Behaviour of meandering two-stage channels

Engineers need methods for calculating river system performance for overbank stages involving curved channels. The need has become more pressing because engineering solutions are increasingly required to retain natural channel features for ecological and amenity reasons. Existing knowledge is deficient for such calculations. The Paper summarizes the scope and preliminary findings of an experimental programme undertaken at very large scale, with the objective of exploring the more important knowledge gaps. High quality data were obtained involving velocity, surface level and discharge observations, employing three channel geometries, two floodplain widths, and a variety of floodplain roughness states. The influence of these conditions on the stage-discharge relationship is presented, and some deductions are made from the velocity data about the flow structures which contribute to the stage-discharge behaviour. Vigorous secondary currents in the channel cross-section, and mass exchange between channel and floodplain flows are the major contributors to the substantial energy loss associated with the sinuosity of the lower channel. The interaction mechanism associated with overbank flow in straight channels has very little influence in this case. Floodplain roughness is important and should be studied further. Channel cross-section and plan geometry (sinuosity) are also influential. Considerably more work is needed to establish a sufficiently robust calculation method to reflect adequately the range of circumstances found in the field. The data acquired in this study will support extensive theoretical work, but further experiments will also be needed.