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Abstract. A shear plate was developed to obtain direct measurements of bottom shear stress under nonbreaking surface gravity waves on a movable sand bed. The experiments were conducted under regular wave conditions in a large wave tank where time histories of bottom shear stress and surface elevation were obtained from a shear plate and wave gauges. Measurements were made at scales approaching those encountered in the field. These data were obtained under steep, vortex ripples where form drag dominated over skin friction for all data presented. Wave friction factors were calculated from measurements and were compared with existing theories as well as with previous laboratory studies. Values of the wave friction factor obtained using the shear plate exhibited reasonable agreement with existing theory. Close agreement between values of the wave friction factor obtained from the shear plate and those measured by previous studies demonstrates that use of a shear plate is a reliable additional method for the measurement of bottom shear stress over a movable bed in the laboratory. The purpose of the present work is the development of a novel experimental approach to obtain direct measurements of the total bottom shear stress for a movable sand bed. Since the measurement of the bottom shear stress is direct, it is not necessary to determine the rate of wave attenuation or to determine the velocity profile over the bed, which can be problematic during sediment suspension. The experiment was conducted in a large wave tank where bottom Reynolds numbers are of the order of 1 x 103-4 x 10 4. Measurements were made at scales that approach those encountered in field conditions. Results are compared with the previous laboratory studies to verify this experimental method. This paper is organized as follows. Section 2 describes the experimental setup, the instruments employed, and the calibration procedures used. The experimental protocol is outlined, and the test matrix is presented. Section 3 describes the measurements obtained. Section 4 discusses the procedure for estimating wave friction factors. The wave friction factors obtained in this study are compared with existing theory and previous laboratory measurements under movable bed conditions. Section 5 summarizes conclusions. Experimental Setup and ProcedureThe experimental work was conducted in Davidson Laboratory's high-speed towing tank. The concrete-sided tank is 95 m long and 3.7 m wide and accommodates water depths up to 17,011

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