Erosion during overflowing from water flowing over piles and onto embankment
Publish date
02 September 2025
Case ID#
3118
Title
Erosion during overflowing from water flowing over piles and onto embankment
Nation
England
Regulator reference no.
308
Legal status
Statutory
Reservoir type
Non-impounding
Reservoir capacity
10 - 24,999m3
Year of construction
1970 - 1989
Main construction type
Earth fill embankment
Dam height
2 - 4.99 metres
Dam flood category
B
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
- Dam or embankment overflowing or overtopping
Describe the incident
In one case, a flood storage reservoir overflowed because its capacity was exceeded. In the other, a river overflowed the defences surrounding it and water flowed into the adjacent flood storage reservoir. In both cases, the embankments had been raised with steel sheet piles. When the water overflowed the sheet piles it dropped vertically onto the earth embankment below causing erosion.
Supporting photos
Natural processes which initiated or contributed to the incident
What were the main contributing factors to the incident occurring?
Dam factors
- None
External factors
- None
Shortcomings
- Design 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
There is no instrumentation
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
The cause of the damage is known and no studies are necessary.
Lesson 1
- Structural
The reservoir was not provided with a dedicated outflow spillway and, had it not been for the need to raise the embankment crest level with sheet piles due to mining subsidence, a well maintained grass embankment may have withstood the flows which have been discharged. The use of sheet piles to raise any embankment which may be subject to overtopping requires very careful detailing and should be avoided, if possible. The emergency reconstruction work involves removal of the sheet piles and raising/re-profiling of the embankment crest with clayey material to an improved profile (1 on 3 side slopes and a 3m wide crest. The crest, a busy public footpath, is to be protected with grasscrete and the grass on the downstream slope is to be strengthened with 'Enkamat' geotextile matting, thus forming a spillway which is designed to withstand any future overtopping events.