Historical events provide a valuable source of information about the risk environment. Risk analysts recognise a substantial degree of variability in the way in which a hazard event evolves. Alternatively, a historical event might have evolved to yield a lower or higher societal loss. These better or worse alternatives are referred to as upward and downward counterfactuals. For enhancing risk awareness, it is instructive to explore downward counterfactuals.
Typically, assessments of extreme events are undertaken by hazard domain experts. However, there is no systematic procedure for spotting missing extreme events. Furthermore, group meetings may be prone to groupthink, or other forms of human cognitive bias. For disastrous river floods, Merz et al. (2021) have suggested that biases of wishful thinking may be avoided by purposefully constructing downward counterfactuals.
An innovative systematic procedure for searching for extreme hazard events has been developed by the author. This procedure can take the form of a round-table game, in which each person takes a turn in suggesting a further downward counterfactual; a way in which the loss might have been worse. This game has some similarities with the traditional Victorian parlour game of consequences, in which each player follows on from the preceding player. This type of parlour game is a socially engaging and instructive way for players to explore the range of extreme events.
This round-table game of exploring downward counterfactuals, which could be played for any geohazard, is illustrated by UK flood risk. One of the most salient near-miss events arose from persistent rain across the Peak District and Yorkshire Dales at the end of July 2019. when half a month’s rain fell. Water poured through the spillway of the Toddbrook Reservoir dam, above the town of Whaley Bridge. The spillway started to erode and concrete ballasts began to rip away. More than 1,500 residents of Whaley Bridge had to leave their homes for six nights after the reservoir dam threatened to breach.
A round-table game would generate a series of notable downward counterfactuals of the 2019 flood risk at Whaley Bridge. These would be informed by meteorological data, and the independent Toddbrook reservoir review report, which concluded it is unlikely that the spillway would have survived the probable maximum flood, and that if the event had been more intense, or extended for a longer period, catastrophic failure of the dam may have occurred.
Few UK catastrophic dam failures have occurred, and fortunately there has been no loss of life due to dam disasters in the UK since 1925. Through downward counterfactual games, exploration of near-misses, such as the severe rainfall of July 2019, can provide important insight into the risk of catastrophic dam failure, and the timeliness of risk mitigation measures.
 Merz B. et al. (2021) Causes, impacts and patterns of disastrous river floods. Nature Reviews, 2, 592-609.
 Woo G. (2019) Downward counterfactual search for extreme events. Frontiers in Earth Science. https://doi.org/10.3389/feart.2019.00340