Art. L. —On Thorough Drainage.
[Read before the Wellington Philosophical Society, September 18, 1869.]
AS the subject of thorough drainage is evidently but little understood in this part of the world, and as I have had some experience in the matter in Scotland, I propose to make a few remarks, and to lay down a few elementary rules on the subject, which I hope may prove of use.
It is often supposed that in drainage it is sufficient to remove water from the actual surface, whereas the beneficial results to be obtained are gained by lowering the water table, or that level at which the underground water rests, to a sufficient depth to allow the roots of plants to get well down, and also to allow rain water to percolate freely through the soil, instead of lying stagnant on it, carrying with it ammonia and portions of atmospheric air, which assist in the decomposition of matter previously inert.
It has been found practically, that a depth for drains of about four feet is that which is economically the best. It becomes very expensive to sink below this depth. If the subsoil is rocky, and presents great obstacles to sinking, a depth of three feet six inches may be considered sufficient.
The main drains ought always to have an additional six inches in depth below that of the small drains.
If a field has an irregular surface half mains are frequently used, so as with more convenience to run the water into the main drain, than if all the small drains were led into it direct.
The distance between each drain generally varies from twelve to thirty-six feet, according to the stiffness of the soil. In heavy clay the short interval of twelve feet is required, in gravelly soil thirty-six feet would be sufficient. The drains should follow the steepest slope.
The main drain is generally taken parallel to the fence along the lowest
side of the field, at a distance of the breadth of a furrow from the fence, and empties by one outlet at the lowest part.
It is of great importance to have as few outlets as possible.
If stones are actually on the ground, it may be found cheaper to use then for filling the drains, but upon the whole, tiles are found the most economical. They are lighter than stones, and therefore require less carriage. The water also runs more freely in them. One to two-inch pipes are generally used, and it is always best to lay them with collars. It is unnecessary and wasteful to have the same sized tile at the upper part as at the lower part, of a drain. Mains vary in diameter, according to the amount of water. From four to six-inch pipes are generally sufficient.
The average expense, in Great Britain, of draining an acre, may be stated at about £5 10s. In this country it would cost considerably more. Until the expense is reduced, the system is therefore not likely to come into very general use, but for small pieces of ground, and particularly gardens, it ought even now to be applied. In garden ground, no doubt, the pipes are apt to get choked with roots, but the damage soon shows itself, and the pipe must simply be lifted and cleaned, and then relaid.
Although there is much excellent natural drainage in this country, yet many districts would be immediately improved by thorough drainage.
Without going far, I might mention Karori, Porirus, and parts of the Hutt.
Drains should follow the steepest slope.
They should, if possible, have a minimum depth of four feet perpendicular.
The main should have a depth of six inches more than the small drains.
The chief main should be cut along the lowest side of the field, parallel to the fence, and should empty by one outlet only.
The interval between the drains should vary from twelve to thirty-six feet, according to the stiffness, or openness of the soil and subsoil.
It is a waste of material to put the same size of pipe at the upper as at the lower part of a drain.
Pipe tiles are far more permanent, and in the long run cheaper, for drainage, than any other material. They ought to have collars. The run of water is more free in them than in any other kind of drain.
A little consideration will show what a difference it will make in the fertility of land, if the water, which now lies, during the winter months, either on the surface, or close below it, and in a stagnant state, is kept flowing at a depth of four feet below. In the one case the roots of plants are perished by the water, in the other they are nourished, and the rain water also, percolating freely, assists decomposition, and removes noxious matters.
Wet land in its natural state is unfit to receive manure, which is wasted if put upon it. When drained it is ready to take advantage of any application of fertilizing material.
In its natural state the ground is hard to work. It is sodden with water in winter, and forms hard clods in dry weather. When drained, it is easily worked at all seasons, and breaks up into fine mould.
The returns from drained land are proportionately great. I can speak from experience when I say that nothing pays better in Great Britain than judicious drainage of land. On the other hand, a large landed proprietor in the Midland Counties informed me that he had thrown away £24,000 upon drainage which would have to be entirely done over again. He had been persuaded that a two-foot drain would be ample, and found that the depth
was quite insufficient. It was at first supposed that the water ought to find its way directly from the surface to the drain, hence the idea of a two-foot drain, but this view was found to be erroneous, and the true principle decided to be as follows:—land is saturated with water rising to a certain height; when the water reaches that height, it will run off if opportunity offers. If no outlet appears, it will rise above the surface, and form a lake or a swamp. If the water table be lowered by the construction of drains, an outlet is offered at a lower level for the subsoil water, or water of capillary attraction, which forms the chief supply of water in the drains. This water being kept flowing, makes room for rain water to get down to the subsoil, instead of lying stagnant on the surface or on the upper soil. No doubt, at times, rain water may pass direct into a drain, but it is not in the usual course. It is found, therefore, that by keeping the subsoil water running at a depth of, say, four feet, that the heaviest rain cannot leave water resting for any time on the surface, but that it must find its way down to the subsoil, thus percolating through, and improving the soil, instead of running violently over the surface, and washing away the finer parts of the ground.