Selection of type of drainage system
In deciding what type of drainage system to install, a farmer is often faced with a very difficult problem, requiring very careful thought and weighing up of the various factors involved, e.g., physical conditions of the area, the source of the excess water, costs, availability of capital, labour, and plant. Advice from an expert is very valuable here, but if this is not available, the farmer must use his own judgment. The essential consideration is that the value of the increased production resulting from drainage should exceed the capital charges for the work and maintenance cost, if any.
Inspection of the area
On his own property, the farmer will be very familiar with the physical conditions, but he should re-examine them to check on the points given below. When inspecting a property which he is considering purchasing and possibly draining, or if attempting to advise or help an inexperienced person, he should make a very careful inspection of the property, and attempt to answer the questions given below.
The inspection should certainly be made when the conditions are the wettest possible. Beware of deciding on drainage questions in summer or autumn, or in a season drier than normal.
(1) What is the nature of the ground surface? Is it flat, gently and evenly sloping, gently undulating, steeply and evenly sloping, broken, or rolling?
(2) Where does the water lie? How did it get there; by direct rainfall, run-off or seepage from a higher level, or from springs? Where has poaching occurred? Where are rushes and other water-loving plants thriving?
(3) What is the nature of the subsoil to a depth of about 4 foot? Examine ditch banks to find this, and dig holes with a spade, or bore with a soil auger. Is the subsoil dense clay, silt, sandy clay, sand, gravel, or peat, or a combination of these? To what height does the water table rise? Yellowish brown staining of the subsoil and ironstone modules often indicate the upper level of the water table. Is it a perched water table, or does the water rise freely from below?
(4) Is there a suitable outfall into an open drain or stream? Is there a sufficient fall to this outfall from all points in the area, even when the stream is at high level? If the fall is not obviously good, levels must be taken to check it.
Choice of system
Decide what type or combination of types of drainage system to install. The following systems should be studied.
(i) Open Drains. Apart from their use as main drains for minor drains to discharge into, or for surface water or flood drainage, open drains would be suitable for subsoil drainage only in fields with very open free-draining soil, in which the drains would be able to draw water from a fairly long distance, or for an area of only a few acres with a fairly steep fall to the proposed drain. Open drains are very useful indeed in draining swamps, provided there is sufficient fall available and the subsoil consists of free-draining material such as certain peats, or peat over gravel, but many peats are not free-draining. These drains are often cut with a dragline excavator. Underdrains can often be put in later when the area has dried up and consolidated.
(ii) Tile Drains. Although tile drain lines spaced close enough will drain tight subsoils, on economic grounds they are best suited to controlling a rising water table in fairly permeable subsoils. In tight clay subsoils, with a perched water table, they are usually employed in fairly widely spaced lines to act as the mains for mole drains, as described in (v) below.
(iii) Brush, Stone, Box, etc. Drains. These would be used as a temporary and possibly cheaper substitute for tile drains, or in the circumstances described in (ii) above but where the subsoil would not support tile drains, e.g., peat swamps.
(iv) Mole Drains. This system alone would only be possible in land with a firm clay subsoil, free from buried timber, stones, and other obstructions, and with a slope of at least 1 in 8o, but preferably 1 in 3o or 4o. If the fall were steeper than this, the moles could be pulled around the slope to get a fall of about 1 in 30 or 40, but care should be taken in mole draining on steep country, particularly if the subsoil is easily eroded, as it would be possible to start “underrunners” and gullies.
(v) Combined Mole and Tile Drains. This system is suitable for gently undulating land with firm clay subsoil, where there is not sufficient fall for moles along the shallow depressions and gullies, but adequate fall down the sides of these depressions. The tile drains are laid along the bottom of the depressions, and the moles pulled up the side slopes. This method is much cheaper than a full tile system, and it has the advantages of the full mole system and of having the bottom of the depressions permanently well drained. Combined mole and tile drainage has also been used successfully on land that is very nearly flat, parallel lines of tile drains being used as mains for mole drains pulled across them.
(vi) Shallow Surface Drains. In areas where for economic or other reasons underdrainage is not practicable, it may be possible to provide only surface drainage by means of broad shallow channels, which act as drains to remove surface water when it accumulates. These channels do not form an obstacle to working the fields, and crops and pasture are grown in them. Surface drainage, although it does not drain the subsoil, may nevertheless bring about a worthwhile increase in production. Surface drainage may also be needed where an underdrainage system has been installed, but where occasional heavy rain causes surface water to accumulate because it cannot percolate down to the underdrains sufficiently rapidly.
(vii) Pumping. Where there is insufficient fall to an open drain, the possibility of discharging the water from the drainage system into a sump, and pumping it through or over a stopbank into the open drain, should be investigated. Special flood pumps capable of handling the water from hundreds of acres are available, and are quite cheap to operate.
Pump drainage has been used very successfully for draining swamp land bordering lakes or rivers, where the land may be actually below high-water level. Stopbanks are constructed around the area, and open drains formed leading to a sump, from which a pump discharges the water through or over a stopbank. A floodgate keeps the water from coming back when the pump is not working.
(viii) Combined Systems. Soil and land surface may vary throughout the area that is to be drained, in which case combinations of one or more of the above types of drainage systems may be necessary.
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