These consents are known as 'flood defence consents'. Previously they were sometimes called 'land drainage consents', after the old legislation that applied.
Consent is required to ensure works do not increase flood risk, damage flood defences, or harm the environment, fisheries, or wildlife. Consent does not confirm that a proposed structure is of sound design, nor that it complies with other legislation, such as planning or health and safety.
The Water Resources Act 1991 and associated byelaws require you to apply for consent from the Environment Agency for works in, over, under or adjacent to main rivers.
Under the Land Drainage Act 1991, you will need the consent of the internal drainage board, or unitary or county council to construct or alter a culvert or flow control structure (such as a weir) on any ordinary watercourse. Some authorities also have their own byelaws.
Contact the authorities as soon as possible to discuss your plans and for application forms. They will want to see full details of the work you propose at least two months before you intend to start. Talking to them early on helps everyone, and avoids delays and wasted effort.
Fees are currently £50 per application, except where they are solely under byelaws where there is no charge. Please note that multiple structures may require multiple fees.
If consent is refused, and you think it has been withheld unreasonably, you may appeal. There is a procedure for such appeals, which are heard by agreed independent arbitrators.
You must not carry out work without consent. If you do, the consequences can be expensive, with the cost of rectifying any damage and possible prosecution.
The Environment Agency’s Living On The Edge booklet explains generally about owning land next to a river, and contains more advice about consents.
The Environment Agency flood map also contains maps showing main rivers.