Deep seated slip in bank
Publish date
02 September 2025
Case ID#
3112
Title
Deep seated slip in bank
Nation
England
Regulator reference no.
301
Legal status
Statutory
Reservoir type
Impounding
Reservoir capacity
10 - 24,999m3
Year of construction
1950 - 1969
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
16 October 2006 - 12:00
Date incident closed
Observations that caused the incident to be declared
- Deformation or instability of crest (settlement, cracking, depressions)
Describe the incident
Settlement monitoring of the dam crest highlighted that movement had accelerated at three points. A deep seated slip in the upstream shoulder is thought to have been caused by operational drawdown of the reservoir in the summer and high pore water pressure within the upstream shoulder of the embankment.
Supporting photos
Natural processes which initiated or contributed to the incident
- None
What were the main contributing factors to the incident occurring?
Dam factors
- Instability
External factors
- None
Shortcomings
- No apparent shortcoming
What was the root cause of the incident?
Impacts on the reservoir
- Settlement / deformation (outside normal or expected parameters)
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?
Yes
Describe any assistance by external parties and impacts on the downstream population
None
Summary of studies or investigations undertaken
Intensive monitoring (weekly level survey, piezometer and toe drain readings etc.), ground investigation (including trial pits on the crest and below the upstream toe beam and boreholes in the upstream shoulder) and preliminary stability analyses.
Lesson 1
- Records and studies
Plotting the settlement data graphically was very useful. The time of the year that the survey is carried out could be important. In our case where the cause of the problem appears to be related to drawdown it is best to carry out the survey in the autumn after the reservoir is drawn down during the summer.
Lesson 2
- Emergency response
The incident has highlighted the need to have adequate emergency drawdown capacity during times of heavy rainfall and inflow to the reservoir.