STAGE DEPENDENCY FOR OVERBANK FLOW IN MEANDERING CHANNELS

Measurements of stage for a range of discharge, and observations of the velocity fields, were made for overbank flow in a small laboratory channel. The velocity field measurements were supplemented by dye visualizations and plaster of Paris explorations of boundary shear stress directions at a typical channel bend. The influence of inner channel sinuosity and crosssection were investigated. Large-scale flow features, unique to overbank flow with a meandering inner channel, are described in the Paper. Prominent among them is a vigorous exchange of water between inner channel and floodplain in and beyond the downstream half of each bend. Floodplain water entering the channel penetrates to its full depth in this region and the consequent channel circulations occupy its whole cross-section. Sinuosity of the inner channel has a pronounced (adverse) influence on system conveyance. This is attributable predominantly to the flow structures outlined above. The influence is moderated by the adoption of natural geometry of the channel bed, as compared with a uniform trapezoidal inner channel. Energy dissipation mechanisms are quite different from those in compound systems with straight inner channels, and much needed further study must acknowledge the difference. Future work must examine the influence of floodplain roughness which is greater than that of the channel, a matter neglected in this investigation but known to be important from the evidence of studies elsewhere.