Drainage Of Land

Drainage Of Land. The verb “to drain,” with its substantives “drain” and “drainage,” represents the O. Eng. dreahnian, from the same root found in “dry,” and signifies generally the act of drawing off moisture or liquid from somewhere, and so drinking dry, and (figuratively) exhausting; the substantive “drain” being thus used not only in the direct sense of a channel for carrying off liquid, but also figuratively for a very small amount such as would be left as dregs. The term “drainage” is applied generally to all operations involving the drawing off of water or other liquid, but more particularly to those connected with the treatment of the soil in agriculture, or with the removal of water and refuse from streets and houses. For the last, see Sewerage; the following article being devoted to the agricultural aspects of this subject. See also the articles Reclamation of Land, Canal, Irrigation, River Engineering, Water Supply and (law) Water Rights.

Agricultural or field drainage consists in the freeing of the soil from stagnant and superfluous water by means of surface or underground channels. It may be distinguished from the draining of land on a large scale which is exemplified in the reclamation of the English Fens (see Fens). Surface drainage is usually effected by ploughing the land into convex ridges off which the water runs into intervening furrows and is conveyed into ditches. For several reasons this method is ineffective, and, where possible, is now superseded by underground drainage by means of pipe-tiles. Land is not in a satisfactory condition with respect to drainage unless the rain that falls upon it can sink down to the minimum depth required for the healthy development of the roots of crops and thence find vent either through a naturally porous subsoil or by artificial channels.

A few of the evils inseparable from the presence of overmuch water in the soil may be enumerated. Wet land, if in grass, produces only the coarser grasses, and many subaquatic plants and mosses, which are of little or no value for pasturage; its herbage is late in spring, and fails early in autumn; the animals grazed upon it are unduly liable to disease, and sheep, especially, to foot-rot and liver-rot. In the case of arable land the crops are poor and moisture-loving weeds flourish. Tillage operations on such land are easily interrupted by rain, and the period always much limited in which they can be prosecuted at all; the compactness and toughness of the soil renders each operation more arduous, and its repetition more necessary than in the case of dry land. The surface must necessarily be thrown into ridges, and the furrows and cross-cuts cleared out after each process of tillage, and upon this surface-drainage as much labour is expended in twenty years as would suffice to make under-drains enough to lay it permanently dry. With all these precautions the best seed time is often missed, and this usually proves the prelude to a scanty crop, or to a late and disastrous harvest. The cultivation of the turnip and other root crops, which require the soil to be wrought to a deep and free tilth, either becomes altogether impracticable and must be abandoned for the safe but costly bare fallow, or is carried out with great labour and hazard; and the crop, when grown, can neither be removed from the ground, nor consumed upon it by sheep without damage by “poaching.”

The roots of plants require both air and warmth. A deep stratum through which water can percolate, but in which it can never stagnate, is therefore necessary. A waterlogged soil is impenetrable by air, and owing to the continuous process of evaporation and radiation, its temperature is much below that of drained soil. The surface of the water in the supersaturated soil is known as the “water-table” and is exemplified in water standing in a well. Water will rise in clay by capillarity to a height of 50 in., in sand to 22 in. Above the “water-table” the water is held by capillarity, and the percentage of water held decreases as we approach the surface where there may be perfect dryness. Draining reduces the “surface tension” of the capillary water by removal of the excess, but the “water-table” may be many feet below. Drains ordinarily remove only excess of capillary water, an excess of percolating water in wet weather.

In setting about the draining of a field, or farm, or estate, the first point is to secure a proper outfall. The lines of the receiving drains must next be determined, and then the direction of the parallel drains. The former must occupy the lowest part of the natural hollows, and the latter must run in the line of the greatest slope of the ground. In the case of flat land, where a fall is obtained chiefly by increasing the depth of the drains at their lower ends, these lines may be disposed in any direction that is found convenient; but in undulating ground a single field may require several distinct sets of drains lying at different angles, so as to suit its several slopes. When a field is ridged in the line of the greatest ascent of the ground, there is an obvious convenience in adopting the furrows as the site of the drains; but wherever this is not the case the drains must be laid off to suit the contour of the ground, irrespective of the furrows altogether. When parts of a field are flat, and other parts have a considerable acclivity, it is expedient to cut a receiving drain near to the bottom of the slopes, and to give the flat ground an independent set of drains. In laying off receiving drains it is essential to give hedgerows and trees a good offing, lest the conduit be obstructed by the roots.

When a main drain is so placed that parallel ones empty into it from both sides, care should be taken that the inlets of the latter are not made exactly opposite to each other. Much of the success of draining depends on the skilful planning of these main drains, and in making them large enough to discharge the greatest flow of water to which they may be exposed. Very long main drains are to be avoided. Numerous outlets are also objectionable, from their liability to obstruction. An outlet to an area of from 10 to 15 acres is a good arrangement. These outlets should be faced with mason work, and guarded with iron gratings.

The distance and depth apart of the parallel drains is determined chiefly by reference to the texture of the soil. In an impervious clay the flow of the water is much impeded and the water-table can be controlled only by frequent lines of pipes. On such land it is customary to lay them about 3 ft. from the surface and from 15 to 21 ft. apart. In lighter soils the depth, and proportionately the distance apart, is increased, but the drains are rarely more than 4 ft. 6 in. below the surface, though they may be 75 or 100 apart. A fall of at least 1 in 200 is desirable.

