Sinkhole from deterioration of the overflow culvert at the interface between different pipe materials
Publish date
02 September 2025
Case ID#
3265
Title
Sinkhole from deterioration of the overflow culvert at the interface between different pipe materials
Nation
England
Regulator reference no.
528
Legal status
Statutory
Reservoir type
Impounding
Reservoir capacity
10 - 24,999m3
Year of construction
1900 - 1929
Main construction type
Earth fill embankment
Dam height
2 - 4.99 metres
Dam flood category
C
Hazard class
High-risk reservoir
Reservoir use
- Conservation (ornamental / heritage)
- Recreation or general amenity
Owner type
Public body
Date & Time of incident
14 March 2024 - 12:00
Date incident closed
01 April 2024
Observations that caused the incident to be declared
- Slope or face deformation (slippage, cracking, slumps, mounds, depressions)
Describe the incident
A sinkhole developed on the dam downstream face some distance from the dam itself. No reduction of reservoir water level was required. The sinkhole was spotted during routine reservoir surveillance by the Undertaker, which triggered consultation with the Supervising Engineer's employer, as the SE was on holiday. The onsite plan had an alternative SE contact, who attended and agreed interim surveillance and actions with the Undertaker. An ARPE was informed and will oversee repairs with the Supervising Engineer. CCTV of the culvert has been undertaken along with some minor excavation. While the whole culvert could not be surveyed by the CCTV, the results indicated the culvert to be the cause at a location of two different culvert materials. It is clear that the incident related to a defect in the secondary overflow culvert. The defect was some distance downstream of the embankment and unlikely to impact the embankment in the short term. further CCTV to better inform what works will be required prior to excavating around the sinkhole to uncover the damaged culvert. The proposed details for the repair has yet to be determined but are likely to involve construction of a better connection detail between the culvert sections and possibly replacement of some sections of the culvert.
Supporting photos
Natural processes which initiated or contributed to the incident
What were the main contributing factors to the incident occurring?
Dam factors
- Deterioration of materials
External factors
- None
Shortcomings
- Construction shortcoming
What was the root cause of the incident?
Poor construction practice at the interface of different materials in the outflow pipe.
Impacts on the reservoir
- Internal erosion (fill deterioration)
- External erosion
Supporting photos
Describe any human factors which influenced the incident
This reservoir has no instrumentation. The site is monitored by freuqnet visual inspections by the undertaker.
Describe any instrumentation at the reservoir and how this was used in warning of the incident or providing monitoring during the incident
Not applicable
Was instrumentation effective leading up to and during the incident?
Not Applicable
Describe any assistance by external parties and impacts on the downstream population
CCTV of the culvert has been undertaken along with some minor excavation. While the whole culvert could not be surveyed by the CCTV, the results indicated the culvert to be the cause at a location of two different culvert materials. After the interim visit the Supervising Engineer discussed the incident with an ARPE and action was required to investigate and undertake repairs in the short-term, which remain to be completed. It is clear that the incident related to a defect in the secondary overflow culvert. The defect was some distance downstream of the embankment and unlikely to impact the embankment in the short term.
Summary of studies or investigations undertaken
Good surveillance after a significant rainfall event was key to spotting the the issue in a timely manner.
Lesson 1
- Surveillance and Monitoring
Consider proactive surveillance (CCTV) of pipeline through the embankment.
Lesson 2
- Emergency response
Having a backup Supervising Engineer contact within the S12 and Onsite plan assisted in gaining quick access to Engineering advice.