Reservoir discharges: consents, permits and risk assessments

When you need a consent or permit to make a discharge from your reservoir and how to carry out a risk assessment.

This guide is for operators of reservoirs, including reservoirs that are owned and managed by water companies.

You must get a consent or an environmental permit from the Environment Agency before you make certain discharges from your reservoir to surface waters.

Before you apply for a consent or permit you need to carry out a risk assessment. This assessment will identify the potential risks before you make a discharge so you can manage them effectively.

You must consider each type of discharge you may make from your reservoir and decide whether you need a consent or permit for the discharge.

Consents for water company operators

Water companies who operate reservoirs can apply for a Water Industry Act 1991 section 166 consent from the Environment Agency.

Water company operators do not need consent for discharges from a pipe that is 229mm diameter or less. However, operators must take all steps to make sure the discharge is as free from pollutants as is reasonably practicable.

Water company operators must have consent from the Environment Agency for discharges:

  • that may contain solids or polluting matter
  • of drawdown releases through the scour valve
  • through pipes that are larger than 229mm diameter

Permits for reservoir operators

You must have an environmental permit from the Environment Agency for all discharges from reservoirs that may contain solids, accumulated deposits or other pollutants.

You do not normally need a permit for discharges that are:

  • uncontaminated surface waters
  • a water transfer and the discharge is non polluting
  • made in an emergency to avoid danger to human health – but you must take all reasonably practicable steps to minimise pollution and you must tell the Environment Agency as soon as reasonably practicable

When you apply for a consent or permit to make a discharge from a reservoir, you need to:

  • carry out a risk assessment of the potential environmental and safety risks of your proposed discharge
  • develop a management system – a written set of procedures that identify and minimise the risks of pollution

Read guidance on how to apply for a bespoke permit.

Water company operators who want to apply for a section 166 consent need to contact the Environment Agency. Email

Carry out a risk assessment

You must carry out a risk assessment for all reservoir discharges that require a consent or permit. Your risk assessment will be part of the technical information that supports your consent or permit application.

Read the risk assessment overview for guidance on how to complete a risk assessment.

You should review your assessment regularly and update it if circumstances change.

If your assessment shows the discharge may represent a significant risk, then you must carry out a detailed assessment. Your detailed assessment should further define the risk and identify mitigation measures. Mitigation measures are actions to avoid or minimise your impact. You may need to employ a qualified specialist to undertake this detailed assessment.

You must plan mitigation measures for all identified risks. You must show, in your management system, how you will make the discharge with minimal polluting impact.

The Environment Agency will consider whether your management system has properly assessed the risks. The Environment Agency will only give you a consent or permit if you have mitigated the risks as far as is reasonably practicable.

Increase in downstream flow

You must assess whether your reservoir discharge will have an impact on the downstream flow.

A large increase in downstream water flow can have serious environmental impacts and safety risks, including flooding.

An increase in downstream flow can:

  • damage property, bridges, boats and bankside structures
  • affect land use, for example block access tracks
  • risk the life and health of humans and animals
  • change the river’s appearance, for example erode the river bank
  • damage the river ecology, for example wash away aquatic plants
  • destroy fish habitats and breeding sites
  • displace fish downstream

To reduce the impact you must control the:

  • rate of your discharge
  • volume of your discharge
  • increase in the depth of the channel downstream

You must carry out routine drawdown operations to prevent or minimise downstream impacts and flood risks, as far as is practicable.

Your risk assessment must identify any properties or situations that your routine discharge operations may affect. For example houses, businesses, caravan and camp sites, and local water users like anglers, boaters and canoeists. You should maintain a contact register and give warning before drawdown operations.

Discharge of sediment

You must consider if the discharge from your reservoir will impact on sediments downstream. Significant deposits of sediments can:

  • damage the river ecology
  • destroy fish habitats and breeding sites
  • change the river’s appearance

Consider the need for a survey to find out how much sediment has accumulated that could be washed through the scour valve. Include details of sediment levels in relation to the scour valve height and sediment sampling, as part of your consent or permit application. Your discharge is more of a risk if:

  • you have not opened the scour valve for over one year
  • there has been an event that could have created large amounts of sediment, for example a flood upstream of the reservoir

If a large quantity of sediment may wash through the scour valve, you must develop a plan to prevent a damaging discharge.

To reduce the impact, consider the following mitigation measures:

  • coincide the discharge with higher natural river flows and increased natural sediment transport
  • open the scour valve gradually
  • make frequent small discharges
  • install and maintain sediment traps to remove trapped sediment

Discharge of polluted sediment

You must consider if pollutants may be present in significant amounts in your reservoir sediment.

Reservoirs in industrial areas may have toxic industrial wastes in the sediment.

If the reservoir sediment contains pollutants these may be carried into the downstream river and can:

  • cause deterioration in river quality – for water users and abstractors
  • damage the river ecology – in the short or long term

If your sediments may contain pollutants you must chemically test the sediment.

To reduce the impact, consider dredging the polluted sediment from the reservoir. You must follow the requirements for disposing of business waste to make sure you dispose of the dredged sediment safely.

Discharge of poor quality water

You must check whether your reservoir is subject to thermal stratification. Thermal stratification is where there are changes in temperature and dissolved oxygen at different depths of the reservoir.

Discharges of stratified water can lower the levels of dissolved oxygen and increase levels of iron and manganese. This can:

  • cause deterioration in river quality – for water users and abstractors
  • kill fish and other aquatic animals

For stratified reservoirs, your risk assessment must include profiling for:

  • dissolved oxygen
  • temperature

The Environment Agency is unlikely to allow a discharge if profiling shows the dissolved oxygen is below 50% saturation in the bottom profile of the reservoir.

If your profiling shows less than 80% dissolved oxygen, you should monitor the downstream watercourse during the discharge. Your risk assessment should set a downstream trigger limit for dissolved oxygen. If dissolved oxygen levels drop below this limit, stop the discharge and notify the Environment Agency.

Algal blooms in reservoirs can cause poor quality discharge water. You should not make planned discharges if there is a significant algal bloom, unless you have shown that the bloom does not affect the discharge quality.

Release of fish

Discharges of water can displace fish from the reservoir into the downstream river.

You must consider the impact on fish if you plan a substantial drawdown. This includes drawdown to a permanently lower water level and also to temporary lower water levels for maintenance before you refill.

You should contact the Environment Agency and ask to speak to your local fisheries team. You may need to rescue the fish so you can relocate them elsewhere or restock them after you’ve refilled the reservoir.

Submit your risk assessment

Send your completed risk assessment and proposed mitigation measures as part of your consent or permit application to the Environment Agency.

Contact the Environment Agency

General enquiries

National Customer Contact Centre
PO Box 544
S60 1BY


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.

The impact of COVID-19 means you may experience some delays in responses.

Published 8 May 2018