Frequency analysis of extreme scour depths at bridge piers and their contribution to bridge collapse risk

Bridge failures due to scour occur more frequently than expected, although bridge design considers conservative scenarios, evidencing a stochastic scour behaviour and the need for improvements in current design procedures. In this paper, a new method for determination of design local scour depth at bridge piers is proposed. The results show that peaks-over-threshold series are better suited than annual-maximum series of maximum scour depths for frequency analysis. Extreme scour depths follow the heavy-tailed Cauchy distribution. Scour depths with a 100-year return period resulted in 22 to 202% of the equilibrium scour depth currently used in design, evidencing the need for a different criterion to select the design scour depth. The proposed method computes the design local scour depth based on its probability of occurrence, introducing a novel probabilistic approach in bridge design for enhanced infrastructure risk management compatible with multi-hazard analysis.