Local scour with angle of attack

Abstract

The purpose of this research is to study the influence of an angle of attack to the maximum depth of scour. An aligned pier at normal discharge may change to an angle of attack at flood flows and vice versa, as a result of the river bed changes. Unfortunately, the maximum depth of scour at an angle of attack will be larger than that of an unskewed pier. This study is limited to blunt-nosed piers since a skewed pier will act as a blunt-nosed pier, eventhough it is a-sharp-nosed pier at zero angle of attack. Theoretical analysis shows that maximum depth of scour depends on the strength of horseshoe vortex system which is initially a function of pier Reynolds number. Therefore, maximum depth of scour should be a function of pier Reynolds number also. The influence of an angle of attack to the maximum depth of scour can be expressed as a coefficient of pier skewness which is defined as the ratio of the maximum depth of scour at an angle of attack to that of unskewed condition. Since the maximum depth of scour is a function of pier Reynolds number, therefore coefficient of pier skewness can be expressed as a function of the ratio of pier Reynolds number, and for the same flow condition it will be a function of the ratio of pier projected width. Experimental results show a good agreement with the theoretical consideration, A general formula for design purposes of the influence of pier skewness to the maximum depth of scour has been derived. It was concluded that the expression from Shen et al (1966) may be used as an upper envelope of the coefficient of pier skewness.