Research and analysis

Post-incident reporting for reservoirs: annual report 2022

Updated 7 October 2024

Applies to England

1. Introduction

Reservoir undertakers (owners and operators) must report to the Environment Agency when a reportable incident occurs at their reservoir.

A reportable incident is:

• when an incident results, or could result, in the uncontrolled release of water
• where emergency measures have been taken to prevent any or any further uncontrolled release of water and to minimise the danger to human life

We classify incidents based on their severity. There are 3 levels:

• 1 (failure)
• 2 (serious incident)
• 3 (incident)

For this report the Environment Agency received final information on 6 incidents.

There was one level 2 incident and 2 level 3 incidents in both 2021 and 2022. We review all 6 incidents in this report.

We include information from incidents which happened more than 1 year ago. This is because reservoir undertakers have 12 months after an incident to provide us with a full and comprehensive post-incident report.

A qualified civil engineer, appointed by the Secretary of State for Environment, Food and Rural Affairs, reviews all incidents. The incidents in this report are either resolved or have plans in place to address any outstanding issues.

Each case study in the report contains important lessons learned for each incident.

We would like to thank the undertakers and engineers who have given permission for photographs they supplied to be used for this report.

2. Incident reporting in England

We are the regulatory body for large raised reservoirs (LRRs) in England. An LRR holds over 25,000 cubic metres of water above ground level. This is like 10 Olympic sized swimming pools.

A small raised reservoir (SRR) holds less than 25,000 cubic metres of water stored above ground level. SRRs do not have to be registered in England, and they do not come under the Reservoirs Act. We recommend that incidents at SRRs are still reported, so that we can learn from them to reduce the risk of similar events happening elsewhere.

Since July 2013, undertakers of LRRs in England must report any incidents to the Environment Agency. This is a requirement of the Reservoirs Act 1975 and Regulation 14 of Statutory Instrument 2013 No. 1677. Undertakers of SRRs can voluntarily report incidents.

As soon as an incident is under control, the reservoir undertaker must provide a preliminary report to us.

This report must contain:

• the date and time of the incident
• the location of the reservoir
• incident details and observations from the site of the incident

The reservoir undertaker must send us a final post-incident report form within a year of the incident.

The report must explain:

• the facts relating to the incident
• the circumstances that led to the incident
• any conclusions
• any lessons learned from the incident

We assign an ID number to each incident to make reporting easier. These may not be in numerical order.

This is because:

• some incidents are not classed as reportable incidents, so the incident number is removed and not re-allocated
• in some cases, we wait until the undertaker completes a full investigation of the causes before allocating an incident number

In July 2021, a ministerial statement accepted the recommendations made in the Independent Reservoir Safety Review Report.  We are working with Defra to carry out these recommendations through our reservoir safety reform programme.

One of the recommendations is about incident reporting. We are improving how we collect incident data and how we report it. This may mean incident reporting looks slightly different in the future. We hope this will help to make learning from incidents faster and reach a wider audience.

It is important to remember that incident reporting is different to incident response. If you have concerns about the operation of a reservoir, you should report it immediately to the Environment Agency. The incident hotline is 0800 80 70 60.

3. Incident classification levels

There are currently 3 incident severity levels.

3.1 Level 1

This is a failure which results in an uncontrolled, sudden release of retained water.

3.2 Level 2

This is a serious incident involving any of the following:

• an emergency drawdown
• emergency works
• a serious operational failure

3.3 Level 3

This is an incident which may involve any of the following:

• a precautionary drawdown
• unplanned physical works
• human error resulting in an adverse change in operating procedures

As part of the incident reporting reform, we are looking into how effective the incident classification levels are. We may change these in the future. If we do, we will adjust any existing information on previous incidents to fit the criteria of the new classification system.

4. Reported incidents

We have received final information on 6 reportable incidents since January 2022:

• 3 incidents in 2021 – 2 level 3 incidents and one level 2 incident
• 3 incidents in 2022 – 2 level 3 incidents and one level 2 incident

The incidents in this report are either resolved or there are plans in place to address outstanding issues. Information from previous incidents can be found in the last post incident report which was published in August 2021.

5. 2021 Incidents

5.1 Incident 482

Dam Type: earthfill embankment

Reservoir status: statutory LRR

Dam height (m): 6.4

Incident type: embankment slip

Incident severity: 3

This incident happened at a flood storage reservoir. A shallow rotational slip occurred on the upstream shoulder of the dam. There was a 5m long tension crack near the embankment crest. The slumped material collected near the toe of the bank.

The undertaker investigated the area and found the damage was relatively shallow. They carried out temporary measures to fill the crack with bentonite mixture. This prevented water getting in during flood events. They also placed bags of material along the base of the slip. This reduced the risk of further movement until a permanent repair could be completed. They also increased monitoring and surveillance of the structure.

Lessons learned

The investigation found that the affected section of embankment was steeper than the design needed to be. Soil creep over time had covered the service road kerb line at the upstream toe of the slope. Photo 1 shows how much soil had accumulated over the kerb line. This had not been considered an indicator of slope movement.

Photo 1: depth of soil accumulated over the kerb line

The incident shows the importance of:

• adhering to construction drawings and specifications when constructing embankments
• identifying and acting on any early indications of slope deformation.

5.2 Incident 484

Dam Type: earthfill embankment

Reservoir status: statutory LRR

Dam height (m): 5

Incident type: pipe/culvert leak

Incident severity: 3

A flood storage embankment was designed with a pumping water main passing through it. There was leakage where the pressure pipe was, in the downstream shoulder of the dam. The reservoir is normally empty so there was no imminent threat of a release of reservoir water.

