Drainage

Drainage, removal of surface or subsurface water from a given area by natural or artificial means. The term is commonly applied to the removal of excess water by canals, drains, ditches, culverts, and other structures designed to collect and transport water either by gravity or by pumping. A drainage project may involve large-scale reclamation and protection of marshes, underwater lands, or lands subject to frequent flooding. Such a project usually involves a system of drainage ditches and dykes; often pumps are required to raise the water into the drainage network.

In cases of large-scale drainage where improvement of outlet facilities is essential to the protection of adjacent property, it is customary to improve natural stream channels to provide required discharge capacity and to excavate main and lateral drains as open ditches or canals to convey the effluent from farm drainage systems to these improved channels. Such connecting drains commonly follow the natural surface drainage pattern of the area, intercepting the normal surface run-off that takes place during periods of excessive rainfall.

Small-scale drainage is often practised by farmers and other landowners who wish to remove surface water from arable fields or to improve water-laden soil. Properly constructed drainage systems can also prevent erosion and gullying of land on slopes by catching the surface water before it reaches the slope. Another important purpose of drainage is to prevent an excessive accumulation in the soil of soluble salts that might be detrimental to plant growth.

The essential principle of any type of land drainage is to provide an open, adequate, and readily accessible channel through which the surface or subsoil water can flow. For this purpose open ditches are sometimes used, but these are not always satisfactory because they may become choked with sediment and vegetation. Underground drains are usually employed, particularly on land that is to be ploughed. Of the different types of closed drains, the most efficient is the so-called tile drain, which is composed of pipes made of sections of hollow earthenware or concrete tiles that are buried at a depth of about 1 to 2 m (about 3 to 6 ft). Excess water in the soil seeps into the pipes through apertures in the tiles.

In draining comparatively flat land, common practice is to lay along one side of the plot a main drain to which a number of transverse laterals are connected. The laterals are often set parallel to the main drain, coming together to join it at the lower end of the field. Local conditions of soil and terrain govern the spacing of laterals and the depth at which they are placed. Laterals may be from 5 to 91 m (15 to 300 ft) apart and from less than a metre to just a metre (2 to 4 ft) below the surface.

To prevent water from higher ground from reaching lower areas, catchment or interception drains are frequently built. They consist of ditches or underground drains, placed across the slope, that catch water and carry it away before it reaches the low ground.

The drains just discussed operate by gravity, but in the drainage of low-lying areas it is not always possible to set the outlet of the drain low enough to obtain a natural flow of water. This situation occurs in the fen country of England, in many areas of the Netherlands and Belgium, in large portions of the southern United States and at some airports in river-bottom areas near large US cities. Where gravity flow is impossible, the water from the drainage system is pumped away into streams or canals, the level of which is often higher than that of the drained land. Drained land frequently settles as its moisture content is lowered, and this increases the difficulty of drainage in low areas. In the English fens this sinking has amounted to an average of 46 cm (18 in). If the soil rests on a water-bearing foundation such as gravel, subsurface drainage may be provided by pumping water from wells, thus lowering the water level in the soil.

The Fens, a large area of eastern England between Cambridge and Lincoln, has been reclaimed from marshland. The reclamation, a continuous process for centuries, is a result of extensive drainage, the building of dykes, and the rechannelling of rivers of the area to prevent silting. In addition, between 24,000 and 28,000 hectares (60,000 and 70,000 acres) have been retrieved from the sea.

The Netherlands has one of the best and most extensive systems of drainage and land reclamation in the world. The greater part of the country is low and flat and must be protected by dykes against inundation by the sea. The soil was originally marshy, and there were many lakes, but drainage and pumping of the excess water by windmills have made the land arable. In 1923 the Netherlands began its largest project, the reclamation of 223,000 hectares (550,000 acres) of land from beneath the waters of the IJsselmeer, formerly known as the Zuider Zee. The reclaimed land has to be repeatedly drained. A similar project, known as the Delta Plan, was begun in the 1950s to protect the southern Netherlands from periodic invasions by the sea. It was completed in 1986 at a cost of $5 billion. Using a reclamation process called poldering, large lakes were created by building dams and dykes. The lakes were then pumped dry and the land reclaimed for agriculture.

In recent years, large drainage projects and reclamation programmes have also been carried out in the Pontine Marshes in Italy and in the Salonica Valley at the mouth of the Vardar River in Greece.

In the United States vast areas are drained, mostly for conversion to agricultural use in the lower Mississippi River Basin; drainage is also extensively practised in Florida, the Carolinas, Minnesota, Michigan, and Iowa.

See also Sewage Disposal.