Research and analysis

Post - incident reporting for reservoirs: annual report 2023

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 currently 3 levels:

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

For this report the Environment Agency received final information on 8 incidents:

• 1 level 1 incident in 2018
• 2 level 2 incidents in 2022
• 4 level 3 incidents in 2022
• 1 level 3 incident in 2023

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 for large raised reservoirs (LRR). Sometimes it takes longer. This could be when more detailed investigations are needed. Owners of small raised reservoirs (SRR) are not legally required to report incidents. However, we encourage this, as they can provide useful learning opportunities.

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.

The independent reservoir safety review  highlighted the importance of reporting incidents (and near misses) to authorities. The reservoir safety reform programme is looking at how we can improve how:

• effectively we receive incident information
• we share the lessons learned from incidents

We would like to thank the undertakers and engineers who have provided information and photographs to be used for this report. We remind undertakers to continue to report mandatory incidents, and encourage the reporting of incidents at SRRs.

We recommend that you should report any incidents that:

• have not caused an uncontrolled release of water
• did not require emergency measures

We call these near misses. Some near misses might result in important lessons learned for the reservoir industry.

If you have a near miss occur at your reservoir and feel there was some useful learning, consider reporting this to us too.

2. Incident reporting in England

We are the regulatory body for LRRs in England. A LRR holds over 25,000 cubic metres of water above ground level. This is equivalent to 10 Olympic sized swimming pools.

A 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 1975.

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
• 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

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 may change the incident classification definitions to help improve the way we learn from incidents. 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 8 incidents since January 2023.

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 annual post incident report which was published in August 2023.

5. 2018 incidents

5.1 Incident 499

Dam Type: Unknown

Reservoir status: non-statutory (SRR)

Dam height (m): Unknown

Incident type: Breach

Incident severity: 1 - failure

A SRR in an upland area of moorland breached when a nearby statutory reservoir released water as part of a test for the drawdown valves. Photograph 1 shows the location of the breach.

It was believed to have stored approximately 2,000 cubic metres of water before the breach. The failure occurred on the same day as the drawdown valve test of the upstream larger reservoir. The exact mechanism of the dam failure is not known. There were no impacts on downstream communities - the site is on open moorland, distant from properties and infrastructure.

Since the failure, the undertakers of the larger reservoir have routinely exercised the draw-off valves. The discharge through the breached reservoir has not posed any concerns. There are no plans to restore the dam. The incident discontinued the structure.

Photograph 1: the location of the breach

Lesson learned

We do not know how the breach occurred as it was not witnessed. It is likely that the overflow facilities could not accommodate the rate of flow from the draw-off system of the upstream reservoir. This may have caused the embankment to fail by overtopping.

It appears that the breach only caused a limited amount of environmental damage. However, the incident highlights the importance of carefully planning drawdown tests. Owners and engineers should consider all likely consequences, especially when the rate of flow released exceeds historical rates of flow.

6. 2022 incidents

6.1 Incident 498

Dam Type: earthfill

Reservoir status: statutory LRR

Dam height (metres): 3.5

Incident type: downstream slip in service

Incident severity: 3 - incident

This incident occurred at a ‘not high risk ‘reservoir. The undertaker observed movement and crack development in the downstream shoulder of a small embankment associated with the site.  The crack depth ranged from 300-500 millimetres (mm). No seepage was visible at the time of the incident. As it was ‘not high risk’, there was no supervising engineer for the reservoir. However, the undertaker requested their attendance to advise.

The staff followed a certified on-site flood plan. The undertaker:

• started weekly surveillance visits
• established a set of monitoring points to survey the banks to detect any further movement
• carried out a monthly survey, with weekly surveillance visits in between
• carried out a site investigation where 4 boreholes of 3 to 6m depth were drilled below the embankment crest

They did these to establish the ground conditions. It appeared that the original construction in 1978 did not remove soft alluvium from the former watercourse under the line of the embankment. They noted possible evidence of low level seepage from the borehole information. The reservoir owner is now planning to carry out stability improvement works.

Lessons learned

It is not clear what caused the incident. The investigations identified the presence of soft ground under the downstream shoulder. This is likely to have been an important factor in the incident. The embankment instability occurred more than 40 years after construction. That fact underlines the value of routine surveillance in evaluating dam safety.

This reservoir was probably not constructed under the supervision of a Construction Engineer. Preparing the foundation appropriately is an important requirement for embankment construction. Any material that could cause instability should be removed at this stage. It also highlights the importance of having detailed record keeping during construction.

6.2 Incident 500

Dam Type: earthfill

Reservoir status: statutory LRR

Dam height (m): 4.5

Incident type: embankment deformation

Incident severity: 3 – incident

This incident occurred at a long embankment forming an off-line flood storage reservoir. It had recently been raised and widened with new compacted clay fill over the existing embankment. This extended the upstream face, with most of the new fill placed on the downstream side.

2 to 3 years after construction, longitudinal cracking was seen on the upstream face. This was along or near the interface between the original fill and the new fill. The most serious cracks extended to a depth of more than 500mm. The longitudinal extent of the cracking extended over a cumulative length of 1.85 kilometres (km).

