Scouring around circular compound pier bridge

The knowledge of the maximum depth of scour around such structures is essential from the point of view of safety of these structures; excessive scour can undermine the foundations and lead to the failure of the structure. Alluvial streams are sometimes partially obstructed by hydraulic structures such as spurs, bridge piers, abutments, guide banks etc. In some other cases high velocity sheets of water from spillways and sluice gates flow over loose alluvial material. In all these cases the bed level in the vicinity of the structures is lowered as a result of interaction between the high velocity flow and the loose bed and consequent modification in the flow pattern, such local drop in the bed level is known as local scour. Proper design requires that the foundation be taken down to a level lower than the anticipated level of the scour hole. It has been reported that since 1950 over 500 bridges have failed in U.S.A. and majority of failures were due to scour of foundation material. In the Indian context, such data on number of failed bridges due to scour are not easily available. However it is known that scour has been the matter of concern to the railways, and detailed hydrologic and scour studies have been undertaken by RDSO, Lucknow. Three major reasons for such a concern can be mentioned namely

(i) Inadequate knowledge of scour phenomenon when these bridges were constructed; (ii) these bridges were designed probably for maximum observed flood upto the time of design. However, since flood is a probabilistic phenomenon, floods of higher magnitude of larger return period must have occurred and for which foundation was no designed.

(iii) lastly the size of wagons, trucks and their frequency has significantly increased which has increased the load on the bridge.