Rural Drainage

The term is commonly applied to the removal of excess water by canals, drains, ditches, culverts, land drains 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 dikes; often pumps are required to raise the water into the drainage network. In cases of large-scale drainage it is essential to improve the discharge capacity of natural channels to protect adjacent properties, and to upgrade the ditches and channels that convey the runoff from farm drainage systems to the improved channels.

Such connecting drains commonly follow the natural surface drainage pattern of the area, intercepting the normal surface runoff that takes place during periods of excessive rainfall. Small-scale drainage is often practiced by farmers and other landowners who wish to remove surface water from arable fields or to improve water-laden soil.

Properly constructed drainage systems 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 plowed. 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 clay, plastic or concrete 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 pipes.

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. 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 100 m (15 to 330 ft) apart and from 0.6 to 1.2 metres (2 to 4 ft) below the surface.

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

A large area of eastern England between the cities of Cambridge and Lincoln, known as the Fens, has been reclaimed from marshland. The reclamation, a continuous process for centuries, is a result of extensive drainage, the building of dikes, and the rechanneling of rivers to prevent silting. In addition many thousands of hectares have been retrieved from the sea.

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 frequently occurs in the Fens where gravity flow is impossible and 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 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.