02 September 2009

Asiantaeth yr Amgylchedd - cartref

Explanation of terms

The meaning of the terms used by the Environment Agency.

Main rivers

Main rivers are usually larger streams and rivers. However, they do include smaller watercourses of local significance. A main river is a watercourse marked as such on a main river map. This is an official document. A main river can include any structure or appliance that controls or regulates the flow of water in, into, or out of, the main river.  Our powers to carry out flood defence works apply to main rivers only, but our other duties and functions extend to all watercourses. In England, Defra decides which are the main rivers. The Welsh Assembly Government does this in Wales. Our local offices have copies of main river maps.  To find out who to contact at your local office call 08708 506 506.

Ordinary watercourse

An ordinary watercourse is every river, stream, ditch, drain, cut, dyke, sluice, sewer
(other than a public sewer) and passage through which water flows and which does not form part of a main river. The local authority, or Internal Drainage Board where relevant, has powers for ordinary watercourses that are similar to those we can use on main rivers. Our powers to carry out flood defence works apply to main rivers only, but our other duties and functions extend to all watercourses.


Moving water wears away riverbanks - causing erosion. It is a natural process that can be made worse by channel narrowing, by inappropriate reinforcement, and by the overgrazing of sheep and cattle. Generally, you the landowner are responsible for any work to reduce bank erosion. You will probably need our consent before carrying out any protection work. We do not usually get more involved unless natural erosion either threatens a flood defence, or may create a significant change to the nature of the river and the land alongside it.

Farmland should be managed in ways that prevent rainwater from carrying off topsoil into the watercourse. This run-off damages the land and the ecology, quality and carrying capacity of the receiving water. For guidance, please see our booklet, Best farming practice: profiting from a good environment (2003).

In certain circumstances, allowing such runoff may constitute a criminal offence. If you are a farmer, this could threaten your single farm payment. For guidance on preventing soil erosion, please see the Defra booklet Single Payment Scheme. Cross Compliance. Guidance for Soil Management (2006), or Welsh Assembly Government Farmer's Guide to Cross Compliance. Under the Environmental Stewardship scheme, (see link below) you could even be paid for improving your management practices.  For construction sites, please see our visit 'Pollution prevention guidance' - see link below.

Flood defence

Some legislation, such as the Acts mentioned earlier, still use the terms 'flood defence' or 'land drainage'. We now refer to the activities that these cover as 'flood risk management' overall, but may use the other terms when referring to legal matters. In law the term flood defence also refers to irrigation (other than spray irrigation) and the management of water levels. The term flood defence can also refer to a structure built for the purpose of managing the flow and storage of floodwater, such as an embankment for example.

Flood plain

A flood plain is an area of land over which river or seawater flows, or is stored in times of flood. Flood plains usually extend beyond the land immediately next to a watercourse. There is often pressure to build on them. However, if buildings or other man-made objects obstruct flood plains, water cannot flow away efficiently and the effects of flooding are made worse.

Flooding can also occur from other sources such as water mains or sewers. These are the responsibilities of other organisations.