A probabilistic approach to the analysis of contraction scour

The analysis of the progression in time of scour depth at the contracted section of a bridge crossing is an important tool to the design and monitoring of bridge foundations. The nature of the riverbed material greatly affects the pace of scour development. In particular, in cohesive soils scour can develop at considerably long time scales and therefore a realistic scour depth analysis should refer to a long sequence of river flows. A method for estimating contraction scour in clear water and unsteady flow conditions is here proposed, which accounts for the cohesion of the bed material. The method is based on the use of the energy balance equation through the contraction. The dependence of scour rate on the bed material cohesion is expressed through a non-linear (empirically derived) scour erosion function. The method has been applied to estimate the scour depth in cohesive soils along different time spans. To this end, a probabilistic framework has been developed to fit a stochastic process to observed mean daily flow record and generate synthetic replicates of the historical series, which are used in a Monte Carlo procedure to calculate the probability density function and exceedance probability of the estimated scour depth values.