Discharging from a reservoir: preparing a risk assessment and management systems

We will ask for the following information when you apply for consent to discharge water from a reservoir. 

Risk assessment

Your risk assessment needs to consider the:

• increase in downstream flow
• discharge of sediment
• discharge of polluted sediment
• discharge of poor quality water
• release of fish

You must carry out a more detailed risk assessment if there is a significant risk to people or the environment. This should include:

• details of the risks
• mitigation measures to avoid or minimise the impact of all identified risks.
  
  

Increase in downstream flow

You must assess whether the discharge from the reservoir 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

All routine drawdown operations must prevent or minimise downstream impacts and flood risks as far as possible. 

Your risk assessment must identify any properties or situations that your routine discharge operations may affect. For example:

• houses
• businesses
• caravan
• camp sites

local water users like:

• anglers
• boaters
• 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 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 the sediment 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 controlled or hazardous waste and dispose of the dredged sediment legally.

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
• harm fish habitats and breeding sites
• kill fish and other aquatic animals.

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

• dissolved oxygen
• temperature

We are 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 us.

Algal blooms in reservoirs can cause poor water quality discharges. 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 us and ask to speak to your local environment team. You may need to rescue the fish so you can relocate them elsewhere or restock them after you’ve refilled the reservoir.

Management systems

Your management system must show how you will make the discharge with minimal polluting impact. And what actions you will take if your activity results in a pollution incident.

You will need to explain additional measures to prevent pollution if the activity is in, or near, a sensitive location.

We will only give you a consent if you have mitigated the risks as far as is reasonably practicable.