandy soil 600mm/sec 1:600

Loam 900mm/sec 1:300

Clay 1.2m/sec 1:150

If a ditch is dug and scour occurs, there are two possible solutions. The capacity of the ditch can be increased by widening the channel, while maintaining or increasing the depth. Alternatively, the slope can be reduced by building small dams or weirs along its length. Each dam holds back water, so slowing the overall velocity. A breakwater or apron is needed at the foot of each dam to prevent erosion, and silt will need removing periodically from behind the dams.

New ditches are much more vulnerable to erosion than established ditches. Ditches through disturbed ground are particularly unstable. Clean water erodes to a greater degree than does water heavily laden with sediment. The shape of the channel may be significant, as deeper water erodes more severely than does shallow water flowing at the same velocity. However, ditches which are too wide for the flow develop shoals and irregular channelling.

Running sand

Running sand or silt is seldom encountered but creates difficult problems where it does occur, most often in alluvial valleys. The sand is so fine and unconsolidated that, when a ditch is dug, the bank slowly slumps no matter how shallow the angle of slope. Preferably relocate the ditch. If this is not possible, brushwood can be stacked against the bank to stabilise it.

Digging ditches and trenches

Wherever possible, it is recommended that machines are used to dig any ditches over 600mm (24") deep. Manual digging is only appropriate for short lengths of ditch, or where machine access is not feasible.

Keep in mind the following points:

The tools required depend on the terrain and soil. Spades are sufficient in topsoil, but usually forks and picks are needed to break up the subsoil. Use a heavy metal fork with long tines. Several specialist types of spade for digging narrow trenches are described in Chapter 5. A bottoming spade or tile scoop is useful for cleaning the bottoms of deep ditches. Keep tools sharp and clean. When digging in clay, dip the shovel into a bucket of water frequently to prevent the clay sticking.
Usually a trench should be dug by a team of two people working as shown.
>
Work from the outflow point uphill. If the outflow is into a waterway which is likely to back up into the ditch, leave the very end of the ditch uncut to act as a coffer dam, until the ditch has been extended above the level of the waterway. Set the turf to one side to be re-used. Unless it is to be backfilled soon afterwards, place the spoil downhill of the ditch where it will not block runoff from reaching the ditch. Place the spoil at least 300mm (1') from the ditch so that it won't slip back in during the first heavy rain.
Stand in the bottom of the trench when digging, not on the sides, except when digging a very deep narrow trench.
Keep the ditch line straight. A line of sand sprinkled on the ground is simpler than pegs and lines, which tend to get knocked.
Where subsidiary ditches are brought into a larger ditch, make sure they enter it at a gradual angle to prevent erosion opposite the inflow point. The bottoms of the inflow ditches should be slightly above that of the larger ditch.
Where the ground is wet or unstable, avoid trampling close to the trench sides.

Shuttering

Any trench dug in unstable ground, or if over 1m (3') deep, should be supported with shuttering to provide safe working conditions. This should be inserted as the trench is dug. Such trenches are not suitable for volunteer work.