Direct Measurement of Bottom Shear Stress under Water Waves

Wave-induced bottom shear stress is one of the most important parameters in modelling of wave hydrodynamics and coastal sediment transport. A new type of shear plate has been successfully developed to measure instantaneous wave bottom shear stress in a laboratory wave flume. The shear plate measures instantaneous horizontal force by applying the Wheatstone half bridge circuit to detect a tiny horizontal movement of the shear plate. There are about 280 individual test runs carried out over one smooth bed and two roughened beds, respectively. In each test run, instantaneous bottom shear stress was measured at a sampling rate of 10 Hz for about 10 minutes. The measured total horizontal force, which consists of wave-induced bottom shear stress and hydrodynamic pressure, is found linearly proportional to wave height in both laminar and turbulent flows. The wave friction factors measured on the smooth bed are shown to agree excellently with the theoretical values derived in laminar boundary layer flow, but those on the rough beds are affected by the bed roughness length estimated. An empirical formula is also proposed to compute the wave bottom shear stress.