Land drainage terms explained

Main rivers

Main rivers are usually larger streams and rivers, but also include smaller watercourses of strategic drainage importance. A main river is defined as a watercourse shown as such on a main river map, and can include any structure or appliance for controlling or regulating the flow of water in, into or out of the main river. The Environment Agency powers to carry out flood defence works apply to main rivers only. Main rivers are designated by the Department for Environment, Food & Rural Affairs in England and by the National Assembly for Wales .

Ordinary watercourse

An ordinary watercourse is every river, stream, ditch, drain, cut, dyke, sluice, sewer (other than public sewer) and passage through which water flows which does not form part of a main river. On ordinary watercourses, Walsall Metropolitan Borough have similar permissive powers as the Environment Agency has on main rivers.

Flood defence

For the purposes of the Environment Agency’s functions, flood defence encompasses not just construction and maintenance of defences against river and sea water flooding, but also flood warning, irrigation (other than spray irrigation) and water level management. The term also refers to a structure built for the purpose of flood defence.

Erosion

Erosion in the context of these pages is the wearing away of riverbanks by the action of moving water. Generally, responsibility for preventing or dealing with erosion lies with the riparian landowner, who will probably need Environment Agency consent before carrying out any protection work. Protection of land against erosion by natural water processes concerns the Environment Agency only where it threatens a flood defence, or where it may create a significant change to the nature of the river and the land alongside it.

Flood plain

A flood plain is an area of land over which river or sea water flows or is stored in times of flood. A flood plain can extend beyond the land immediately adjacent to a watercourse. Flood plains are often under pressure for development, but if buildings or other man-made objects obstruct flood plains, water cannot flow away efficiently and the effects of flooding are made worse.