ABSTRACT
We report here a laboratory study on the effects of bed roughness and emergent rigid vegetation on the unsteady flows associated with flash floods and extreme flood events. The flow structure of unsteady open-channel flows over rough bed with and without emergent rigid vegetation has been investigated in an 18m long and 3m wide laboratory flume. Steady uniform flows were also studied and served as reference flows. For both steady and unsteady flows, two geometries are tested: (1) uniform bed roughness (uniform dense synthetic grass modelling meadow) along the flume and (2) a uniform staggered distribution of emergent wooden circular cylinders (model of rigid vegetation) set on the bed roughness. Transient flow depths are simultaneously measured at six longitudinal positions using ultra-sonic sensors. Transient velocities are measured at one longitudinal position over the water column using a side looking ADV probe to estimate depth-averaged velocity. In order to compute ensemble averages of the flow parameters, 109 runs of the same hydrograph are carried out repeatedly at the flume entrance. Two consecutive runs are separated by a base flow. The ensemble averages of the measured discharge, flow depths and velocity are found to be converged after 72 runs. For the two geometries: at the peak flow of the hydrograph, the vertical profiles of mean streamwise velocity and horizontal Reynolds shear stress are comparable to the profiles obtained for steady uniform cases of same flow depth; and accelerated and decelerated velocity profiles are obtained during the rising limb and falling limb of the hydrograph, respectively. Reynolds shear stresses are also found to be higher during the rising limb than during the falling limb for a fixed flow depth. The hysteresis in the depth-averaged velocity/flow depth relationships is comparable in shape and size for the two geometries highlighting the weak effect of the rigid vegetation compared to the effects of un-stationarity.
