Guidance

Abstract or impound water: hydrological information needed for your application

Updated 13 December 2023

Applies to England

The Environment Agency grants most licences to abstract or impound water using the flow or level restrictions in local catchment abstraction licensing strategies. If they cannot do this they may grant your proposal with a local flow or level condition. Where they need to use a local condition they may ask you to produce flow estimates to understand the flow in the river. This will help them to set conditions to make sure your proposal leaves enough flow in the river for people and the environment.

The flow conditions the Environment Agency produces must be accurate to make sure the abstraction or impoundment licence is not:

• damaging to the environment
• over precautionary

When the Environment Agency assesses your application they may need:

• flow duration statistics including the mean flow value
• details of how you have produced the flow duration statistics
• flow measurements to validate the flow duration statistics
• hydrographs showing the impact of your proposal

This document explains what these terms mean.

The Environment Agency recommends that you get enhanced pre-application advice before you apply so that they can tell you what they need. If you submit a formal application without doing an enhanced pre-application first and the Environment Agency find they need hydrological information from you, they may need to return your application to you until you can provide it.

1. Flow data

1.1 Hydrographs

The Environment Agency does not need a hydrograph to determine every proposal, but they will need them if they have concerns about the impact your proposal may have on the environment. The Environment Agency can tell you if they need them if you ask for enhanced pre-application advice.

The flow duration curve provides flow statistics to describe flow rates on an annual basis. Flow duration statistics do not show the variation of flow in a day, or part day basis. They need a hydrograph for this.

A hydrograph plots daily (or sub daily) flow on the y (vertical) axis over a specific period shown on the x (horizontal) axis. Flow rates may be:

• daily mean flows - the mean of flow measurements taken over a day
• sub daily flows recorded at period time intervals, for example every 15 minutes

Hydrographs show:

• any impact on flow variability
• any impact on summer low flows and winter peaks
• the reliability of the abstraction

You may need to provide 3 hydrographs to show a:

• wet year
• dry year
• average year

The hydrographs must compare actual (current) flows with the modelled impact of the proposal.

1.2 Flow duration statistics

Flow duration statistics are a:

• table of data
• graph, known as a flow duration curve

They show the statistical availability of any given flow. That is the amount of time for which a given flow is equalled or exceeded in a set period of time. The x (horizontal) axis shows the percentage of time that a flow is equalled or exceeded. The y (vertical) axis shows flow rates.

Q is the symbol used for flow. For example, Q30 is the flow that will be equalled or exceeded for at least 30% of the time. Or, to express another way, for 110 days in an average year (30% of 365 days).

Flow duration statistics must use best available data and cover a long period of record to represent the long-term average flow of the river.

Normally the Environment Agency needs natural flow duration statistics, but in some cases, they will ask for influenced (or actual) flow statistics. The abstraction licensing strategy confirms the type of flow statistic you need.

You must send the flow duration statistics as a graph and table of data. The Environment Agency will use the data to confirm the hands-off flow conditions on your licence.

1.3 Mean flow

The mean flow is the average of all the flow measurements taken over a period of time at a particular point in the river. Mean flow can range between Q30 and Q40 depending on the type of river.

The mean flow, as with all flow duration statistics, will vary from year to year. You must make sure that the mean flow produced represents the long-term average.

2. Producing flow statistics

Use data from within the catchment. This is likely to produce more accurate flow statistics. You can produce flow statistics using naturalised data from their gauging stations. You will then need to scale to the ungauged site. The Environment Agency have a large network of gauging stations so there is likely to be a gauge within the major catchment of your proposal. They have naturalised sequences for many of their gauging stations. Contact the Environment Agency to find out if the data exists and if you can have it.

Note: the Environment Agency produces naturalised sequences based on the best available data at the time.

Where gauged data is unavailable, you can use other hydrological methods to estimate flow statistics at an ungauged site. You may choose to use a computer model.

Note: computer models can produce flow estimates with high levels of uncertainty depending on the size and type of river. You must consider this when choosing to use a model. It may be appropriate to discuss it with the Environment Agency before getting the flow estimates.

3. Validation of flow statistics

The quality and accuracy of the flow statistics will vary depending on the method used to produce them. Every flow estimation method has limitations and uncertainties and these can affect the flow value estimations. It is not always obvious which technique will produce the most accurate flow estimates for a particular site.

If the Environment Agency needs more confidence in your flow duration statistics, they will ask for flow measurements to validate them. This will depend on the:

• technique used to produce your flow statistics
• environmental sensitivity of the catchment
• potential impact of the proposal

Validation compares measured data with the estimated data and will show how reliable your flow duration statistics are. Contact the Environment Agency to find out if they have any flow measurements that you could use for validation. They will tell you how to get the data and if there is any cost. If they do not have suitable flow measurements, you will need to take the measurements.