There are various forms of under-drainage, some of them alluded to in the historical section below, but by far the commonest is by means of cylindrical or oval pipes of burnt clay about 1 ft. in length, sometimes supplemented by collars, though nowadays the use of these is being abandoned. Pipes vary in bore from 2 in. for the parallel to 6 in. for the main drains.

In constructing a drain, it is of importance that the bottom be cut out just wide enough to admit the pipes and no more. Pipes, when accurately fitted in, are much less liable to derangement than when laid in the bottom of a trench several times their width, into which a mass of loose earth must necessarily be returned. This is easily effected in the case of soils tolerably free from stones by the use of draining spades and the tile-hook which are represented in the accompanying cut. The tile-hook is an implement by means of which the pipes may be lowered from the edge of the trench and laid at the bottom. An implement, sometimes propelled by steam, known as the draining plough, can be used for opening the trenches. Draining can be carried on at all seasons, but is usually best done in autumn or summer. A thoroughly trustworthy and experienced workman should be selected to lay the pipes, with instructions to set no pipes until he is satisfied that the depth of the drains and level of the bottoms are correct. The expense of tile-drainage may vary from about £2:10s. per acre on loose soils to £10 an acre on the most tenacious soils, the rate of wages and the cost of the pipes, the depth of the trenches and the ease with which they can be dug, all influencing the cost of the process.

Drainage is not a modern discovery. The Romans were careful to keep their arable lands dry by means of open trenches or covered drains filled with stones or twigs. It is at least several centuries since covered channels of various kinds were used by British husbandmen for drying their land. Walter Blith (see Agriculture) about the middle of the 17th century wrote of the improvement which might be effected in barren land by freeing it from the excess of stagnant water on or near the surface by means of channels filled with faggots or stones, but his principles, never generally adopted, were ultimately forgotten. In the latter half of the 18th century, Joseph Elkington, a Warwickshire farmer, discovered a plan of laying dry sloping ground that is drowned by the outbursting of springs. When the higher-lying portion of such land is porous, rain falling upon it sinks down until it is arrested by clay or other impervious matter, which causes it again to issue at the surface and wet the lower-lying ground. Elkington showed that by cutting a deep drain through the clay, aided when necessary by wells or auger holes, the subjacent bed of sand or gravel in which a body of water is pent up by the clay, as in a vessel, might be tapped and the water conveyed harmlessly in the covered drain to the nearest ditch or stream. In the circumstances to which it is applicable, and in the hands of skilful drainers, Elkington’s plan, known as “sink-hole drainage,” by bringing into play the natural drainage furnished by porous strata, is often eminently successful.

Draining Implements.

During the subsequent thirty or forty years most of the draining that took place was on this system, and an immense capital was expended in such works with varying results. Things continued in this position until about 1823, when James Smith of Deanston, having discovered anew those principles of draining so long before indicated by Blith, proceeded to exemplify them in his own practice, and to expound them to the public in a way that speedily effected a complete revolution in the art of draining, and marked an era in agricultural progress. Instead of persisting in fruitless attempts to dry extensive areas by a few dexterous cuts, he insisted on the necessity of providing every field that needed draining at all with a complete system of parallel underground channels, running in the line of the greatest slope of the ground, and so near to each other that the whole rain falling at any time upon the surface should sink down and be carried off by the drains. A main receiving drain was to be carried along the lowest part of the ground, with sub-drains in every subordinate hollow that the ground presented. The distances between drains he showed must be regulated by the greater or less retentiveness of the ground operated upon, and gave 10 to 40 ft. as the limits of their distance apart. The depth which he prescribed for his parallel drains was 30 in., and these were to be filled with 12 in. of stones small enough to pass through a 3-in. ring—in short a new edition of Blith’s drain. Josiah Parkes, engineer to the Royal Agricultural Society, advocated a greater distance apart for the drains, and, in order that the subterranean water might be reached, a depth of at least 4 ft.

The cultivated lands of Britain being disposed in ridges which usually lie in the line of greatest ascent, it became customary to form the drains in each furrow, or in each alternate, or third or fourth one, as the case might require, or views of economy dictate and hence the system soon came to be popularly called “furrow draining.” From the number and arrangement of the drains, the terms “frequent” and “parallel” were also applied to it. Smith himself more appropriately named it, from its effects, “thorough draining.” The sound principles thus promulgated by him were speedily adopted and extensively carried into practice. The great labour and cost incurred in procuring stones in adequate quantities, and the difficulty of carting them in wet seasons, soon led to the substitution of “tiles,” and soles of burnt earthenware. The limited supply and high price of these tiles for a time impeded the progress of the new system of draining; but the invention of tile-making machines removed this impediment, and gave a stimulus to this fundamental agricultural improvement. The substitution of cylindrical pipes for the original horse-shoe tiles has still further lowered the cost and increased the efficiency and permanency of drainage works.

The system introduced by Smith of Deanston has now been virtually adopted by all drainers. Variations in matters of detail (having respect chiefly to the depth and distance apart of the parallel drains) have indeed been introduced; but the distinctive features of his system are recognized and acted upon.

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