(7a) Analysis of Risk Due to Compound Extreme Events

Extreme weather events often include compound, connected events that individually amplify the impacts to process safety in the industrial sector. During Hurricane Harvey (2017) an extreme event triggered by prolonged rainfall over the Houston metropolitan area was made more extreme due to compound influences of coastal storm surge, which was not extreme in itself but contributed to reduced discharge of water over the inland area. Compound effects led to significant operational, safety, financial, and regulatory ramifications, such as the dozens of chemical release events documented after Harvey.

Often hazard risk assessments focus on one factor rather than compound, multiple forcing mechanisms. It is possible that extreme event analysis in a univariate context may underestimate probabilities and impacts of extreme events. In this study, a new framework addresses multivariate analysis of risk from compound hazards. Observations and simulations are used to identify expected frequencies of chance extreme events forced via connected individual events. The framework emphasizes the statistical dependence of multiple physical variables that contribute to extreme compound events when individual events are not extreme. Finally, the use of a compound hazard model as input to a full performance-based methodology that includes structural, damage, and loss analyses is defined.