The amount and quality of flow measurements needed will be proportionate to the:

• scale of the proposal
• environmental risk

You need to allow for the time needed and any costs:

• the Environment Agency may need measurements over a range of flows, which can take many months, even years, depending on the data required
• flow measurement requires expertise - the Environment Agency advises that you use consultants to take the measurements if a more specialised gauging technique is necessary

Questions the Environment Agency will ask you to decide if flow measurements are needed

Is there high confidence in the method used to produce the flow duration statistics?

• Yes - flow measurements are not needed
• No - go to next question

Do appropriate flow measurements already exist near the proposal? For example, are there any recent Environment Agency flow measurements taken nearby that are representative of the site?

• Yes - additional flow measurements are not likely to be needed
• No - go to next question

Are there any species or habitats present nearby that are sensitive to changing flows?

• Yes - site specific flow measurement programme is required
• No - go to next question

To answer this question you will need to consider:

• the Water Framework Directive status
• proximity to any designated site including sites protected by the Habitats Regulations
• any species that the Environment Agency has identified

Are there any protected rights downstream of the proposal?

• Yes - site specific flow measurement programme is required
• No - go to next question

Is the site complex? For example, are there multiple channels or sluice controls on the river that need to be understood to determine the application?

• Yes - site specific flow measurement programme is required
• No - go to next question

Will there be a significant reduction in flow, including depleted reaches if it is a non-consumptive abstraction?

• Yes - site specific flow measurement programme is required
• No - risk of proposal is low, no flow measurements are needed

3.1 Flow measurement techniques

This section discusses 3 flow measurement techniques. To understand more about these techniques, you can read:

• ISO 748:2021 - Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using point velocity measurements
• ISO 1438:2017 - Hydrometry - Open channel flow measurement using thin-plate weirs

When confirming that flow validation is needed, the Environment Agency will discuss and advise the appropriate measurement technique with you. When agreeing the method and number of measurements they will consider:

• sensitivity of the river catchment
• impact of the proposal
• proximity of other river users

When deciding the measurement method, you must consider:

• the size of the channel and the flow in the river - some techniques are more appropriate for smaller rivers
• whether there are sensitive species nearby - for example how sensitive the species are to changing flows
• whether there are other river users or protected rights nearby
• the potential impact of your abstraction or impoundment proposal

You may be able to use simple methods for smaller proposals that are in non-sensitive river catchments, for example:

• float and a stopwatch
• bucket and stopwatch (for small streams)

You will need to use specialist flow measurement equipment following British Standards for:

• large proposals
• proposals that fall within environmentally sensitive catchments

Flow measurements need to be accurate to validate the flow statistics. To get accurate measurements the selected sites should:

• be well defined
• have stable banks
• be free from weeds and other obstructions

When taking flow measurements you must:

• carry out health and safety checks and select appropriate personal protective equipment
• secure suitable landowner permissions
• assess for and avoid sensitive habitats and species

Whenever possible, confine the flow to a single channel. If this is not possible then gauge both channels and sum the flows.

Look for a straight approach channel with reasonably even flow and depth. Ideally find a straight section 4 times the channel width for rivers less than 50m wide and 200m for rivers greater than 50m wide.

Confirm the technique with the Environment Agency before taking the measurements to make sure they are appropriate to the site.

3.1.1 Volumetric gauging using a calibrated container

You may be able to use simple methods, using non-specialist equipment:

• for smaller proposals
• in non-sensitive river catchments
• when flows are very low

For very small watercourses you could use a calibrated vessel and stopwatch. You need to time how long it takes to collect a set volume of water. For example, if it takes 30 seconds to fill a 10 litre bucket, the flow will be:

10 (litres) ÷ 30 (seconds) = 0.33 litres per second

Capture all water flowing past the measurement into the vessel otherwise you will underestimate the flow.

The final flow measurement should be an average of 2 or 3 consecutive measurements. This type of measurement should last at least 10 seconds, so the vessel must be large enough to fill for 10 seconds. The Environment Agency is unlikely to accept this technique in rivers that:

• have a special designation
• contain protected species
• where there is fish migration

3.1.2 Floats (area multiplied by velocity method)

You could use floats and a stopwatch for smaller proposals in non-sensitive river catchments. However, this method can be unreliable because velocity changes with depth and across the channel. Do not use this technique in a turbulent river.

For an explanation of this method and more information see ‘ISO 748:2021 Hydrometry - Measurement of liquid flow in open channels using current-meters or floats’ for comprehensive guidance on flow measurement.

Measure the mean cross sectional area and mean velocity to calculate the flow. Use the formula:

Q (flow) = A (cross sectional area) × [V (velocity) × 0.84 to 0.9 velocity reduction coefficient factor]

The velocity reduction coefficient factor is applied to address the change in velocity over different river depths. The smoother the riverbed the higher the coefficient will be.

