Laboratory measurement of bottom shear stress on a movable bed

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. Copyright 2000 by the American Geophysical Union.