An assessment of the seriousness of the cracking was undertaken. This determined that the cracks posed a risk to the safety of the embankment through several possible mechanisms of failure. It could introduce water pressure into the body of the old and new embankment sections. The qualified civil engineer responsible for the works instructed the undertaker to fill the cracks with bentonite sand.

Lessons learned

The incident highlights the design challenges associated with embankment raising works. Soils of different properties must perform in a homogenous manner without the formation of cracks at the interface.

Investigations concluded that the cracking was unlikely due to a single cause.

Instead, it was likely due to a combination of causative factors. It is believed the main factor was the asymmetrical widening and associated differential settlement of the new fill across the width of the raised embankment. The new fill was deeper on the downstream side of the raised embankment than on the upstream side of the raised crest. It is possible that loading the new fill on the downstream face of the original embankment caused movement in the upstream direction.

This may have led to cracking at the interface.

Other contributory factors considered were:

• lateral spreading of the new fill during consolidation
• a difference in the plasticity of the clay fill - the clay used for the new embankment had a higher plasticity than the clay used for the original embankment

The undertaker scoped remedial works to fill the cracks with bentonite sand. By the time they started the works, the cracks had reduced in width so they used a different strategy. A trench was excavated on the line of the cracks and backfilled with a capping layer of bentonite sand. They created a monitoring and surveillance strategy and will infill any remaining or new cracks with bentonite slurry.

6.3 Incident 501

Dam Type: earthfill

Reservoir status: statutory LRR

Dam height (m): 12

Incident type: uncontrolled flow through gate/valve

Incident severity: 3 - incident

This incident occurred at a large impounding reservoir. The mechanical failure of a 450 mm diameter scour valve led to the reservoir emptying through the scour pipe.

The valve is a butterfly valve, located within a ‘wet shaft’, which is normally under water. A higher level compensation water pipe and valve discharges into the same 1.9m diameter shaft. There was no secondary valve or guard valve fitted to the scour main.

During a routine exercise of the valve, the valve could not be re-closed and was stuck in the 80% open position. As there was no means to isolate the valve from the reservoir, the reservoir substantially drained down through the scour pipe. Fortunately, this did not cause any flooding to occur.

The undertaker:

• used environmental protection measures against sedimentation
• undertook fish rescue
• engaged with a qualified civil engineer to review the incident and develop an investigation plan

A confined spaces entry team assessed the valve. The team advised that the gearbox had failed. They decided to replace the gearbox and refurbish the valve and the valve spindle. There was insufficient space available within the shaft to introduce a guard valve. 

Lessons learned

Investigation of the valve gearbox found that the bearings had collapsed on the worm shaft. It was believed that there were two reasons for this.  Firstly, the gearbox was not designed for a submerged application. Secondly, the weight of a 12m long solid mild steel spindle was considerable. There are several lessons to learned from this incident.

A specialist had visually inspected the valve and gearbox two years before the incident. This inspection failed to identify that the specification of the gearbox was inappropriate. If there is any doubt over the appropriate specification of valve gearboxes, suitable checks should be completed.

The undertaker had regularly exercised the valve, but it had become stiff to operate before the incident. They had not recorded this in the reservoir safety records or taken pre-emptive action. A reservoir may have scour facilities that rely on the use of a single valve. In this situation it is important that operators have a mitigation plan for a scenario where the valve cannot be closed after opening. This could be included as part of the statutory flood plan.

6.4 Incident 503

Dam Type: earthfill

Reservoir status: statutory LRR

Dam height (m): 3.17

Incident type: erosion and leakage into draw-off

Incident severity: 3 - incident

This incident occurred at a small earthfill dam. The reservoir has a circular overflow shaft and scour valve which is only accessible by boat.  A 600mm diameter concrete pipe drains the overflow shaft. This extends over a length of 34m to the downstream toe of the embankment.

Leakage was detected flowing into the pipe when the reservoir was not spilling. This indicated either a problem with the scour valve or leakage into the overflow shaft or outlet pipe.

The Supervising Engineer directed the undertaker to make daily observations under Section 12(6) of the Reservoirs Act. There was evidence of eroded fill material deposited near the pipe outfall. A CCTV survey of the pipe revealed leakage at many pipe joints upstream of the crest of the dam. It appears that internal erosion had occurred on the outer face of the draw-off pipe. The pipe was lined with a cured-in-place liner, preventing further leakage and deterioration.

Lessons learned

The incident highlights the value of regular surveillance. The seepage through the pipe joints had increased over several months. It is important to keep the Supervising Engineer informed of any changes at a dam. This is particularly important in relation to leakage flows.

In this case the installation of a liner appears to have been effective. It stopped the seepage before the extent of the erosion led to any significant problems with the dam embankment.

The incident also highlights the challenges posed by overflow or draw-off facilities which have no bridge access from the dam crest. At one point ice formation on the reservoir hindered investigations, preventing safe access to the top of the overflow shaft.

