Flow structures in a compound meandering channel with flat and natural bedforms
Detailed experiments were conducted on a meandering compound channel, with a sinuosity of 1.384, in a 13m long 2.4m wide flume. Two cases were examined, where the main channel contained flat and natural bedforms. Measurements recorded include stage-discharge, sediment transport and bed shear stress. A three-component Laser Doppler Anemometer measured the velocity and turbulence in the flow and the bedform was measured using Digital Photogrammetry. It was found from the stage-discharge data that at most depths the effect of the bedforms is to reduce the discharge capacity of the channel. The maximum reduction in the discharge capacity was at the bankfull flow depth where the discharge was reduced by thirty percent. The sediment transport rate was found to decrease at relative overbank flow depths of 0.2-0.3. The velocity and turbulence measurements were used to examine the flow structure. It was found that the formation of bedforms in the main channel significantly affects the flow structure of the flow in the main channel, although the flow on the floodplain is similar. Significant secondary flow circulations were found in the natural bed case, particularly at higher flow depths. The secondary circulations are caused by centrifugal force, flow entering the main channel from the floodplain and reverse flows as the flow passes over ridges in the natural bed case. A new method for predicting velocity and discharge in meandering channels has been introduced based on the two-dimensional curvilinear equations for strearnwise motion. The turbulence terms were found to be insignificant and the method was applied to data sets at different scales.