Spring 2005

  Computation of Local Scour Around Bridge

Ahmed Asif Reza


There is a worldwide concern on damage of bridge caused by local scour around bridge pier. Structural failures of bridge induced by local scour usually occur in extreme cases of flood flows interacting with pier or when considerable sediment transport occurs. Experiences has shown all too often that scouring can progressively undermine the structure. A through study has been conducted to understand the state of art. So, to calculate the maximum locals scour depth at bridge location of Ghior Khal in north central region three methods (FHWA, Laursen’s and Breuser’s) for piers and two methods (Liu et al Method and Simon & Sentruk’s) for abutments are selected in the present study.

Local scour at piers has been studied extensively in the laboratory. As a result of the many laboratory studies, there are numerous pier scour equations. Scour depth depends on various parameters (such as depth of flow, shape of pier, angle of attack of flow, sediment size etc.). Different engineers have evaluated their equations to predict scour depth using different parameters and field of application. Thus to investigate the behavior of local scour against different parameters a sensitivity analysis is being made in the present study for circular pier.  

To protect water structures such as bridges from failure it is very important for the engineers to find out the most dominating parameters that are affecting the maximum local scour depth. The obtained dominating parameter should be given majority during design and construction of the structure. In the present study it is found that the Laursen’s Method gives the highest scour depth, FHWA’s Method gives the lowest scour depth and the Breusers Method gives the value in between. From which the maximum scour depth is selected. It is also found that velocity is of greater sensitive than pier width followed by flow depth and angle of attack of flow is insignificant if the structure (Pier) is circular.