Field Drainage Maintenance

The performance of any field drainage system will depend on the frequency and quality of maintenance undertaken on the system.

It is very likely that fields in the UK with clay soils or in areas of heavy rainfall already have existing drainage systems installed. If there are problems with waterlogging in these areas the existing drainage system may not be functioning correctly.

The CivilWeb Field Drain Design Spreadsheet can be used to accurately and quickly design all kinds of field drains.

Ditches and Outfalls

Ditches and outfall need to be maintained in good condition in order for the rest of the field drainage scheme upstream to function. This can also increase the deterioration of upstream components such as lateral drains which can suffer from decreased performance and become blocked more quickly. A completely blocked outfall can cause significant flooding and waterlogging of large areas in wet weather.

Outfalls are a likely area where sediments are able to slowly build up over time. Clearing an outfall is usually a very simple procedure and can be undertaken very quickly with a spade. Ditches, swales and outfalls should be cleared in autumn to minimize the risk of soil and crop damage.

Ditches should be fenced off from livestock. This will prevent bank damage and erosion due to livestock. It also improves the water quality by preventing animal manures from entering the water.

Pipes

Any lateral and main drainage pipes included in the field drain design will need to be maintained in order to ensure that they continue to perform as designed.

Tree Roots

Tree roots should be prevented from entering the pipe. Any trees or hedges within the tree rooting zone or around 1.5m from the pipe should be removed before the pipe is installed if possible. If this is not possible a sealed pipe should be used for these areas rather than a perforated pipe.

If a pipe becomes blocked by tree roots it may be possible to expose the pipe and clear using rods. It is likely that the affected section of pipe will need to be replaced with a sealed pipe to prevent further blockages occurring in the future where tree roots are present.

Siltation

Siltation can occur in pipes. This is more likely to occur when the pipes have been left submerged or blocked for a period of time. This can also occur where a downstream blockage affects the hydraulic performance of the pipe. When this occurs the water in the pipe will be slowed down increasing the amount of sediments which are deposited. This can also be an issue where poor design or installation has allowed sediments to enter the pipe.

If there are maintenance accesses to the pipes they can sometimes be jetted or rodded to remove the sediments. If this is not possible the pipes may need to be replaced. When pipes are replaced they should be replaced with pipes of the same diameter in order to ensure they will perform as required. Increasing the pipe size is sometimes appropriate but can lead to flooding or erosion issues downstream.

Ochre

In addition to general siltation filed drains can also be at risk of ochre blocking. This occurs when soluble iron is able to leach out of surrounding soils and oxidises within the drainage pipe. This can lead to blockage of either the pipe itself or blockage of the pipe perforations preventing water from entering the pipe. Similar to siltation blocking this can sometimes be cleared using jetting or rodding, and sometimes the pipes will need to be replaced.

Where siltation or ochre blockages are expected, maintenance accesses to the pipes should be included in the field drain design and regular rodding or jetting of the pipes should be undertaken as a maintenance activity. The frequency of these visits should be determined by undertaking regular inspections of the pipes performance.

The CivilWeb Field Drain Design Spreadsheet can be used to accurately and quickly design all kinds of field drains.