If you own or manage a large, raised reservoir, you must register it with us. You can register your reservoir here. You don’t need to register if the reservoir is exempt.

Who needs to register a large, raised reservoir?

If you or your organisation is an undertaker for a large, raised reservoir you must register it. An undertaker is the legal term for a person, company or any other legal entity who carries out an activity at the reservoir and has control of it. For example, a company who operates the reservoir to produce hydro-electric power would normally be considered an undertaker.

If there is no person or company that manage or operate the reservoir, the owner or lessee is the undertaker.

Some activities are not sufficient for a person to be considered an undertaker, for example a person who exercises their legal right to fish but otherwise has no control of the reservoir would not normally be an undertaker.

There can be more than one undertaker. If you are one of several undertakers, you should agree the limits of your responsibilities to each other in writing.

You do not need to register your reservoir if it is exempt.

What is a large, raised reservoir?

A reservoir is large and raised if there is a dam, wall, earth embankment or similar structure which is designed or capable of storing 10,000 cubic metres of water, or more above the natural level of the surrounding land. A lake or other area which has been artificially raised may also be a large, raised reservoir if it meets the same criteria.

10,000 cubic metres is equal to 100 metres long x 100 metres wide x 1 metre depth, or the equivalent of four Olympic sized swimming pools.

Register your reservoir

Information you need to provide

When you register, you'll need to have the following information available.

The details of the legal entity to be recorded as an undertaker.
If the legal entity is a Limited Company, registered charity or other organisation you will need the principal or Head Office address and any company or charity registration number.

Authorised person
You’ll need to give us the contact details for an authorised person who can act on behalf of the undertaker and with whom we can discuss the reservoir.

Emergency contact
The 24 hour contact details for someone that we can call if there is an emergency at the reservoir

The name and location of the reservoir.

The grid reference for the centre of the reservoir.
Give us the coordinates for the centre of the reservoir. You can use gridreferencefinder.com or www.what3words.com to provide a British National Grid reference, what3words, eastings and northings etc.

The Local Authority area the reservoir is situated in.

The type of reservoir. You will need to tell us which of the following types your reservoir is.

  • Impounding reservoir - an impounding reservoir is created by blocking a natural flow of water with a dam or embankment. The flow may be from a stream or river. They may also be fed by groundwater springs.
  • Non-impounding reservoir- a non-impounding reservoir is filled by diverting or pumping water. The inflow can be controlled even if this is just by fixing different levels at the inflow weir.
  • Service reservoir - a service reservoir is covered and sealed to supply treated drinking water. 

The date or approximate year the reservoir was first constructed.

Top Water Level (TWL). The TWL must be given in metres above Ordnance Datum (mAOD). TWL is the level to which water can be stored before it overflows the lowest spillway or overflow pipe:

  • For reservoirs with a fixed overflow sill, TWL is the lowest crest level of that sill
  • For reservoirs where the overflow is controlled by moveable gates or siphons, TWL is the maximum level to which water may be stored behind the gate or siphon
  • For flood storage reservoirs, TWL is the maximum level to which floodwater may be stored during a flood event to the overflow level.

Reservoir surface area.
The surface area should be given in square metres. You can estimate surface area using www.gridreferencefinder.com or arrange a professional survey. If are not able to provide TWL, you must still register the reservoir. You must then provide the TWL within six months of registration.

Reservoir capacity.
The capacity must be measured from the hard bed of the reservoir to Top Water Level. It must include any silt which has accumulated.

Dam location.
Give us the coordinates for the centre of each dam or embankment. You can use gridreferencefinder.com or www.what3words.com to provide any of the following coordinates: British National Grid reference, what3words location, eastings and northings.

Dam construction type.
You’ll need to tell us what type of dam or dams retain the reservoir from the following list: earth-fill, rock-fill, gravity dam, buttress dam, arch dam, or another type.

Maximum dam height.
Dam height is measured in metres from the toe to the crest. The toe is the downstream edge of the dam where its base meets the lowest natural ground level of the surrounding land. The crest is the level of the top of the dam but does not include the height of any wave wall designed to protect the dam from wave overtopping.

