Protecting water from agricultural run-off: water retention measures (TIN099)

Soil bunds

This specification is only a guide and may not be appropriate for all sites.

If the subsoil from the excavation contains sufficient clay it can be used to construct the bund. The soil should have minimum clay content greater than 20% and ideally above 30%. Where the clay content of the soil is not known you can send representative soils samples for analysis at an accredited laboratory.

A key trench should be excavated at least 450 mm into the bottom and sides of the ditch. Using selected soil with at least 20 to 30 per cent clay content, back fill and consolidate the key trench and build up the bund in thoroughly compacted 150 mm layers until the required height is achieved.

Side slopes should generally be no steeper than 1.5:1. The height of the bund should always be finished slightly proud of ground level to allow for minor settlement. The top should be at least 2 m wide. Trim the bund faces to a smooth finish.

To control water levels, a 200 – 250 mm diameter pipe should be used as in most circumstances this should be enough to prevent overtopping of the bund. An un-perforated land drainage pipe or rigid plastic pipe can be used. The crown of the pipe should be set at least 30 cm below the top of the bund.

Photo 4 A bunded ditch

To reduce the risk of bund erosion in the event of a storm, the pipe should be extended beyond the base of the earth bund on the downstream side. In some circumstances a spillway may need to be constructed to prevent erosion of the bund face and the ditch downstream.

A right angled bend can be attached to the upstream end of the pipe, so that it can be twisted to control the water level in the wetland area.

Any accumulations of rubbish or silt must be removed from under the pipe where it enters the bund. To further help stabilise the bund sow a grass seed mix at 25 g/m2. For details see Table 2.

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