Ž . Coastal Engineering 37 1999 529–543 www.elsevier.comrlocatercoastaleng The estimation of shear stresses from near-bed turbulent velocities for combined wave–current flows J. Wolf ) CCMS-Proudman Oceanographic Laboratory, Bidston ObserÕatory, Birkenhead, Merseyside, L43 7RA, UK Abstract High frequency near-bottom velocities measured by a combined pressure gauge and flow-meter are analysed with a view to understanding and modelling waves and sediment transport in shallow waters. Data from five stations in different water depth, wave and current conditions are compared. Wave, tidersurge and turbulence components have been separated from the bottom velocity data and are used to derive a friction velocity and examine the bottom stress parameterisa- tion. The horizontal component of turbulent velocity fluctuations near the bed is found to be well-correlated with the bed shear stress from a combined wave and current model. The model of w Christoffersen and Jonsson Christoffersen, J.B., Jonsson, I.G., 1985. Bed friction and dissipation x in a combined current and wave motion. Ocean Engineering, 12, 5, 387–423 is in reasonable agreement with the observed data. The observed turbulence can shed light on the wave bed shear stress as well as the shear stress due to the current. A fitting procedure gives a derived Nikuradse length scale for each station which is similar to that obtained independently from bed grain size estimates. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Wave-current; Shear stresses; Near-bed velocities 1. Introduction Ž During the EC MAST SCAWVEX Project Surface Current and Wave Variability . Experiment , several datasets were collected using bottom-mounted InterOcean S4DW Ž . Directional Wave current meters, in the near-shore region of the North Sea. Three ) Fax: q44-151-653-6269; E-mail: judith_wolf@compuserve.com 0378-3839r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0378-3839 99 00042-3