Depth-Ratio Effects on Flow in a Partially Vegetated Compound Channel

Laboratory experiments were conducted to investigate the effects of the relative depth ratio, Dr, on flow in an asymmetric compound channel with a partially vegetated floodplain. Five cases covering Dr=0.15−0.52 were examined. Partial-width vegetation produced two lateral shear layers: the main-channel/floodplain (MCFP) layer and the non-vegetated/vegetated-floodplain (NVV) layer. As Dr increased, streamwise velocity and discharge were redistributed from the main channel toward the floodplain: the main-channel discharge fraction decreased from 83.7% to 56.6%, while the combined floodplain fraction increased from 16.31% to 43.33%. The dimensionless shear parameters λMCFP and λNVV decreased from 0.584 to 0.064 and from 0.741 to 0.316, respectively, with λNVV>λMCFP in all cases. The maximum local Reynolds shear stress occurred near the NVV interface. For the four cases measured using an acoustic Doppler velocimeter (ADV), mean transverse advection dominated the total depth-averaged transverse momentum exchange, and its case-wise maximum magnitude exceeded those of the Reynolds-stress and dispersive contributions by factors of 71.4–356.3 and 74.2–556.7, respectively. Spectral analysis indicated that large-scale coherent motions over the floodplain were strongest under shallow-flow conditions and weakened as Dr increased. These findings show that Dr regulates the relative roles of the two shear layers.