A Reliability-Based Scour Depth Estimation in Indian Alluvial Rivers

The inherent uncertainties in the estimation of bridge pier scour depth suggest that the reliability of bridge piers must be assessed to ensure desirable safety of bridges against scour. Use of reliability tools is an important step in examining the relationship between pier safety factor and risk, which are considered to be the key parameters for decision-making in bridge foundation design. The Lacey's scour equation is one of the earliest formulas for the scour depth prediction. It is still in practice in Indian subcontinent although many other scour prediction equations exist. In the present study, a procedure for the reliability assessment of bridge piers against local scour is presented using Lacey's scour depth prediction equation. First Order Reliability Method (FORM) and Monte Carlo simulation technique have been applied for this purpose with due consideration of correlation among the design variables. A simplified relationship between safety factor and reliability index has also been proposed for direct application of reliability values in the design process. Effect of flow discharge and sediment size on pier reliability against scour is investigated. In order to highlight the application of presented methodology, reliability of Elgin Bridge over an Indian river called ‘Ghagra’ is estimated and the estimated reliability is used to explain root cause behind sequence of problems faced by this bridge since its construction. Some parametric studies have also been included to obtain the results of academic and practical interest.