Experimental evaluation of the friction slope in unsteady flows

Fent, Ilaria [UCL] Soares Frazao, Sandra [UCL]

(eng) Experiments of moderate unsteady flows over mobile beds are used to determine the friction slope from the measured flow conditions. The experiments concern the progressive bed evolution in a supercritical flow over steep slopes made of coarse sand and a dike-overtopping case, where the dike is made of fine sediment. Bed and water elevation were measured by digital imaging techniques. Using these measurements, the friction slope Sf is derived from three shallow-water models used for the description of such flows. The resulting Sf by these three models are compared in order to assess the validity of the proposed approaches. Then, the meas-ured friction slopes are used to calculate an equivalent Manning friction coefficient for these flows. The pre-liminary results tend to indicate that some unsteady flow effects can be identified through the comparison be-tween the Manning coefficients obtained from the measurements and the values obtained using empirical for-mulas based on grain diameters.