The undertaker stopped pumping and carried out an excavation. They found a pipe joint coupling had moved and become displaced. This led to the leakage from the joint. Photo 2 shows the displaced pipe coupler.

Photo 2: the displaced pipe coupler

Lessons learned

Pressure pipelines should not pass through dam embankments if this can be avoided. Any leakage through joints or defects in the pipe itself can cause leakage and erosion. This could lead to embankment failure.

5.3 Incident 485

Dam Type: earthfill embankment

Reservoir status: statutory LRR

Dam height (m): 3

Incident type: damaged spillway

Incident severity: 2

The undertaker was repointing some masonry blocks on the surface of a spillway chute. When they removed a block, they found soft ground and water. The undertaker carried out further investigation, directed by a panel engineer.

They found that there were voids between the masonry blocks and the backing concrete. Photo 3 shows a void. The mass concrete backing layer was poor quality and the ground below the concrete was soft.

Photo 3: the void between the masonry blocks and the backing concrete

Emergency services and stakeholders held an emergency meeting. Nearby residents were evacuated until the reservoir level was lower.

There was no bottom outlet available to start emptying the reservoir. This meant pumps were needed to reduce the level, by pumping water over the dam crest. It took nearly a day to get and set up the temporary pumps at the site. Up to 9 6-inch pumps pumped the water over the crest over a few days.

The undertakers built a sheet pile cut-off wall immediately upstream of the spillway as a temporary measure. A new spillway is being planned.

Lessons learned

The spillway showed no signs of structural problem prior to the incident. The undertakers found the defects during maintenance work. The incident highlights the value of ground investigations. These can confirm the construction arrangement and physical conditions of dam structures. This is particularly important for spillways which must safely convey flood events without structural damage happening.

6. 2022 Incidents

6.1 Incident 489

Dam Type: earthfill embankment

Reservoir status: statutory LRR

Dam height (m): 4.6

Incident type: embankment leakage

Incident severity: 3

The incident happened during heavy rainfall. The overflow spillway on an earth embankment dam was in operation. Monitored seepage flows greatly increased at a location near the spillway. The undertaker informed the supervising engineer two weeks after seepage rate increased.

Initially, attempts to reduce the reservoir level using the high-level outlet valve did not work. The undertaker used a boat and found the intake screen had become blocked by a log and other debris. This screen is normally under the water level and is not visible.

They cleared the debris which lowered the water level. The seepage also reduced. The path of the seepage was believed to be behind the side wall of the cascade spillway structure. Continued seepage behind the spillway wall could have led to failure by internal erosion.

Lessons learned

Undertakers should immediately tell the supervising engineer when they notice any significant change in seepage. Where possible, the design of intake screens should allow the undertaker to monitor it and clear any blockages.

Undertakers can refer to the Environment Agency ‘Blockage Management Guide’.

6.2 Incident 488

Dam Type: concrete gravity

Reservoir status: statutory LRR

Dam height (m): 10.2

Incident type: human error

Incident severity: 3

This incident occurred at a non-impounding reservoir formed by a concrete perimeter wall. The undertaker found seepage at the toe of the dam. When they shut the reservoir inlet pumps the seepage stopped.

Investigations found that the source of the water was a leak on the scour main. The main runs around the edge of the reservoir close to the outer toe of the dam wall. When exercising the valves at the inlet/outlet tower, a staff member had not closed a cross-connection valve. The valve was from the inlet main to the scour main. It was part of a sequence which needed to be closed before switching the inlet pumps on. Pumped inflow had then flowed directly into the scour pipe causing the pipe to leak at the toe of the dam.

Lessons learned

Human error caused this incident. The undertaker carried out an investigation and put in place measures to stop the incident happening again.

These included:

• physical measures within the tower - improved lighting and a valve schematic
• improved operational documentation and procedures

6.3 Incident 487

Dam Type: earthfill embankment

Reservoir status: non-statutory SRR

Dam height (m): 10

Incident type: damaged spillway

Incident severity: 2

A flood event destroyed the spillway chute for a 10m high embankment dam. This reservoir was a SRR and therefore unregistered. A member of the public noticed a problem and informed a panel engineer, who they knew personally. The panel engineer then alerted us.

The chute was on the left downstream mitre between the embankment and natural ground. This left a void. Some parts of the downstream shoulder of the embankment collapsed into the void. Photo 4 shows the damaged spillway chute.

Photo 4 : the damaged spillway chute, from downstream looking up at the crest

We used our permissive powers to attend site and manage the response. A panel engineer directed our response.

Contractors used 2 6-inch pumps to reduce the reservoir water level. They attempted to use the low level outlet pipe, which was unsuccessful. They also tried to use an upstream valve but there was no flow, suggesting a collapse or blockage.

We treated this incident as an emergency because the reservoir was upstream of a village in a steep valley. The dam was not maintained or managed to the standard of a registered LRR.

Lessons learned

This incident occurred at an ancient non-statutory reservoir. It had not benefitted from professional engineering oversight in recent times.

Evidence showed that the spillway chute was made of unreinforced mass concrete and was very thin in places. There was no evidence of a right side wall. A side wall would have protected the dam embankment from erosion.

There were indications of previous repair work to reduce erosion from floodwater. Nobody had tested the low level outlet for a long time. The downstream end of the pipeline was buried, and efforts to clear the outlet still failed to generate any flow. The incident highlights the poor physical and operational condition of some non-statutory reservoirs.