Field drainage: Environmental impact assessments

It is important to carry out an environmental assessment before installing or carrying out maintenance on field drainage systems.

Minimising environmental impacts

Field drains can help decrease surface run-off and soil erosion, reducing loss of sediment and associated pollutants to water.

Conversely, field drains can also provide a rapid route for water enriched with pollutants, such as ammonium, phosphorus, and fine sediment – especially newly installed drains or in fields with deep-cracking clays.

Although it is often better for both agricultural production and the environment to remove excess water by field drainage, it is important to assess environmental risks associated with the installation or maintenance on field drainage systems.

Plan field drainage to avoid:

• Negative impacts on water bodies used for drinking water abstraction or fisheries
• Raising nitrate levels in sensitive Sites of Special Scientific Interest (SSSIs)
• Diverting water away from areas that may need it for drinking, washing or habitat
• Diverting flows that increase the risk of flooding and infrastructure failure

Field drains and outfalls could be designed to discharge into a wetland buffer area before flows enter a watercourse or be directed away from sensitive water bodies.

Sustainable Drainage Systems (SuDS) or other approaches, such as bioreactors, can be used with field drainage systems to trap sediment and slow water/soil run-off and filter pollutants in drainage water.

Best practice

Follow best practice when applying manures, fertilisers, and agrochemicals to avoid losses via surface run-off or field drains.

• Do not apply organic manures within 12 months of pipe or mole drainage installation
• Do not apply organic manures to drained land when soils are wet and drains are running
• Do not store organic materials (slurries or silage) within ten metres of a field drain

Enhancing the environment

New drainage schemes can provide an opportunity to create conservation features.

Silted-up farm ponds could be reopened to provide a habitat and catch pit for eroding soils. Ditches could be over-dug into ponds and shallow pools to provide a habitat for aquatic plants, invertebrates, amphibians, birds and small mammals.

Timing of clearance operations or ditch maintenance may have implications for wildlife. Avoid disturbing breeding or nesting animals.

Waterlogged land

In some situations, it may be better to allow field drainage to deteriorate. For example, to act as a sediment trap and reduce the need for costly activities, such as watercourse dredging.

Although such land may be of low value agriculturally, it may have biodiversity benefits or help to reduce flooding risk.

Areas suited to this may be adjacent to watercourses or natural wetlands, as well as ribbon areas at the base of steep slopes − particularly on intensive grassland on heavy soils in the centre and west of the UK.