1 INTRODUCTION

Most of the low-land rivers can be considered as shallow water flows. The width to depth ratio is generally larger than 50. This shallowness has two consequences: the flow is vertically confined, resulting in quasi-two-dimensional flow features, and bottom friction plays an important role. Especially in case of transverse differences in streamwise velocities the resulting flow pattern contains length scales much larger than the water depth. This has severe consequences for the lateral and longitudinal transport of mass and momentum as well as for mixing processes.

An example of a shallow shear flow is given in Fig. 3-1. In this laboratory experiment a compound channel has been modelled, with on the right-hand-side the deep main channel (length 30 m, width 3 m, depth 12 cm) and on the left-hand-side the shallow floodplain (length 30 m, width 7 m, depth 4 cm). Dye was injected upstream at three positions: in the main channel, on the floodplain and on the interface. In the main channel and on the floodplain the dye band is slightly broadening, notably stronger in the main channel than on the floodplain, due to the higher velocity and larger water depth in the main channel. On the interface the dye band pattern looks completely different. As the velocity in the main channel is higher than on the floodplain, a transverse difference in streamwise velocity...