Ulley Spillway Incident
Publish date
02 September 2025
Case ID#
3117
Title
Ulley Spillway Incident
Nation
England
Regulator reference no.
307
Legal status
Statutory
Reservoir type
Impounding
Reservoir capacity
10 - 24,999m3
Year of construction
1850 - 1899
Main construction type
Earth fill embankment
Dam height
10 - 14.99 metres
Dam flood category
A
Hazard class
High-risk reservoir
Reservoir use
- Other
Owner type
Public body
Date & Time of incident
25 June 2007 - 12:00
Date incident closed
Observations that caused the incident to be declared
- Deformation or instability of embankment or dam wall (slumps, cracks, depressions, etc.)
- Slope or face deformation (slippage, cracking, slumps, mounds, depressions)
- Water flowing outside of engineered channels
Describe the incident
The dam had a masonry spillway channel at the toe of the embankment. Following heavy rainfall, a large volume of water flowed down the spillway channel. It is believed that turbulent water overflowed the spillway walls and also plucked masonry blocks out of the wall. This led to the spillway walls collapsing, which exposed the downstream face of the dam to erosion. As a result of existing fluvial flooding and the threat from the reservoir, many people living downstream were evacuated and major roads, such as the M1, were closed. The Fire Service used their high volume pumps to draw down the reservoir and temporary repairs were made. Refer to Bulletin No.1 for more details.
Supporting photos
Natural processes which initiated or contributed to the incident
What were the main contributing factors to the incident occurring?
Dam factors
- Instability
External factors
- None
Shortcomings
- Design shortcoming
What was the root cause of the incident?
Impacts on the reservoir
- Spillway or overflow - failure or instability
- 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
The incident was one that could not be anticipated by instrumentation. There are crest pins which are levelled annually. Readings taken after the event showed marginal change and the conclusion is the crest was unaffected by 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
The Country Park manager contacted the police. The police declared an emergency at about 10pm. The fire brigades from around England and Wales brought a total of thirteen high volume pumps (HVP). These were used, initially, to reduce the water level back down to the normal operational top water level (NOTWL) and then to reduce to 1m below NOTWL. On Wednesday 27th June, the HVP's were replaced by commercial pumps. The public and press were interested and wanted to have a look. The incident is in effect a construction site (multiple hazards normally associated with construction sites plus deep water, fast running water and darkness). An off-site press office was needed to field questions and provide escorts for visitors when the situation is safe. A member of the public expressed concern about the lack of an emergency telephone number at the reservoir. Given the flooding in the middle of the nearby city, it would not have helped to have a telephone number. A solution is to ensure that in the emergency plan, the police have contact numbers and that the public are encouraged to dial 999. It means the lines of communication are kept simple without overlap.
Summary of studies or investigations undertaken
The problem of the erosion at the end of the spillway is not possible to investigate in detail because the masonry walls on the embankment side of the spillway were completely destroyed and since then the area has been covered with granular fill.
As a result of the incident, the Supervising Engineer has recommended that a Section 10 Inspection be undertaken. This will include a number of studies.
Lesson 1
- General design and construction
Floods should be able to be directed not just over the weir and along the spillway but away from the toe of the embankment.
Lesson 2
- Structural
Where a channel is lined using masonry walls, what is the integrity of the masonry wall. If the flow gets out of the channel will the walls collapse leading to further erosion or will they stay upright and limit any erosion. The walls need to act monolithically which means that different skins of masonry need to be tied together. Also, any voids need to be filled to stop water getting into the masonry and pressurising the voids. There are examples from the incident of jets of water issuing from the walls and of individual masonry units being plucked out of the walls.
Lesson 3
- Structural
Where there is a change in gradient from steep to shallow, are the spillway walls on the embankment side high enough to direct the flow away from the embankment even if this causes erosion away from the embankment.