Comparison of three different methods for assessing in situ friction velocity: A case study from Loch Etive, Scotland

Three approaches, Eddy Correlation (EC), Turbulent Kinetic Energy (TKE), and Inertial Dissipation (ID) methods, were compared to evaluate their potential for estimation of friction velocity in a Scottish sea loch. As an independent assessment parameter, we used simultaneous O2 recordings of the diffusive boundary layer (DBL) that were compared with theoretical distribution as derived from the respective friction velocity estimates. Friction velocities were calculated based on the three approaches using continuously measured turbulent properties, and values estimated by the TKE method were significantly higher than values of the two other approaches. Time series of calculated friction velocity were subsequently employed as input parameters for nonsteady calculations of the O2 distribution within the DBL. The friction velocity is a controlling factor for the eddy diffusivity distribution immediately above the sediment surface and for the DBL thickness, and friction velocity values of the TKE method derived significantly higher theoretical O2 concentration while the EC and the ID approach provided results that were not significantly different. Overall differences from measured O2 concentration in the DBL were 0.2% for the EC method, 9.8% for the TKE method, and 0.7% for the ID method. The results reveal that the EC method appear to be the best approach for estimating friction velocities though not significantly different from the ID method, whereas the TKE method was unreliable at the ~70 m deep, relatively calm study site. This information is important for future research in similar sounds, fjords, and sea-lochs.