Observations of bottom stress in combined waves and currents

Observations of currents, waves and sediment transport were made during the LEACOAST2 experiment. The objective is to understand the sediment transport in this energetic environment in combined waves and currents. For this it is necessary to calculate the bed shear stress. The use of observed data for Reynolds’ stress calculations is problematical for combined waves and currents. Direct Reynolds’ stress calculations in combined waves and currents are contaminated by waves even though the instrument should be outside the wave boundary layer (although within the tidal current boundary layer) even when the observations are corrected for tilt. In theory there should be no contribution from the irrotational wave motion in the covariance method of obtaining the bottom stress but in practice a very small error in tilt can produce an error contribution from the wave motion which can overwhelm the turbulent contribution. It is not straightforward to remove the effect of waves, since waves and turbulence co-exist at wave frequencies. Various methods for separation of waves and turbulence are examined.