Resistance to Flow on a Sloping Channel Covered by Dense Vegetation following a Dam Break

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 wavefront, 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 were 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 wavefront 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 wavefront was quasilinear 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, nonuniformity, transient waves, and other flow disturbances were more likely to play a role when compared to conventional sheltering effects.