Mass and momentum exchange in compound open-channel flows

Compound open-channels consist of a main channel that carries the bank-full, more frequent discharges and a floodplain that is inundated during flood flows. This channel morphology is ubiquitous in the natural and man-made environment and has recently found renewed attention among hydraulic engineers due to an increase in flood extent and frequency and the concomitant flood damages. As a result of the floodplain flow being substantially slower than the one in the main channel, a shear layer forms at the interface, triggering turbulent and advective exchange of mass and momentum between deep and shallow parts of the compound channel. Large Eddy Simulations (LES) of turbulent flow and tracer transport in an asymmetric compound open-channel with a shallow floodplain depth were carried out for different floodplain roughness conditions with the goal to provide a fundamental understanding of the relevant physical processes. Besides the provision of a thorough statistical description of the flow, mass and momentum exchange at the interface between deep and shallow regions of the flow are quantified.