Dam crest level.
You must give the level of the dam crest above Ordnance Datum. The crest is the level of the top of the dam but does not include the height of any wave wall designed to protect the dam from wave overtopping. If are not able to provide the dam crest level, you must still register the reservoir. You must then provide the information within six months of registration.

Wave fetch.
This is the longest distance over which waves may form upstream of the dam. Provide wave fetch in metres. You can use www.gridreferencefinder.com to measure the distance.

Spillway level.
You’ll need to give us the lowest level of the spillway or overflow on this dam in metres above Ordnance Datum. If you're not able to provide the spillway level, you must still register the reservoir and then provide the information within six months of registration.

Spillway construction.
You’ll need to tell us what the spillway or overflow is made from, for example: brick, masonry, concrete, plastic or metal pipe, reinforced grass, cut through rock, or something else.

Bottom outlet or scour pipe.
Tell us if there is a bottom outlet or scour pipe may be opened to release water from near the bottom of the reservoir.

Historic Documents.
If you have any certificates, reports or statements relating to the reservoir you must tell us about them. Examples include preliminary certificate, interim certificate, final certificate, certificate of efficient execution of works, inspection certificate, inspection report, supervising engineer statement.

Reservoir use.
Tell us what you use the reservoir for.

Empty, silted-up and disused reservoirs

If your reservoir was originally designed to hold more than 10,000 cubic metres but is now empty, silted up or no longer used it must still be registered with us. You may be able to decommission the reservoir using a qualified civil engineer for advice.

Exemptions from registration

You do not need to register the following structures as reservoirs:

  • Mine lagoons regulated under the Mines Regulations 2014
  • Quarry lagoons or tips regulated under Quarry Regulations 1999 or Mines and Quarries (Tips) act 1969
  • Canals
  • Sea defences
  • Road and rail embankments, unless they are designed to store water.

If you are uncertain about the definition of a large, raised reservoir, the exemptions or your responsibilities as a reservoir owner, operator or manager, please contact us.

Calculating the capacity of your reservoir

There are two common ways of knowing the capacity of your reservoir.

  • Documented evidence from when the reservoir was constructed, for example, design drawings, planning applications and water impoundment licenses.
  • Arrange for a bathymetric survey. This should be undertaken by a person who understands the definition and terminology of the Reservoirs Act 1975.

You may want to measure or estimate the capacity yourself first to see how likely your reservoir is to hold more than 10,000 cubic metres. Reservoirs with completely vertical sides, like concrete or metal tanks, can be calculated using the formula:

Surface Area x Dam Height

If you have a reservoir created by damming a watercourse, you can allow for the sloping basin to reduce the overall capacity. You can estimate this using the formula

(Surface Area x Dam Height) x 0.33

This calculation is only an approximation to help you decide if your reservoir should be registered. A professional survey will give you certainty and provide the information you will need to register. If you do not know the capacity, you will need to provide a reliable calculation within six months of registration.

Reservoir registration and notification form

If you are registering a reservoir for the first time, you need to pay a registration fee.

All our fees are set out on our website: Natural Resources Wales / Reservoir charges

To pay by BACS, you will need to give your bank the following information:

Our bank: RBS National Westminster Bank PLC, 2½ Devonshire Square, London, EC2M 4BA

Account name: Natural Resources Wales

Account number: 10014438

Sort code: 60-70-80

Reference: Please reference the payment “Reservoir—" followed by the name of the first reservoir you have given above, e.g., “Reservoir Big Lake”. This reference will appear on the transaction so we can verify your payment.

To pay by credit or debit card please phone us on 03000 65 3000 or we will telephone you to take payment. 

What happens after I register?

We will record your reservoir on the register of reservoirs.

If you have not been able to provide all the information required, you will need to provide this within six months.

We will review the information and may contact you if any information is missing or unclear.

We have a duty to consider whether your reservoir should be designated as a High-Risk Reservoir. This process will inform us, and you, whether formal supervision and inspection of the reservoir by a qualified civil engineer is required. Understand-your-reservoir-risk-designation.

You should read the rest of our guidance on the responsibilities you may have.

What happens if I don’t register my reservoir?

It is a criminal offence to fail to register a large raised reservoir or provide the information required.

Read a list of reservoir offences and our response options

Last updated