Discussion of "Unobstructed and Obstructed Turbulent Flows in Gravel

The paper by Tritico and Hotchkiss is based on river measurements and contributes to the understanding of fluid mechanics in gravel-bed rivers. It deals with processes related to sediment transport, river morphology, pollutant mixing, and fish habitats. The importance of boulders protruding from the riverbed that can locally influence the turbulence structure of the flow is also assessed. The discussers recently carried out field measurements in gravel-bed rivers, using a 3D acoustic Doppler velocity profiler ADVPRolland and Lemmin 1997. The turbulence structure was investigated in unobstructed flows and those with submerged obstructions. Here, we would like to complement some of the authors’ findings with results of measurements taken in the Swiss lowland river Venoge, whose hydraulic conditions are given by Franca 2005a. We measured 15 instantaneous velocity profiles forming a rectangular grid with five points in the transversal direction spaced at 10 cm and repeated in three planes spaced at 15 cm in the downstream direction. The blockage ratio in the river was h /D50= 2.9, whereas the authors’ measurements were made with blockage ratios of 5.4 to 8.3. A common feature of the two studies is the spatial variability of the riverbed roughness. Such river flow conditions as mean flow velocity, friction velocities, flow depth, and representative grain diameters are similar in both cases. We will discuss unobstructed flow, obstructed flow, and make some comments.