LAND-DRAINAGE.

New Zealand Journal of Agriculture, Volume XV, Issue 2, 20 August 1917, Page 91

LAND-DRAINAGE.

SOME NOTES ON OPEN DRAINING.

L. J. B. Grant,

Fie’ds Inspector, Tauranga.

When increased crop-production or stock-carrying capacity on wet or swamp lands is aimed at, drainage naturally becomes the first consideration for removing the surplus water. Open channels, whether natural streams or .artificial ditches, receive water that passes through the soil as well as the water that flows over the surface of the - ground. In ground composed largely of sand, or soil with a gravel subsoil, effective and sufficient drainage can be accomplished by open- channels. . The open nature of the soil permits free percolation of water, first from the surface, then into the gravel, and thence into the channels. But in a heavy loam or clay open channels afford less effective drainage, owing to the slow percolation of water through the soil, and the walls of the ditch becoming puddled, thus impeding the passage of water into it..

Tiled drains and- underdrains are better . than open - ditch drains, but are' quite impracticable on swamp country of a peaty formation, such as that around Te Puke and extending .to Otamarakau, also the Rangitaiki Swamp. On such land shrinkage takes place when the water is drained off, and open-ditch draining must be resorted to . until the ground becomes sufficiently consolidated to permit tile drainage.

Main outlets being cut, draining the farm section can be taken in .hand. It will be advisable to keep in mind certain cardinal principles which, although simple, are important in the construction of drains. The drain should be located where nature would provide the natural watercourse, for the obvious reason that the surface slope of' land leads the water. in that direction, and the soil will naturally permit the flow of water towards lower ground. Cut the drain in a straight line as far as possible. Any change in direction should be made in long curves, avoiding as far as possible carrying the ditch through where no drainage is required. This is where one may have to depart from the rule of placing the drain where nature would have placed it. • Straightening by short cuts here and there makes the method less expensive (and perhaps more ' effective) without impairing its value as a ; drain compared, with a natural course. The subdrain should follow the line of drainage as far as • possible, and laterals in a line of

the greatest slope. There may be exceptions to this principle, but they apply to particular cases when it is necessary to intercept water which percolates through the soil from a high level.

Experience shows that deep drains and fewer of them are better than smaller ones and more of them when tapping the source of water-supply, such as main springs. Under other con-

ditions, however, the land can be made dry too quickly, causing crevices, and thus interfering with capillary action. This may make the land as valueless as when too wet. A gradual draining of water should be aimed at.

The accompanying diagrams illustrate different systems of drainage designed for various natural conditions.

Fig. 1 indicates the plan upon which all drainage-systems should be based, and is in itself sufficient for draining small areas. The main drain is dug along the line of lowest levels of the swamp, and the side drains A, B, C are dug along the principal gullies or most suitable points where they will drain the largest possible area.

Fig. 2.This system may be used alone to drain an elongated, strip of swamp, or as an extension of the system in Fig. T. The main is to be dug along the line of the lowest level (the natural watercourse), and the laterals 1 to 13 almost at right angles to the main— is, along the greatest slope. '

Fig. 3. —This is perhaps the most economical system for draining very flat lands. By adopting a similar method the laterals Ito 8 and a to d can be graded sufficiently where the ground would be too flat for any other system.

Fig. 4. By this very useful system a number of laterals are run into a short mainland discharged through a single outlet. This system can easily be extended by using it in conjunction with that shown in Fig- 3-