River (fluvial) flooding

A watercourse is a flowing body of water including rivers, streams and brooks. During times of heavy rainfall watercourses' capacity can be exceeded resulting in flooding to land, infrastructure and homes.

In the natural environment rainwater is transported through a number of processes before it reaches the watercourse, including percolation through the soil or interception by vegetation. These processes slow the flows of water reaching the watercourses and reduce the volume and speed of run-off. The flow rates in natural watercourses are typically slower than those in engineered systems. This again results in further attenuation of water.

In the urban environment rainwater falls on hard surfaces causing increased volumes of rapid run-off. The water enters the watercourse soon after a rainfall event. This causes increased peak flows which may exceed the capacity of the channel and lead to flooding.

The responsibility for a watercourse normally rests with the 'riparian owners', that is the owners of land through which the watercourse flows.

This principle applies to both statutory main rivers and to smaller rivers and streams – often referred to as ordinary watercourses. The Environment Agency has permissive powers to carry out works on main (larger) rivers.

On ordinary watercourses the lead local flood authorities (LLFAs) responsibilities have been reinforced. The main legislation relating to ordinary watercourses is the Land Drainage Act 1991 (LDA) which has been amended by the FWMA.

Ordinary watercourse regulation ensures that activities that might affect ordinary watercourses do not increase the risk of flooding on a particular site or further upstream or downstream and do not adversely affect the environment. Regulation consists of issuing consents for acceptable work and undertaking enforcement action to deal with unacceptable activities.

The amendments mean that almost all of the ordinary watercourse regulation relating to consenting is transferring to the LLFAs. However, in areas where there is an Internal Drainage Board (IDB), they will continue to be responsible for ordinary watercourses.

Certain works may be eligible for grant funding from central government.

Consent of the Environment Agency may be needed for certain activities affecting ordinary watercourses and flood plains. More details about this and riparian responsibilities are outlined in the Environment Agency's 'Living on the Edge' booklet.

Living on the Edge – on the Environment Agency website


A culvert is essentially a covered channel or pipe that is used to continue a watercourse or drainage path under an artificial obstruction.

Culverts are one of the most common forms of structure on a watercourse, particularly in urban areas and may have been built for a variety of reasons such as:

  • to hide a polluted watercourse
  • to attempt to make an open watercourse safer
  • to allow for landfilling in a low-lying valley area
  • to create more space for other purposes around a development
  • to allow roads, railways, canals and other infrastructure to cross streams.

Culverts can range in size and length and are capable of carrying large streams under industrial developments, residential areas and motorways.

Good design, construction and maintenance of culverts can ensure that they function appropriately. However, neglect of these issues can result in them becoming ‘pinch points' on a watercourse. It is often necessary to install grills or screens at the entrances and exits of culverted sections of watercourses. This is done to try and reduce the risk of blockage in the culvert by debris and because of the danger culverts present to children. These grills and screens are a major cause of flooding when they are poorly maintained or trap large amounts of debris in times of high flow.

The fact that a watercourse has been culverted does not alter its legal status or the responsibilities of riparian owners. It is not uncommon to find the situation where a watercourse – which formerly flowed through open countryside with perhaps a single riparian owner – becomes culverted as the result of property development and may then have numerous riparian owners, many of whom will not be aware of the culverted watercourse or of their riparian responsibilities.

The Environment Agency is in general opposed to the culverting of watercourses because of the adverse ecological, flood defence and other effects that are likely to arise. It will therefore only approve an application to culvert a watercourse if:

  • there is no reasonably practicable alternative
  • or the detrimental effects of culverting would be so minor that they would not justify a more costly alternative.

In all cases where it is appropriate to do so, adequate mitigation must be provided for damage caused. Wherever practical the Environment Agency will seek to have culverted watercourses restored to open channels.