Unsteady bed shear stresses induced by navigation: Laboratory observations

Time-dependent bed shear stresses induced by the passage of a barge tow have been measured with hot film shear stress sensors in a 1:25 scale model. Conditions typical of those observed for Upper Mississippi River navigation traffic were simulated in the experimental facility. Two sets of experiments were carried out: the first set consisted of simultaneous shear stress measurements at different locations for a variety of flow depths and boat operating conditions, providing space-time distributions of ensemble average wall shear stresses. The second set included a large number of realizations gathered for one particular flow condition at a single position, allowing analysis of the time evolution of the turbulence characteristics (i.e., standard deviation) of the bed shear stresses. The results of the first set of experiments show that for all the experimental conditions the basic patterns of the shear stress are similar, with two regions of high shear stress associated with the passage of the bow and the stern of the barge tow, respectively. Analysis of the second set of experiments showed that, as a result of the passage of the barge tow, the bed-shear stress standard deviation departs from the values commonly observed under steady, uniform, open-channel flow conditions. This behavior has important implications for sediment transport.