Case studies

Deep seated slip in bank

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.

16 October 2006 | England

Breach of SRR from flood

This dam failed due to flood inflow. The dam overflowed causing erosion of the downstream shoulder and eventual breaching of the dam. The embankment was inadequately designed, as it was constructed of an uncontrolled mixed fill material, which contributed to the failure. The spillway provision was also inadequate

22 May 2006 | England

Leakage from poorly sealed joints

Increased leakage from a point two-thirds down the downstream face, adjacent to a spillway structure, was noted. The water level was lowered as a precaution and the leakage stopped. Regular surveillance of the toe area had not been carried out. Leakage was attributed to poorly-sealed joints in the spillway structure.

28 December 2006 | England

Leak in clay core

The leaking reservoir was confirmed to be the non-impounding reservoir. During 8 June they also decided to make contact regarding use of the New Dimension pumps as a contingency. The Fire & Rescue Service have deployed one New Dimension pump, which has removed 10 Ml. Another pump will be arriving tonight. These will remain in place until the undertaker's contractor arrives with their pumps. The undertaker has signed up to pay for the use of the ND pumps. The Inspection Engineer was called on Wednesday evening when the problem first arose and was present on site from Thursday morning, providing advice and agreeing actions. The undertaker is currently monitoring the leak and dropping the level of the reservoir. They are monitoring for any change in the leak, e.g. an increased amount of water, or in the physical conditions associated with it, e.g. development of a pile of spoil. The water flow is reduced and the water is running clear. There was an existing leak but the patch became wetter and it was not possible to mow the grass as the ground has become boggy. Divers went in on 9 June to check the leak in the clay. They have possibly found the leak, a puncture in the clay. If this is the case it will be fixed with a membrane. They are also lowering the water level. The reservoir is fed solely by a large aqueduct and therefore this feed has been stopped. The water treatment works is run when necessary and not all the time. The undertaker started it up during 8 June but it was not working properly. This is why the New Dimension pumps are being considered. The water treatment works are now up and running and have removed 40 Ml from the reservoir. In addition the scour valve has removed a further 10 Ml.

| England

Seepage from voids in masonry and tree roots

The owner of the reservoir noticed leakage passing around the outlet structure at the toe of the embankment. The reservoir level was lowered by pumping water over the crest. Leakage through the masonry wall forming the upstream face of the dam and through the sides of the drop shaft on the line of the dam crest had been occurring for many years. Drilling was carried out vertically from the dam crest and from within the spillway drop shaft to fill voids and stem the leakage. The voids behind the drop shaft wall, possibly together with rotten tree roots within the embankment, probably contributed to the new seepage path developing between the drop shaft and the downstream toe.

05 July 2006 | England

Ulley Spillway 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.

25 June 2007 | England

Erosion during overflowing from water flowing over piles and onto embankment

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.

25 June 2007 | England

Erosion during overflowing from water flowing over piles and onto embankment

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.

26 June 2007 | England

Series of SRR weirs came close to breaching in flood event

A series of weirs, already in a poor condition, were damaged during a flood event. One weir in particular, about 1.5m high and impounding about 12,000m3 , came close to breaching due to erosion around an abutment. It was not possible to access the sluice gate to lower the water level until the flood subsided

02 July 2007 | England

Leakage from within tunnel in bank

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.

08 April 2006 | England