6.5 Incident 504

Dam Type: earthfill

Reservoir status: statutory LRR

Dam height (m): 3

Incident type: other

Incident severity: 2 – serious incident

Works were being carried out to provide a new spillway structure. A cofferdam had been installed to isolate and protect the works area.

The cofferdam construction comprised:

• sheet piles
• reinforced PVC sheeting
• gravel fill

During a period of sustained high reservoir water level, leakage through the base of the cofferdam to one side of the sheet piles occurred. This inundated the working area. Installing pumps over the dam crest and using a bypass channel reduced the reservoir water level.

The on-site flood plan was successfully used, along with the advice of the Supervising and construction engineer. The contractor reviewed the design and reconstructed the cofferdam using sheet piles. This enabled the permanent spillway works to be safely completed.

Lessons learned

The incident highlights the importance of temporary flood protection works for any major construction on spillways.

Cofferdams must be effective for the safety of those working downstream and for the security of the dam. Designers must ensure cofferdam works are appropriate and safe. Any changes to the design must be effectively communicated.

6.6 Incident 505

Dam Type: earthfill

Reservoir status: statutory LRR

Dam height (m): 5

Incident type: internal erosion

Incident severity: 3 - incident

This incident occurred at an old embankment with a history of internal erosion problems.

The embankment had been modified in the past. It features a:

• shallow concrete pile wall
• capping beam at the upstream face

Buried within the crest but set back from the pile wall is a concrete beam. The beam once supported concrete kerbs.

Previous modifications created an overtoppable section acting as an auxiliary spillway, with the kerbs removed from the beam. A sinkhole was discovered close to the spillway channel near the right abutment. Photograph 2 shows the sinkhole. The hole measured up to 800mm in diameter and was just over 1m deep.

Water could be seen flowing at the base of the hole towards the downstream toe of the dam. Stoplogs removed from the spillway weir reduced the reservoir water level. When the water level was 200mm lower, the leakage stopped.

Investigations identified some settlement immediately behind the wall forming the upstream face of the dam. Excavation found leakage had been occurring through a crack or joint in the pile wall. It had progressed under the base of the concrete beam that had supported the kerbs.

Photograph 3 shows the excavation through the crest. It shows the piled upstream face on the left, and the buried concrete beam under the crest. Photograph 4 shows the area of leakage and erosion below the concrete crest beam.

The undertaker appointed a qualified civil engineer to oversee remedial works. The fill material on the line of the leakage and sinkhole was replaced with fresh compacted cohesive fill material. The repair was successful. 

Photograph 2: the sinkhole

Photograph 3: the excavation through the crest

Photograph 4: the area of leakage and erosion below the concrete crest beam

Lesson learned

Investigations are being planned to further explore piping incidents. Unless carefully implemented, concrete pile walls can tend to crack and leak. The narrow crest and steep downstream face of this dam probably contributed to the development of internal erosion.

It is likely that there were indications of deformation or leakage prior to the sinkhole formation. Surveillance visits had not, however, been effective in detecting it. The incident highlights the value of frequent surveillance visits by staff with appropriate technical training. They should be able to detect problems as soon as possible.

In this case, reducing the reservoir water level was effective in managing the incident.

7. 2023 incidents

7.1 Incident 502

Dam Type: earthfill

Reservoir status: statutory LRR

Dam height (m): 6

Incident type: culvert damage

Incident severity: 3 - incident

Passage of a flood event damaged a 450mm diameter brick-lined spillway culvert under an earthfill dam embankment. The undertaker found a sinkhole on the line of the culvert just downstream of the downstream toe of the embankment.

The culvert had become blocked, but the available draw-off facilities use this common outlet culvert. This meant these outlets could not lower the reservoir water level. A pump reduced the reservoir water level and the flow passing through the culvert. Photograph 5 shows the blocked culvert.

Fortunately, the dam did not overtop because of the incident. Visual inspection from a chamber immediately downstream of the damaged culvert found that the brick lining had completely collapsed. Excavation revealed the presence of tree roots within the culvert. When clearing the blockage, tree roots were found through the crown of the culvert.

As a temporary repair, a section of pipe was installed to act as a sleeve past the area of damage. A permanent repair or replacement pipeline is being planned.

Photograph 5: the blocked culvert

Lessons learned

The culvert conveyed water from two draw-off pipes and the overflow.  The dam may have failed if the culvert collapsed immediately under the dam embankment. Blockage of a spillway can also lead to failure by flood overtopping.

In this case, the undertaker found the damage through a routine surveillance visit. Investigations found that tree roots had penetrated the culvert which would have weakened the crown of the culvert.

The incident highlights:

• the value of routine surveillance visits
• the need to periodically inspect culverts by CCTV or other appropriate means to assess structural condition
• one of the risks with mature trees growing on dam embankments

If you need to inspect culverts or assess their structural condition it may be necessary to isolate the culvert from the reservoir. Reservoir lowering is likely to allow for this. These inspections are particularly important where pipelines or culverts pass directly under the dam.