Sarfaraz A. Ansari

Influence of cohesion on scour around bridge piers

Experimental results on temporal variation of scour around circular bridge piers founded in cohesionless and cohesive sediments under steady clear water flows are reported. The difference between scour patterns in cohesionless and cohesive sediments is brought out. Considering the horse shoe vortex to be the prime agent causing scour, a procedure is developed for computing the temporal variation of scour depth in cohesive sediments. Empirical relationships have also been obtained for maximum scour depth around bridge piers in cohesive sediments.

Scour of a cohesive soil by submerged plane turbulent wall jets

The paper presents an interesting study into local scour in cohesive sediments. Experiments with cohesive sediments are not easy to conduct. However, the experiments are well documented and the Authors have to be congratulated for their contributions on such a significant problem of scour due to submerged plane turbulent wall jets in cohesive sediments. The database acquired will have lasting value for future researchers. The Authors have presented an interesting phenomenon of erosion characteristics (p. 198) in the form of small cohesive clods or lumps and they have found that the shape and size of the lumps of a cohesive sediment vary with respect to flow characteristics. For example, at higher velocities (of the order of 12.75 m/s) large chunks of clay with a size of 30^4-0 mm were found to occur and circular flakes 2-3 mm in diameter and less than 0.5 mm in thickness were found to occur at lower velocities (of the order of 5.16 m/s) for the same clay tested. Recently, a similar study on scour due to a submerged horizontal jet issuing from a sluice opening in a cohesive sediment has been also reported by Dey and Westrich (2003). Based on the range of jet velocity from 0.484 to 1.398 m/s, they have reported that the lumps were almost spherical shaped having approximate sizes of 1-5 mm diameter. Both the studies are concerned with the influence of flow characteristics on geometry of lumps for a cohesive sediment. The Discusser is indeed in agreement with the findings of the Authors. However, on the basis of a major experimental programme for study on the influence of cohesion on local scour, details of which are given in Ansari (1999) and Ansari et al. (2002a), the Discusser would also like to present interesting observations concerned with the influence of sediment characteristics on geometry of lumps for constant flow conditions. Observations revealed that the shape and size of lumps also depend upon several factors concerned with sediment characteristics such as type and quantity of clay content present in cohesive sediments, plasticity index, antecedent moisture content, dry density, etc. Figure 1 is presented for some typical cases as illustration. It is evident that for constant flow parameters the