Estimation of Bed Shear Stress Using Inertial Dissipation Method Under the Presence of Wave Motions

Bed shear velocity controls not only erosion and deposition of sediment at the bed, but also diffusion of sediment into the water column. Accurate estimation of bottom shear velocity therefore is important to predict the rate of sediment transport. As a part of National Research Laboratory sediment transport study along the coast of Southwest Korea, we deployed a benthic boundary layer tripod at about 15 m depth in the inner shelf off Jindo from April 6 to April 10, 2004. Flow velocity was measured using an Acoustic Doppler Velocimeter at 35 cm above the bed. Measurements of velocity fluctuations were used to estimate the shear velocity using the inertial dissipation method. The inertial dissipation method with a standard burst-averaged velocity as the dominant flow velocity underestimated the shear velocities more than 50 percent. During the experiment, the study area was under the influence of significant wave energy. In such a condition, kinetic effects of wave motions on the estimation of shear velocity must be considered. A method to account for the wave effects is to use a root-mean square wave speed for the dominant advection speed. This method improved the estimation of shear velocity substantially.