Fundamental Properties of Flows in Open Channels with Dead Zone

Abstract

The fundamental behavior of unsteady flows in an open channel with a rectangular dead zone is investigated experimentally and numerically to clarify the mass-exchange mechanism near an embayment along a river bank. Typical flow features such as generation of a coherent vortex due to shear instability in the mixing layer between two flow regions (main-channel flow and dead zone) and water-surface oscillation (seiche) in the dead zone are shown by the findings of laboratory tests. These flow features can be reproduced numerically by plane two-dimensional (2D) open-channel flow equations in the variable grid system. Model performance was examined in detail by changing the grid size and constants. The calculated values for temporal and spatial variations in velocity and depth are compared with observed values. Examination of the calculated results shows that temporal velocity variations at the interface can be decomposed into two components due to seiche and shear instability; during one seiche, cycle-velocity variations caused by instability are amplified to the large-scale vortex selectively by the interaction between seiche and instability components. This selective amplification process can be detected in a series of photographs of the surface flow pattern as well.