The effect of hydraulic resistance on the downstream evolution of the water surface profile h in a sloping channel covered by a uniform dense rod canopy following the instantaneous collapse of a dam was examined using flume experiments. Near the head of the advancing wave front where h meets the rods the conventional picture of a turbulent boundary layer was contrasted to a distributed drag-force representation. The details of the boundary layer around the rod and any interferences between rods was lumped into a drag coefficient Cd. The study demonstrated the following: In the absence of a canopy the Ritter solution agreed well with the measurements. When the canopy was represented by an equivalent wall friction as common when employing Manning's formula with constant roughness it was possible to match the measured wave front speed but not the precise shape of the water surface profile. However upon adopting a distributed drag force with a constant Cd the agreement between measured and modeled h was quite satisfactory at all positions and times. The measurements and model calculations suggested that the shape of h near the wave front was quasi-linear with longitudinal distance for a constant Cd. The computed constant Cd(≈0.4) was surprisingly much smaller than the Cd(≈1) reported in uniform flow experiments with staggered cylinders for the same element Reynolds number. This finding suggested that drag reduction mechanisms associated with unsteadiness non-uniformity transient waves and other flow disturbances were more likely to play a role when compared to conventional sheltering effects.
