Drainage

Published 11 April 14

James O’Loughlin from Teagasc spoke about the objectives of land drainage, the correct way to investigate drainage issues, as well as types of drainage systems and their maintenance at the DairyCo/BGS/RABDF Spring Farm Walk in Dumfries on 26 March.

There are many reasons for poor performance on heavy soils: poor soil fertility, low levels of perennial ryegrass and poorly maintained existing drainage and all these issues should be addressed before major investment in drainage is considered. Underlying all drainage work must be the economic returns to the farming business.

Objectives of land drainage

The objective of any form of land drainage is to lower the water-table, providing suitable conditions for grass growth and utilisation. A controlled water table promotes deeper rooting of plants, in this case grass, which improves sward productivity. It also improves the load-bearing capacity of the soil and lessens the damage caused by grazing and machinery.

When planning any drainage programme, the potential of the land to be drained needs to be assessed first, to determine if the costs incurred will result in an economic return through additional yield and utilisation of the grass or other crops grown.

Some thought is needed in deciding on the most appropriate part of the farm to drain. From a management point of view it is better to drain land which is nearer to the farmyard and work outwards. However, it may be more beneficial to decide where to commence works once the drainage potential has been established by site investigation. This ensures a better return on the investment.

Drainage investigations

Through effective land drainage, there is  enormous potential for developing our land resources. Drainage problems tend to be as a result of two major factors; high excess rainfall and a complex geological and glacial history.

The land drainage problems encountered in Ireland and Scotland are complex and varied and a full understanding of the issues involved is required before commencing works.

The first step is a detailed investigation into the causes of poor drainage, knowledge of previous schemes in the area and their effectiveness, will often guide the investigations.

A number of test pits (at least 2.5m deep) should be excavated within the area to be drained. The test pits should be dug in areas which are representative of, area as a whole. As the pits are dug, the faces need to be observed, soil type should be established and the rate and depth of water seepage into the pit (if any) recorded. The depth and type of the drain to be installed will depend on the interpretation of the characteristics revealed by the test pits.

Types of drainage system

There are two principle types of drainage system:

  • Groundwater: A network of piped drains establishing a deep drainage base in the soil
  • ·Shallow: These are used where soil is clayey (heavy) and infiltration of water is impeded at all depths.

Outfalls/Maintenance

Every drainage scheme is only as good as its outfall. Cleaning and upgrading of open drains acting as outfalls from land drains is an important step in any scheme. Before commencing land drainage the proposed outfall should be assessed and where necessary upgraded. Open drains, running in the direction of maximum slope, should be established to as great a depth as possible. This will maximise the potential for land drainage, with associated benefits.

When a drainage scheme has been completed, the layout should be drawn and noted on a farm map. This map can then be used as a guide when maintaining the works, as well as a record of the works. Land drain outlets should be regularly cleaned and maintained, especially if open drains are cleaned/upgraded, as this may result in blockages at the outlet.

Drainage is costly, therefore, it is imperative that the correct drainage solution is chosen – it must improve the levels of grass production and utilization – and once installed it must have regular maintenance.