INSTABILITY OF DEVELOPED ALTERNATE BAR ON NON-UNIFORM SEDIMENT BED

摘要

In flume tests, it was found that there are some hydraulic conditions in which alternate bars can hardly attain their equilibrium; developed bars fade with decrease of wave height or upstream bars catch up with downstream ones. Such alternate bars were not observed in flume tests using uniform sediment. It is supposed that longitudinal and transverse sediment sorting is responsible for such instability of developed alternate bars. In the present study, instability of developed alternate bars on non-uniform sediment bed is discussed by means of numerical analysis and flume tests. The results suggest that alternate bars formed on non-uniform sediment bed do not approach an equilibrium state under some hydraulic conditions. Time lag between the time necessary for accomplishing equilibrium bed geometry and the time for sediment sorting adjusted to the bed geometry is responsible for instability and unsteadiness of alternate bar bed. Furthermore, we found that developed alternate bars formed on non-uniform sediment bed become unstable under the hydraulic conditions where the time scale of sediment sorting development is almost the same as that of alternate bar formation. The hydraulic conditions in which bars become unstable are discussed by means of linear bed stability analysis. The results suggest that the disturbance amplitude of mean diameter of sediment may be a parameter for predicting of bar bed instability.