In this paper, shear plate was mounted on the bottom of a wave flume and direct measurements of the smooth and rough bed shear stress under waves were done in the term of the horizontal force exerted on the shear plates by the bottom shear stress in the wave boundary layer. It was conducted under immobile bed condition; grains of sand were glued uniformly and tightly onto the shear plate, which prevented the movement of sand with currents and the generation of sand ripples. The distribution of the bottom mean shear stress varying with time was measured by the interaction between the shear plate and shear transducers. The relationship between the force measured by the shear transducers and the voltage shown was linear. Simultaneous measurements of the bottom velocity were carried out by ADV. While the whole process was completely controlled by computers, bottom shear stress and velocity were measured synchronously. The 1/3 characteristic shear stress value under irregular waves was considered as the main factor for analysis. Compared with theoretical results and previous experimental data, it is shown that the experimental method is possible and applicable to the laboratory studies on the initiation of sediments and the measurement of the shear stress when sediments initiate.
Research on the wave boundary layer at present mainly focuses on the regular waves. You et al.(1991, 1992), You(1992, 1994, 1995) studied on velocity profile, bottom shear stress and friction coefficient inside the turbulent wave boundary layer. Research on irregular waves is relatively less but actually in the ocean area real waves are mostly irregular. Sledth et. Al(1987) and Jensen et. Al(1989) made measurements on the bed friction coefficient in the rough and smooth turbulent bottom boundary layers respectively. Myrhaug(1995) studied on the distribution of the maximum bottom shear stress under irregular waves in the term of the probability.
