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Design Blamed for England Spillway Failure

Monday, March 23, 2020

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Recently, Executive Technical Director from international engineering company MWH, David Balmforth, published an 85-page independent review report regarding last year’s partial collapse at a Toddbrook Reservoir dam in Whaley Bridge, England.

Balmforth is also a visiting professor at Imperial College London in the Department of Civil and Structural Engineering, and was previously the President of the Institution of Civil Engineers.

What Happened

In 2019, between July 27 and Aug. 1, heavy rains plagued Whaley Bridge, causing a spillway on the Toodbrook Reservoir to fail. According to Balmforth’s report, the weather was well forecasted and fell in two separate events: first occurring between July 27-29 and again, more severely, from July 30 - Aug. 1.

At the time of the incident, The Guardian reported that an excess of water ran over the spillway above the town on Thursday (Aug. 1), and as a result, the spillway began to erode, also causing the concrete ballasts to move away.

According to reports, the dam was built in 1831, and the auxiliary spillway is thought to have been added in 1969.

A Canal and River Trust employee tried to reduce water levels by opening valves, as well as clearing debris and continuing to track the valves, but there was too much overflow to account for, and the clay wall began to be washed away. Authorities were notified of the situation soon after.

Around that time, police, the Canal and River Trust and the Environment Agency have all said that there was a risk the dam could collapse. During the first few hours of tackling the water level, only one pump was available. Others were brought in and a local civil engineering company built a temporary road to facilitate pump transport. Among tools used, 11 steel pipe hoses—each 500 feet long and 12 inches in diameter—were implemented.

As a result of the incident, roughly 1,500 residents were evacuated after the dam ruptured, although a few residents refused to leave regardless of warnings.

To mediate the infrastructure’s possible collapse, a RAF Chinook helicopter delivered 400 tons of aggregate—which largely consists of sand, gravel and stone—to help stem the flow of water into the reservoir. The same helicopter was also used to shore up the damaged face of the structure.

Additional best practices at the time included draining the dam of 5.7 meters of water over a course of five days, putting it at 46% holding capacity. However, officials reported that the reservoir would need to be lowered to 25% capacity before residents could return home.

Reservoirs with a capacity of over 25,000 cubic meters above ground level located in England and Wales are required to comply with the Reservoirs Act, which includes having a supervising civil engineer and an inspecting civil engineer file annual safety reports. The Trust maintains that the Toddbrook Reservoir was “absolutely fine.” An independent inspection also takes place once every 10 years.

The Trust reported that the last independent inspection took place in November 2018 and was signed off by the independent Panel Engineer and CRT Supervising Engineer in April 2019.

The Report

Appointed by the Secretary of State, Balmforth lead the review and was assisted by Peter Mason, an All Reservoirs Panel Engineer, and Paul Tedd, a specialist in earth embankment dams. The panel of three reviewed evidence from the incident, visited the reservoir and inspected the spillway in detail, in addition to reviewing documentation from the Canal & River Trust and the Environment Agency.

In conducting a complete report, the panel also interviewed Supervising and Inspecting Engineers, CRT Engineering staff, the engineers from CRT, staff from the EA who deal with regulations and enforcement, and Mott MacDonald who responded to the emergency.

From the findings, the panel reports that the most likely cause of the infrastructure failure was the structure’s poor design, which was then worsened by intermittent maintenance over the years, causing the spillway to deteriorate.

“The lack of an effective cut-off between the spillway crest and the impermeable core of the dam would have allowed water to pass into the embankment fill under the spillway chute. While some of this will have drained downwards through the permeable fill, it is likely that some will have flowed beneath the slabs of the spillway chute causing erosion of its foundation,” the report states.

“In addition, seepage of water through a construction joint in the crest had been observed from time to time flowing down the face of the spillway, and this is likely to have seeped into the longitudinal joints which were not fitted with water bars and had only received intermittent maintenance.”

Since the incident, the inspecting engineer of the structure has required that CRT complete repair maintenance within 18 months, while Balmforth has issued 22 recommendations to improve safety across the entire reservoir network.

His recommendations have since been accepted by the government.

Environment Secretary George Eustice added, “'This review provides the whole reservoir industry with important lessons following the incident that took place at Toddbrook Reservoir last year, and I hope it also reassures the public that we remain committed to ensuring the safety of reservoirs across the country.

“There is nothing more important than the safety of our communities, and I expect the Environment Agency and reservoir owners to immediately take action on these recommendations in order to ensure the continued safety those living and working near reservoirs.”

Moving forward, Eustice has requested that Balmforth lead a second stage of the review, conducting an even larger assessment of reservoir safety legislation.

   

Tagged categories: Environmental Controls; EU; Europe; Infrastructure; Infrastructure; Inspection; Locks and dams; Reservoir; Safety

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