THE very heavy rains since November 2015 to recent days have been very challenging for horse owners and breeders on heavy soils. Grazing conditions are challenging, grass growth is poor and spreading of farmyard manure and slurry has not been possible. Given this background it is understandable that there is now considerable interest in drainage.

It is very important that soil compaction caused by machinery or horses is not confused with poor drainage. It is also important that the drainage infrastructure currently on the farm is examined, as far too often poor maintenance which causes restricted water movement and an apparent breakdown of the drainage can be resolved by cleaning water courses and flushing drainage pipes.

WHAT CAUSES IMPEDED DRAINAGE?

The rate at which water moves through a soil, hydraulic conductivity, varies enormously depending on the soil type and management. Open gravelly soils have a capacity for water flow that is hundreds of thousands of times that of compacted heavy clay. In free draining soils the rate at which water flows downwards through the soil is always greater than then that being supplied by rainfall. In poorly drained soils the rate of infiltration at the soil surface is regularly exceeded by the rainfall rate due to:

OBJECTIVES OF LAND DRAINAGE

To achieve effective drainage the works will have to solve one or more of these problems and possibly a combination of all three. The objective of any form of land drainage is to lower the water-table providing suitable conditions for grass growth and utilization. A controlled water table promotes deeper rooting which improves sward productivity. It also improves 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 first assessed 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 the most appropriate part of the farm to drain. From a management point of view it is better to drain that land which is nearer to the stable yard 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.

INVESTIGATION

The first step of any drainage works is a detailed investigation into the causes of poor drainage using test pits. Knowledge of previous drainage schemes in the area, and their effectiveness will often provide an insight into the causes. 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 that are representative of area as a whole. As the test pits are dug, the faces of the pits are observed, soil type should be established and the rate and depth of water seepage into the test pit (if any) recorded.

Visible cracking, areas of looser soil and rooting depth should be noted as these can convey important information regarding the drainage status of the different layers. The depth and type of the drain to be installed will depend on the interpretation of the characteristics revealed by the test pits.

The difficulties of drainage problems in Ireland are largely due to our complex geological and glacial history. Glacial processes lead to the formation of rolling and undulating landscapes, made up of haphazardly sorted rock and soil materials. Layers of varying texture and composition have the effect of irregularly distributing groundwater flow, with fine textured soils acting as a barrier to movement, impeding drainage, and lenses of gravels and sands promoting water flow, transmitting groundwater over large areas with resulting seepages and springs on lower ground.

TYPES OF DRAINAGE SYSTEM

Two principle types of drainage system are distinguished:

SHALLOW DRAINAGE SYSTEMS

Where a test pit shows little ingress of water at any depth a shallow drainage system is required. These soils that have no obvious permeable layer and very low hydraulic conductivity are more difficult to drain. Shallow drainage systems are those that aim to improve the capacity of the soil to transmit water. These include mole drainage and gravel mole drainage. The aim of these drainage techniques is to improve hydraulic conductivity by fracturing and cracking the soil and to form a network of closely spaced channels.

Mole drainage is suited to soils with high clay content which form stable channels. Mole drains are formed with a mole plough comprised of a torpedo-like cylindrical foot attached to a narrow leg, followed by a slightly larger diameter cylindrical expander.

The foot and trailing expander form the mole channel while the leg creates a narrow slot that extends from the soil surface down to the mole channel depth. The success of mole drainage depends on the formation of cracks in the soil that radiate from the tip of the mole plough at shallow depths as the soil is displaced forwards, sideways and upwards.

The effectiveness of mole drains depends on the extent of suitable cracking during installation. As such the ideal time for carrying out mole drainage is during dry summer conditions, this will cause maximum cracking in the upper soil layers as well as facilitating adequate traction preventing wheel-spin on the surface.

Gravel-filled moles employ the same principles as ordinary mole drains but are required where an ordinary mole will not remain open for a sufficiently long period to render its application economical. This is the case in unstable soils having lower clay content. Sub-soiling is used effectively where an iron pan or cemented layer impedes drainage. The effect is to break the layer and crack the soil. A stable outlet channel will not be formed.

Collector drains, which are installed across the slope at 0.75m BGL, are required for all mole drains. Depending on the topography and slope the collector drains will be at a spacing of 10–60 m. A larger spacing reduces costs but results in a higher chance of failure. The mole drains themselves are drawn at right angles to the collectors (up-slope) at spacings of 1.0-1.5m and a depth of approximately 0.4-0.5m. Stone backfill for collectors should be filled to within 250mm of the surface to ensure interconnection with the mole channels. It is important that after mole drainage or ripping, paddocks are either reseeded or allowed settle as the sod can protrude and cause danger for horses.

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 drainage scheme. When a scheme has been completed, the layout should be drawn and noted on a farm map. This 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 drain outlet. The use of a concrete or un-perforated plastic pipe over the end of the drain pipe, minimum 1m in length, will protect the outlet from damage and will make locating and maintaining it easier.

CONCLUSIONS

The drainage problems in Ireland are a result of two major factors: excess rainfall and a complex geological and glacial history. There is a need for a better understanding of the underlying causes and of the design and implementation of appropriate drainage systems. There are many reasons for poor performance on heavy soils: poor soil fertility, low levels of ryegrass and poorly maintained existing drainage, all issues should be addressed before major investment in drainage is considered. Underlying all work must be the economic returns to the farming business. Drainage is costly, ranging from €3,500/Ha for shallow drains to €7,000/Ha for gravel mole drainage, therefore it is imperative that the correct solution is chosen and it must improve the levels of grass production and utilisation. A drainage system once installed must have regular maintenance.

James O’Loughlin and Pat Tuohy are based at the Animal & Grassland & Innovation Centre, Teagasc, Moorepark, Fermoy, Co Cork.