A cascading risk model for the failure of the concrete spillway of the Toddbrook dam, England during the August 2019 flooding

Dam break is considered as a major catastrophe with significant negative economic, social, and environmental consequences, and thus must be prevented at any cost. Here, we report and analyze a near-miss dam break incident in Toddbrook dam, England during the August 2019 flooding, where the spillway of the dam failed putting the entire dam at the risk of failure. A combination of field surveys, desk studies and numerical modelling is applied to analyze the incident and to develop a cascading risk model for the first time. Our hydraulic modelling showed that the spillway was under fast-flowing water having a speed of up to 15.0 m/s. Such a high-speed flow played a major role in the failure of the spillway through facilitating water injec-tion beneath the spillway slabs. The spillway suffered from poor maintenance and was densely vegetated, which most likely undermined the foundation. The spillway was poorly designed as the concrete slabs were relatively thin and unreinforced, the profile of the spillway was not fit for purpose, and the spillway lacked a stilling basin. Due to rapid drawdown, a landslide was generated on the upstream slope of the dam, which was reconstructed through our geotechnical modelling, indicating that a slower pace must have been taken during the process of emptying the reservoir. We developed a cascading risk model which begins with three primary causes of insufficient maintenance, design shortcomings, and the torrential rainfall leading to flooding. Our risk model, which is among the first of its type, would help preventing future dam failures.