The advancing wave front on a sloping channel covered by a rod canopy following an instantaneous dam break

The drag coefficient C-d for a rigid and uniformly distributed rod canopy covering a sloping channel following the instantaneous collapse of a dam was examined using flume experiments. The measurements included space x and time t high resolution images of the water surface h(x, t) for multiple channel bed slopes S-o and water depths behind the dam H-o along with drag estimates provided by sequential load cells. Using these data, an analysis of the Saint-Venant equation (SVE) for the front speed was conducted using the diffusive wave approximation. An inferred C-d = 0.4 from the h(x, t) data near the advancing front region, also confirmed by load cell measurements, is much reduced relative to its independently measured steady-uniform flow case. This finding suggests that drag reduction mechanisms associated with transients and flow disturbances are more likely to play a dominant role when compared to conventional sheltering or blocking effects on C-d examined in uniform flow. The increased air volume entrained into the advancing wave front region as determined from an inflow-outflow volume balance partly explains the C-d reduction from unity.