Calculate the velocity of the river by measuring the time it takes a float to travel a specified distance. Repeat with several floats distributed across the stream width. For example, if it takes 30 seconds for a float to travel 10 metres, the velocity will be:

10 (metres) ÷ 30 (seconds) = 0.33 metres per second

Apply the velocity reduction coefficient factor to the velocities of each float. Calculate the mean velocity from the results.

Calculate the cross sectional area by measuring the width and mean depth of the channel. Then calculate the flow. For example, if the area is 0.04 metres squared, the flow will be:

0.33 (metres per second) × 0.04 (metres squared) = 0.13 cubic metres per second

3.1.3 Using specialised equipment

Use specialist equipment for:

• larger proposals
• proposals which fall within sensitive catchments

For more information see ‘ISO 748:2021 Hydrometry - Measurement of liquid flow in open channels using current-meters or floats’ for comprehensive guidance on flow measurement.

The principles of the technique are similar to those described in section 3.1.2. Use consultants to take the flow measurements if specialised gauging techniques are needed.

The instruments used must be:

• serviced
• calibrated
• appropriate for the flow conditions - some are more suitable at low flows or shallow depths
• maintained and operated in line with the manufacturer’s instructions

The velocities measured must be between the maximum and minimum calibration for the instrument used. Submit instrument calibration certificates with the flow measurement data.

3.2 Number of measurements required

The number of measurements needed will depend on the:

• scale of the proposal
• type of river
• environmental risk
• presence of other river users and protected rights
• how conclusive initial measurements are

For most rivers, the Environment Agency will ask for 3 to 5 measurements. They will use these to assess the quality of the flow duration statistics. If the initial measurements are not sufficient to do this, they will need more.

Consider the flows that need validation. If you need to validate a Q95 flow estimation, concentrate measurements around lower flows. The Environment Agency understands that flows may not fall to Q95 during the period of measurement. If you provide measurements between, for example Q70 and Q85, they will use expert judgement and risk based decisions to confirm if the Q95 flow estimate is acceptable.

3.3 Using the flow measurements to validate the flow statistics

Once you have the flow measurements, link them using the long-term flow duration exceedance for that day. To do this use a local gauging station. It should be:

• in the same catchment as the proposal
• hydrologically similar - have the same catchment response which means have similar geology, rainfall, area, land use and so on

The Environment Agency operates a large network of river flow gauges. Contact them for information about their gauging station network.

Then plot the measurements against the proposal site’s estimated flow duration curve.

Send your flow measurements (including individual flows if it is a multiple channel site) to the Environment Agency.

This must be presented:

• in tabular form
• on a flow duration curve (graph)

When you send your flow measurements, include:

• the date and time of the measurements
• an accurate grid reference of where you took the measurements
• width and depth of the river at the point of your flow measurements
• details of how you measured the flow
• how you used the flow measurements to validate the flow duration statistics

The Environment Agency will review your data. They will discuss the options with you if the flow measurements suggest that the flow statistics are inaccurate. This will include whether:

• you need to use a different method to produce the flow duration statistics
• you need to take more flow measurements
• they need to adjust the flow estimation based on the validation

The Environment Agency will reject flow statistics if they think the technique used is not robust.

They will use the confidence limits used in the hydrological model or technique where:

• there is low confidence in the flow duration statistics
• there are no measurements to validate the flow duration statistics

In these cases, they will use a precautionary approach for estimating the impact of the proposal to make sure the environment is protected. The flow conditions set are likely to result in less water being available for abstraction.

3.4 Continuous data

Sometimes the Environment Agency needs short periods of continuous data.

You can collect this data by installing a flow measuring structure such as a thin plate weir or producing a rating curve for the site to convert the level to flow.

In most cases you will not need a licence or a flood risk activity permit to install a temporary flow measuring weir. Check if you need a licence to impound water and check what activities are exempt from needing a flood risk activity permit to decide whether you need to apply for either.

The Environment Agency recommends using at least 10 flow measurements to produce a rating curve. Take these flow measurements across the range of recorded levels.

Note that in larger rivers a weir may not be suitable so you would need to rent an ultrasonic area velocity meter.

For either method, you will need to record the ‘head’ of water at the measuring site. You can install a gauge board and take manual readings daily or twice daily, but in smaller, flashier catchments you could miss some of the high-flow data you will need to support your application. In these situations the Environment Agency recommends using a data logger to record water levels at 15 minute intervals.

4. Storage reservoirs

Storage reservoirs are becoming more important as water availability is becoming increasingly limited in some catchments. Refer to the Environment Agency’s water reservoir guidance for more advice on planning your storage reservoir.

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.