Some characteristics of the interaction between turbulent flows and movable rippled bottom in the river mouth

摘要

Interaction of turbulent flows with the river bed being eroded constitutes a complicated process of formation, under the influence of turbulent velocity fluctuations, of sand waves and ripples on the sandy bottom. These forms, in their turn, exert considerable back influence upon the structure of turbulent flow. To study this phenomenon, an extended program of experiments has been conducted, using devices that registered average velocities and their turbulent pulsations. The method of fluorescent tracers was used to study the dynamics of sediments in the flow. Experiments were conducted in the field conditions (at the Jordan River's inflow into Lake Kinneret, Israel), (a) A correspondence was established between the longitudinal size of the turbulent eddies and the size of bottom ridges, (b) The higher the bottom ridge, the stronger is the flow's eroding potential: the more bottom particles become suspended. This may be related to the periodic expansion and compression of the transit flow above the ridges. Such hydraulic cycles "expansion-compression" cause tangential accelerations, which exceed the acceleration of gravity by 2-3 orders of magnitude. This is probably why bottom particles en masse become suspended in the flow, (c) The higher the bottom ridge, the smaller flow velocity is needed to cause suspension of the bottom sediments (their transition to the suspended state). This occurs regardless of the stage of development of the bed forms: active or passive as long as they exist on the bottom of the flow, this is observed, (d) In the contact zone the jet river flow and water body, this process, along with other factors, contributes to the formation of large accumulative structures - river bars.