Low risk water impoundments

With the exception of some exempt activities, by law you must have and comply with an impoundment licence before you start work to:

construct, change or remove any impounding works on any inland water
cause the flow of any inland water, including some lakes, reservoirs and ponds, to be obstructed by impounding works.

An impoundment is any structure within inland waters that can permanently or temporarily change the upstream or downstream level or flow of water.

Inland waters include any lake or pond, river, stream or other watercourse, whether natural or artificial, and whether tidal or not, any reservoir or dock, or any channel, creek, bay, estuary or arm of the sea.

Impoundments may include, but are not limited to, dams, weirs, fish passes, sluices, penstocks, stop logs, lock gates, low level infrastructure / access crossings, retaining walls, reservoir embankments and temporary diversions during construction work. Impoundments may be constructed using man-made or natural materials.

Find out if you need an impoundment licence.

There may be low risk impoundment activities for which NRW will not normally take enforcement action against you.

You can still apply for an impoundment licence if you prefer to have one. You may need to do this for some accreditation schemes.

This guidance and checklist will help you decide if your impoundment activity is low risk.

If you conclude that you do not need to apply to us for an impoundment licence, you should keep a copy of your checklist and supporting evidence (for example, before and after photos and measurements) for your own records. It is evidence that you have carried out an assessment and documents your decision that you do not need to apply for an impoundment licence. We may request that you provide a copy to us.

You may still need to apply for other consents or permits.

Find out about other permits and permissions.

Before you start work on an existing structure you will need to find out:

if there is a licence already issued or going to be issued - we can help you do this
who owns the structure and seek their permission to do the work or take over an existing licence

Will your activity result in a change in the flow or level of the water?

You need to consider how your proposal will change water flow, quantity or level.

If your impoundment, or amendments to or removal of an existing impoundment, could cause a change in the water level, quantity or continuity of flow, you will need to apply for an impoundment licence.

Impoundments, or alterations to impoundments, may:

alter the flow going downstream, so there might be less water available for others or the environment;
change the pattern of how the water flows over a structure;
change the water level and physical features of a river;
change the physical form and processes of erosion and deposition of a river and its associated habitats;
make fish passage, either upstream or downstream, more difficult and so reduce habitat connectivity and ecological resilience.

If your impoundment, or amendments to, or removal of an existing impoundment, could cause a change in the water level, quantity or continuity of flow, you will need to apply for an impoundment licence.

For example, building a new weir, operating old structures which require any alteration (including rebuilding or removal of part or all of the works), or adding structures, such as hydropower turbines, sluices, penstocks and lock gates may cause a change in water level, quantity or flow.

Natural Flood Management (NFM) measures which are not truly “leaky”, consist of placing solid logs, planks or bunds in the watercourse, will change the flow within a watercourse (which includes ditches), particularly at lower flows, and are likely to require an impoundment licence.

Be aware that while your impoundment may be considered low risk and not need an impoundment licence, if it is intended to act as a facility to abstract over 20 cubic meters of water each day, alone or in combination with an existing abstraction, you will need to apply for a new, or vary an existing, abstraction licence.

Find out if you need an abstraction licence.

Migrating or spawning fish

In-river works have the potential to disturb or destroy migrating or spawning fish and their eggs and/or juveniles. To avoid the risk of contravening the Salmon and Freshwater Fisheries Act, in-river works should not occur during the fish spawning and migration season.

Contact the Senior Officers (Fisheries) in NRW’s People and Places Teams for detail on the timings for in river works.

Impoundments we will not pursue a licence for

New activities

fish passes that have formal written approval by Natural Resources Wales and do not significantly affect the distribution of flow over the weir
placing a notch in a weir to encourage fish migration
placing (and where necessary securing) natural material, similar to that which would be found in the catchment (e.g. branches), across part of the channel or part of the cross-sectional area of the channel to diversify flows, for example in river restoration projects.
fixed control or passive flood prevention structures within the channel that only hold back, slow the flow of, or divert, flood water. Structures with movable gates or sluices will require an impoundment licence.
flap valves that only operate during flooding.
small, temporary (i.e no more than 6 months) measurement structures, such as v-notch or small rectangular weirs that will not accumulate a significant quantity of sediment.

Maintaining an existing structure

Maintaining a structure relating to it's upkeep, care or preservation but not increasing the structures dimensions, changing the design or function of the structure, or altering the upstream or downstream water level or flow.

For example:

routine sluice maintenance
re-pointing (block work structures)
repairs, for example to expansion joint, stone pitching or bank/wall
void filling (in 'solid' structures)
managing vegetation
routine de-silting of part of the structure
re-positioning (rock, rubble or block work bank protection structures)
replacing elements that do not contribute to phased replacement of the whole structure
re-facing
skimming/covering
blockage removal, including screen clearing
embankment grouting.

Some of the above activities or maintenance works that are not regularly carried out may require other permits and consents.

The removal or disturbance of sediment and silt can cause pollution and affect downstream transport of sediments. To cause or knowingly permit any polluting matter to enter a watercourse, including silt and sediments, is an offence and may also be in breach of the conditions of any permits you hold.

Diversion works

Works that temporarily divert water in the immediate vicinity during construction or maintenance of a structure or other authorised in-channel activities and do not affect any other water users, rights or the environment.

Other activities

Other activities which are considered low risk can include:

any activity outside the channel, for example on top of the river banks, such as installing or maintaining bridge supports or flood defences
work in the river, parallel and adjacent to the bank that doesn’t significantly narrow the channel, for example installing sheet piling, rip rap, block work and revetments for erosion protection
installing a wave wall on an existing reservoir dam that is not intended to impound any extra water
increasing the height of spillway sidewalls (that does not alter the impounding level)
altering, moving or constructing a spillway without increasing the impounded level or changing the downstream flow regime to the detriment of the environment.

Please note that all of the above activities are likely to require other permits or licences, including:

an Ordinary Watercourse Consent from your Local Authority

a Flood Risk Activity Permit from Natural Resources Wales

Land Drainage Consent from Natural Resources Wales.

Hydropower

You must have an impoundment licence for turbines installed on or within a weir because they significantly modify the flow over the weir, can cause significant risks to the environment and may cause potential impacts for other water users.

If you are maintaining, altering or installing a weir to provide flow to your hydropower turbine, you are likely to need to apply for an abstraction and/or impoundment licence depending on the site layout.

Find information on the type of licence you may need on our hydropower web pages

Checklist to help you decide if your impoundment is low risk

Use this checklist to help you decide whether you need to apply for an impoundment licence.

If you answer yes to any of the questions, you may have to apply for an impoundment licence. If you answer no to all the questions, or answer yes but can provide supporting evidence or justification why your proposal will not have a negative impact, you will not normally need to apply for an impoundment licence.

You should keep a copy of your checklist and supporting evidence (for example, before and after photos, measurements and any specialist advice received) for your own records.

It is evidence that you have carried out an assessment and documents your decision that you do not need to apply for an impoundment licence. We may request that you provide a copy to us.