Evaluation of friction velocity in unsteady flow experiments

The paper presents estimates of the friction velocity in unsteady flow using two formulas that have been derived from the two-dimensional Reynolds and one-dimensional Saint-Venant equations. Experiments on unsteady flow were performed to collect data on the spatio-temporal evolution of the water level and velocity necessary to apply the formulas. The key feature of these experiments is high temporal resolution of the data. The application of the formulas is presented with an in-depth analysis of the flow conditions, methods for estimating the input variables and an uncertainty analysis of the results. The evaluation of the input variables focuses on the spatial derivative of water depth, which is the most problematic variable in this study. A comparison of the formulas for the friction velocity shows that they are equivalent for the cases presented in this paper.