Discussion of 'Response of Velocity and Turbulence to Sudden Change of Bed Roughness in Open-Channel Flow' by Xingwei Chen and Yee-Meng Chiew

摘要

The authors have experimentally studied the variation of velocity profiles, development of the boundary layer, and the variation of bed shear stress in an abrupt change of bed roughness from a 1 mm sand bed to a 11.2 mm artificially roughened bed with marble. Velocity profiles in different sections as well as turbulence characteristics were measured with both a micro acoustic Doppler velocimeter (ADV) and a laser Doppler velocimeter (LDV), and bed shear stress was determined from velocity profiles as well as Reynolds stress measurements. After analysis of data, the authors concluded that in contrast with Nezu and Tomi-naga (1994), who reported an overshooting in stress as flow enters the rougher bed, the variation of shear stress was found to be gradual in their analysis. They also concluded that the response of the equivalent roughness height to the sudden change occurs gradually over a transitional length.