Experimental study on local scour around bridge piers and its reduction

Abstract

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 and bed forms produced below the threshold condition due to very fine sediments. An additional complexity is the composition of the sediment, which is usually a mixture of alluvial sand and clay. Experience has shown all too often that scouring can progressively undermine the pier, as complete protection against scouring is usually prohibitively expensive. So, the designer must seek the ways to guide and control the process to minimize the risk of failure. Regarding all the complexity mentioned above, the present experimentation IS performed under mobile bed conditions with the aim of investigating the general characteristics of local scour, considering the riverbed materials and the pier shapes commonly employed in Bangladesh. Also experimental study is conducted to investigate the effect of cohesive sediment on local scour. An additional series of experiments is undertaken as the measures of reducing the potential local scouring around bridge piers. The total numbers of forty-five experimental runs are performed for this present case. In evaluating the general behavior of local scour, non-cohesive ripple forming sediment is used. In this case a flat bed cannot be maintained even under the critical flow velocity. Although critical flow velocity is considered a demarcation point for distinguishing clear water and live bed condition by most of the earlier investigators, this consideration is not valid for this particular condition. The scour map is constructed and analyzed to investigate the extent, slope and location of maximum scour hole. Also, parameters related to flow characteristics and pier geometry, which influence the maximum depth, are studied. To investigate the effect of cohesive sediment content in riverbed, five sediment beds with different percentage of cohesive and non-cohesive materials are undertaken. The scour map as well as the sediment property is analyzed. A decreasing tendency of maximum scour depth with increasing degree of cohesiveness is found due to increase in inter-molecular bondage. Reduction of scour is further enhanced by increasing the non-uniformity of sediment sizes, which leads to armouring phenomena in the bed and scour hole. Dimensionless sediment number (0*), a more representative parameter is introduced in this present study to express the characteristics of sediment mixture in terms of mean sediment diameter, sediment gradation, and percentage of both cohesive and non-cohesive sediment. Scour around different pier shapes, pier with collar and pier with splitter are also experimentally investigated to observe their efficacy as reduction measures in noncohesive sediment bed under live bed conditions. Circular and round-nose pier with different length width ratio, keeping a constant effective width shows a considerable reduction in scouring due to stream aligning effect. Collar, an effective reduction measure in case of clear water condition, is also applicable for live bed condition as it reduced the down flow, but the effectiveness decreases at a certain amount in this case. Collar with round-nose pier is much more effective as both stream lining and reduction in down flow occurs simultaneously. Splitter, by stream lining the flow, also reduce scour in considerable amount if it is constructed at the upstream face of the pier. Finally, a design guideline is proposed to estimate maximum scour in ripple-forming sediment bed where live bed condition typically prevails.