ON THE UPSTREAM-MIGRATING SAND BARS

Abstract

 Free bars developed in a straight river reach generally migrate downstream direction. A recent field observation, however, has implied that under a specific condition, sand bars could migrate upstream direction. Here, we simulate upstream-migrating sand bars using a two-dimensional morphodynamic model and, discuss how such kind of bars could develop. The sand bars developed in a straight channel is numerically simulated under the repeated cycle of a simple discharge variation, which satisfies the super-resonant bar regime. The result shows that in the early stage of the computation, downstream-migrating sand bars develop due to a free bar instability. Subsequent long term morphodynamic processes, however, cause significant bar elongation, resulting in upstream-migrating sand bars. The mechanisms of this upstream-migrating sand bars are discussed based on the phase differences between the bed variation and bedload transport rate on the bars.