Probabilistic bridge scour estimates

Scour at bridges is generally estimated as a deterministic value. Other loads at bridges, such as vessel impact and wind loads, are determined as probabilistic values. In an effort to use probabilistic estimates as a tool in decision making, a probabilistic framework was developed for estimating scour using deterministic methods given in the Hydraulic Engineering Circular (HEC-18). Uncertainties in the HEC-18 model, in determination of the parameters and in estimating the hydraulic variables for a large event storm, were included in the analysis. The probabilistic framework can then be used as the basis for determining the likelihood of achieving various scour depths, probabilities of failure for various foundation designs, pile depths necessary to achieve a specified probability of failure for a design bridge life span, and for comparing designs based on various storm events. The Bonner Bridge in North Carolina is used as an example. Pile depths appropriate for various design life spans are calculated based on both 100- and 500-year storm events.