Leakage and breach of washland flood storage embankment
Publish date
02 September 2025
Case ID#
3249
Title
Leakage and breach of washland flood storage embankment
Nation
England
Regulator reference no.
509
Legal status
Statutory
Reservoir type
Impounding
Reservoir capacity
10M+ m3
Year of construction
< 1800
Main construction type
Earth fill embankment
Dam height
2 - 4.99 metres
Dam flood category
A
Hazard class
High-risk reservoir
Reservoir use
- Other
Owner type
Public body
Date & Time of incident
09 August 2024 - 12:00
Date incident closed
Observations that caused the incident to be declared
- Leakage or seepage from a new leakage point
- Unexpected leakage or seepage from a known leakage point
- Water flowing outside of engineered channels
Describe the incident
Two incidents occurred at this site, which is washland flood storage reservoir. The embankment affected was a cradge bank. This is a permanent bank constructed at a lower level to the main dam embankment. It would be overtopped before the main defence bank in a flood event. For the first incident, the undertaker noticed leakage during a planned annual inspection with the supervising engineer. The undertaker regularly monitored: the rate of leakage, any turbidity in the leaking water level of water in the river from where the leak was suspected At the time of the first incident, the reservoir had not impounded water for a considerable time. If the cradge bank were to fail, the ability of the dam to safely sustain a sudden hydraulic impact load from the inflow was uncertain. Monitoring continued. A member of the public reported the second incident, which occurred 3 months later. The noticed a leak through the cradge bank between the river and the reservoir the reservoir was filling because of the leak. By the afternoon, the bank had breached. The undertaker inspected the bank. They found it was overtopping in many places, resulting in scour damage to the crest and slips on the downstream face. The undertakers inspected the bank using safe vantage points nearby and drone footage. Access to the dam during impoundment is not possible. The breach happened at a location which is designed to overflow and allow water into the reservoir. Due to this fact, it was not deemed a reservoir safety issue. It did impact the operation of the reservoir and could alter the flow regime of the river. The breach of the cradge bank did not impact the structural integrity of the main dam. The photos show the breach developing, during the event, and after the eventThe undertaker investigated the cause of the incidents. The affected embankment is 400 years old and 20 km long. The structure is made from local material that has a low cohesive material content. There has been significant historical activity from badgers. Their former setts are often collapsed, which cant be seen from visual inspection. They are sufficiently porous to enable water to enter the embankment and lead to piping failure.
Supporting photos
Natural processes which initiated or contributed to the incident
- None
What were the main contributing factors to the incident occurring?
Dam factors
- None
External factors
- Other external factors (describe below)
Shortcomings
- Process or procedural shortcoming
What was the root cause of the incident?
Impacts on the reservoir
- External erosion
Supporting photos
Describe any human factors which influenced the incident
Describe any instrumentation at the reservoir and how this was used in warning of the incident or providing monitoring during the incident
Not applicable. There is no instrumentation present that would have indicated this issue.
Was instrumentation effective leading up to and during the incident?
Not Applicable
Describe any assistance by external parties and impacts on the downstream population
None
Summary of studies or investigations undertaken
Investigations are at an early stage and are focused on understanding how the construction works could occur within a public highway on the crest of the dam (under the control of the highway authority) and without any consultation with the Environment Agency by the water company installing the pipe.
Lesson 1
- Surveillance and Monitoring
Engagement of the local communities, farmers and other third parties to actively report any issues they observe should be encouraged. It was an important and valuable source of information for these incidents.
Lesson 2
- Operation and maintenance
Burrowing animal activity can lead to vulnerabilities in dams. Surveillance and early action is important to identify when animals may be burrowing in dams and mitigate any impacts. If historical activity is known about, repair works may be required to reduce the risk of piping failure.
Lesson 3
- Surveillance and Monitoring
This highlights the practical difficulties associated with surveillance, monitoring and physical intervention on very long internal washlands type banks. This is particularly the case during high flow conditions
Lesson 4
Closing comments
Acting to investigate and rectify leakage early may reduce the risk of more serious events happening afterwards.
Supporting photos
