The causes of the Whaley Bridge dam failure

Back in August 2019, the Whaley bridge dam partially collapsed and officials evacuated a town in fear of a total failure. Now, the causes of the dam's failure have been revealed.

The dam is established in Toddbrook Reservoir which is located near the small town of Whaley Bridge in Derbyshire, United Kingdom. Made out of earth material, it was highly damaged after an intense storm struck the area. Authorities tried to ameliorate the conditions by pumping water out of the dam while the residents of the Whaley Bridge were evacuated.

After the failure, UK's government issued a detailed investigation to figure out the causes of the partial failure. This case study would then be used to propose measures in order to prevent similar incidents in the future. The project was led by David Balmforth, visiting Professor at the Department of Civil and Environmental Engineering at Imperial College London.

According to the report, issued by Prof. Balmforth on February 10, 2020, the dam's failure was a result of the spillway's inadequate design. In addition, the structure's deficiency deteriorated over the years since maintenance works were not properly conducted. The report mentions that if the spillway was efficiently maintained it would have withstood the rainfall event. Nevertheless, it would still be deficient during a maximum flood scenario.

The particular facts that made the spillway's design problematic are the following (all of them contributed to the failure):

1. The spillway's concrete slabs were designed thinner and with less steel reinforcement than necessary.
2. The transverse joints (grooves in concrete slabs that create weakened vertical planes) used insufficient dowel bars.
3. No dowel bars were utilized for the longitudinal joints.
4. There was no underdrain option to the dam.
5. There were no barriers between the dam's crest and the clay core of the dam, a fact that allowed water to deteriorate the spillway when the water level was high.

The scientific team focused on measuring settlements of the dam by manipulating satellite data which, according to the report, are not currently widely utilized to access the condition of similar structures. An unexpected subsidence of the crest was observed in July 2018 while the spillway chute starting settling in January 2019 and continued until the time of the failure.

The report presents a 22-point summary of recommendations on improving the current inspection, supervision and regulation states in the UK's dams. 

Prof. Balmforth stated that "the incident at Toddbrook Reservoir is a sharp reminder that all our reservoirs need to be looked after and managed well."

You can find the full report issued for the Toddbrook Reservoir failure here