Guidance

Abstract or impound water: hydraulic information needed for your licence application

Updated 29 August 2025

Applies to England

Structures are sometimes used in rivers to control and measure river flows and abstractions. If your plan to abstract or impound water involves a structure, the Environment Agency may need you to provide hydraulic information about it. This will help them decide if the structure is appropriate for the purpose it is designed for before granting your licence. 

Your proposal may involve a structure to:

• measure flows
• ensure the hands-off flow or level is maintained
• ensure the appropriate abstraction is maintained

When the Environment Agency assess your application they may need:

• drawings of your structure including details of the dimensions – this may include a detailed survey if the structure already exists, especially if it is not built to British or other recognised equivalent standards (known as non-standard)
• calculations using the appropriate equation to work out the river flow or abstraction rates
• a list of assumptions you have made during these calculations
• flow measurements to understand flow around and over structures at the site

After your abstraction or impoundment licence has been issued, the Environment Agency may need you to:

• confirm that the structures were built in line with the drawings we agreed during licence determination
• take flow measurement to check (validate) the calculations provided during determination
• ensure ongoing maintenance and calibration of structures to ensure you continue to comply with your licence conditions

These requirements will be detailed in your licence. The Environment Agency will visit you to check that you understand the conditions of the licence and the structure is operating correctly.

This guidance explains what these terms mean.

The Environment Agency recommends that you request enhanced pre-application advice so they can tell you what information you will need to gather. If you submit a formal application without requesting enhanced pre-application advice first:

• the data you collect may not meet the Environment Agency’s needs
• the Environment Agency may ask you to send more hydraulic information

They may need to return your application until you can provide this. 

Hydraulic drawings and structure equations are a specialised discipline. If in doubt, you can employ a specialist to help advise whether your chosen structure is appropriate for your intended purpose and location.

1. Structure data

1.1 Drawings and structure dimensions

Plan and cross section view drawings of your structure to show how the structure will alter the river flow. The drawings must clearly show the height, length or diameter of any:

• weirs and associated notches
• sluices
• orifice plates
• pipes

Information about all structures at the site is required, for example fish pass channels.

For existing structures, you must provide a detailed survey upstream and downstream of the structure including the weir crest. You must take photographs of the structure and any useful features to help the Environment Agency determine the structure’s suitability.

A consultant can help advise whether your chosen structure is appropriate for your intended purpose and location.

1.2 Structure calculations

The Environment Agency may need your structure calculations to ensure that the structure will comply with the licence you are applying for.

There are equations to calculate river flow or abstraction rate for standard structures depending on the water depth.  

You must explain where the structure deviates from British Standards or other recognised equivalent standards.

1.3 Assumptions made

You will need to decide on various variables (such as channel roughness or smoothness of pipes) for your structure calculations. If the structure is non-standard the assumptions made on these variables may be greater, leading to increased uncertainty. The assumption is why you chose the variable you did.

The Environment Agency may need to apply stricter conditions to your abstraction or impoundment licence if there is environmental risk.

If the Environment Agency asks you for structure calculations, you need to identify all assumptions you have made and the impact this may have on the calculation, for example the discharge co-efficient.

1.4 Validation of structures  

The accuracy of your structure will vary depending on the assumptions you have made during the design stage.

If your structure is built to British or other recognised equivalent standards, the Environment Agency is unlikely to require validation unless it is in an environmentally sensitive area. However, they may need you to resurvey the structure occasionally to ensure it continues to meet appropriate standards. This survey must be done by an independent, suitably qualified professional.

If your structure is non-standard or does not meet all the criteria set out in the British Standard, you must provide validation to the Environment Agency that it will meet the conditions in your licence and continue to do so throughout the duration that your licence is valid.

For advice on validation flow measurements, check the guidance abstract or impound water: hydrological information needed for your application.

2. Construction and ongoing maintenance and calibration

The Environment Agency will put a condition in your licence to ensure the structure is built and maintained for the purpose it is designed for. You may need to produce a maintenance plan to show how you will ensure that structures will remain fit for purpose.

During determination, the Environment Agency will only advise on elements of your design that look wrong. You are responsible for designing the scheme.   

Certain conditions on your licence may change over time, such as hands off flows or levels. If this happens you will need to change your structure to accommodate these changes. You will need to consider the risk of this when you design your structure. 

3. Calculating abstraction volumes for transfer licences 

The Environment Agency normally will not include annual abstraction volumes on a transfer licence. However, annual abstraction volumes are needed to:

• understand how much water may be taken when assessing the impact of the proposal
• calculate the application charge

The Environment Agency understand that it can be difficult to calculate annual abstraction volumes in some instances. When calculating your annual abstraction volumes, you will need to provide as much information as possible to verify your annual abstraction calculation.

3.1 Where structures exist

Where structures exist, you must supply structure drawings and equation calculations to tell the Environment Agency what the maximum possible abstraction is at any given time.

You must also list the assumptions you have made as explained in section 1.3.

3.2 Where structures do not exist

Where no structure exists, for example the flooding of land, information about the area and depth of land being flooded is required with a calculation of the volume.

You must include a detailed site plan that captures all features that will be flooded with water, including lengths and depths.

You must also list the assumptions you have made when calculating the abstraction volume.

Contact the Environment Agency

General enquiries

National Customer Contact Centre
PO Box 544
Rotherham
S60 1BY

Email enquiries@environment-agency.gov.uk

Telephone 03708 506 506

Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) +44 (0) 114 282 5312

Monday to Friday, 8am to 6pm.