For Surface Drainage.
Surface water is often removed from fields by shallow furrows ploughed through the low places. This is a useful method, but the furrows are a nuisance when working implements over the fields. A better method is to form broad but shallow channels or ditches with a grader or ditching plough while the paddock is under cultivation. Implements can work across such channels. In the system of surface drainage known as “water bedding”, used on almost flat land with a tight clay subsoil, parallel lines of broad shallow channels are formed, from I to 2 ch. apart, running in the direction of such slight fall as the field may have. The areas between the channels, which are known as “lands”, are slightly crowned so that surface water will run into the channels, which carry the water off the field. The channels and lands can be formed easily with a grader. Pasture and crops will grow in the channels, as they carry water only during and shortly after rain. This is a very effective method of surface drainage.
Surface drains assist in getting rid of surface water after heavy rain, but do not drain the subsoil. For this purpose underdrainage, to lower the ground-water level, is necessary.
The topsoil and subsoil are drained by either open drains or underdrains. These, in addition to lowering the water table, will of course also remove surface water.
Open drains, or ditches in the same position and of the same depth, serve the same purpose in draining the topsoil and subsoil as do underdrains. As they will also carry a great deal of water, even on flat grades, they are also very useful indeed as main channels for surface water, thus preventing flooding, and they are essential for carrying the discharge from underdrains. They also have the advantage of being very easy to construct, requiring little skill, and are capable of being dug comparatively cheaply entirely by machines such as dragline excavators.
Compared with underdrains, however, they have certain disadvantages that definitely limit their usefulness :
(i) They obviously cannot be cut through paddocks, so must run along the fence lines. Thus, they cannot lower the water table over an appreciable portion of the field unless it is very small or unless the soil is very free draining. A slope towards the ditch will, of course, assist them to drain the field by gravity, and so they are best placed along the lower boundary if possible. Here they can also act as outlets for underdrains.
(ii) They cause a strip of land to be wasted. This often needs to be fenced to prevent stock pushing the banks down and even being drowned in the ditch. The fenced strip and the banks of the drain also harbour weeds.
(iii) They need frequent cleaning, which is expensive, and very laborious, slow, and unpleasant work if it has to be done by hand.
(iv) The banks form a path for machinery, men, and stock. This traffic packs the soil and reduces the efficiency of the drain. The raised bank formed in cleaning the drain also interferes with its effectiveness.
Underdrains may be run beneath the surface of the fields, particularly under the lower wetter areas. They are used particularly where the fields are large or where the soil is not very free draining. In such cases, open drains around the fence line are ineffective in lowering the ground-water level.
Underdrains are of three general types :
(i) Tile Drains and Pipe Drains.
These are cylindrical pipes commonly in diameter from 3 inches upwards and 1 foot in length. They are usually made of burnt unglazed clay but concrete pipes may also be used. They are laid end to end in trenches without in any way sealing the joints. Practically all of the water enters the tile lines through the gaps between successive lengths. They can be laid on fairly flat grades, e.g., a minimum of 1 in 400 (2 inches per chain) for the most common size of tile (which is 4 inches in diameter) up to grades as steep as 1 in 3o, and are used in all types of soil that is solid enough to support them. They are laid in parallel lines 1/2 ch. apart or more, depending on soil conditions, or merely through the low areas in undulating fields. If well planned and laid, tile drains are highly efficient and will continue to work efficiently for a long period; there is probably no limit to their useful life.
(ii) Substitutes for Tile Drains.
These are drains laid in trenches in a similar manner to tile drains, but consisting of such materials as gravel, bundles of brushwood, boards in the form of a square or triangle, stone slabs, small tree trunks, etc., placed in the bottom of the trench, which is then filled up. If the material is easily available they may be cheaper than tile drains but they tend to fill up with soil and become inefficient fairly quickly, and so can be regarded only as temporary drains, or as an inferior substitute for tile drains. They have been used with some success in peat where the soil will not support tile drains.
(iii) Mole Drains.
A mole drain is a tunnel formed in the subsoil by the passage of a torpedo-shaped “mole” attached to a vertical blade, which is carried by the frame of the mole plough. A tractor provides the motive power. Mole drains are usually from 2 to 3 inches in diameter, and are usually satisfactory only in film clay subsoils, free from stones and other obstructions. In silt or sands the moles collapse rapidly and block, but in good firm clay they may have a life of seven years or more. Another essential requirement for satisfactory mole drains is that the ground should have an even surface and a slope of at least 1 in 8o, preferably more. This is because the mole drains run more or less parallel to the ground surface, and if there is insufficient slope they will silt up and block rapidly. However, very short drains, not more than 2 ch. long, have been found to remain effective for several years on slopes that are very nearly flat. Mole drains are usually spaced much closer than tile drains, at from 6 to 15 ft. apart, and are drawn at depths varying from 54 to 3o in. Compared with tile drains, they suffer from the disadvantage that they have a comparatively short life, but their great advantage is their relatively low cost.
All of the above types of drains may discharge direct into open drains, or into other underdrains, e.g., minor tile-drain lines into major tile-drain lines, minor moles into major moles, moles into tiles, etc.