DRAGUIGNAN, the chief town of the department of the Var in S.E. France; 51 m. N.E. of Toulon, and 28½ m. N.W. of Fréjus by rail; situated at a height of 679 ft. above the level of the sea, at the southern foot of the wooded heights of Malmont, and on the left bank of the Nartuby river; pop. (1906) 7766. It possesses no notable buildings, save a modern parish church, a prefecture, also modern, and a building wherein are housed the town library and a picture gallery, with some fair works of art. In modern times the ramparts have been demolished, and new wide streets pierced through the town.
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 For several reasons this method is ineffective, and, where possible, is now superseded by underground drainage by means of pipe-tiles.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.