Leakage from within tunnel in bank
Publish date
02 September 2025
Case ID#
3121
Title
Leakage from within tunnel in bank
Nation
England
Regulator reference no.
311
Legal status
Statutory
Reservoir type
Non-impounding
Reservoir capacity
10M+ m3
Year of construction
1970 - 1989
Main construction type
Earth fill embankment
Dam height
15 - 29.99 metres
Dam flood category
A
Hazard class
High-risk reservoir
Reservoir use
- Other
Owner type
Limited company
Date & Time of incident
08 April 2006 - 12:00
Date incident closed
Observations that caused the incident to be declared
- Leakage or seepage from a new leakage point
Describe the incident
Significant water leakage was noted in the road adjacent to the dam toe. This was due to leakage from a wet tunnel under the dam, and the reservoir water level was lowered as a precaution. The owner is still assessing the exact cause of the problem with the tunnel. In order to stabilise pressure within the tunnel a shut down and isolation of the Pumps at the Inlet Pumping Station was undertaken along with closure of the Valves at the Inlet Tower to the Reservoir. A precautionary drawdown was undertaken until the situation was fully assessed. The precautionary drawdown was stopped once the condition was assessed as stable via a CCTV Submarine Survey of the wet tunnel was completed.
Supporting photos
Natural processes which initiated or contributed to the incident
- None
What were the main contributing factors to the incident occurring?
Dam factors
- Failure or damage to pipes or culverts
External factors
- None
Shortcomings
- No apparent shortcoming
What was the root cause of the incident?
Impacts on the reservoir
- Failure or damage to pipework
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
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
A Forensic Investigation of the Failure was undertaken whilst repairs were in progress. In addition to this a technical review of all Wedgeblock Tunnel Assets was undertaken.
Lesson 1
- Surveillance and Monitoring
Ensure that all tunnels under dams are inspected on a regular basis and records made of their repairs signed off by a competent person. The frequiency of the inspections should be as often as technically required as part of the understanding of the initial design assumptions and any subsequent condition assessment from previous inspections
Lesson 2
- Records and studies
Ensure that all design information regarding design of tunnels and inpection reports under dam structures are kept with the reservoir records and form part of the inspection
Lesson 3
- Emergency response
The quick isolation of the tunnel was made possible as a result of the owners practices regarding regular key valve testing and maintenance along with the existance of isolation procedures which were fully understood by local operatives