The Structure of Movable Bed Configuration

Abstract

In this paper, the authors discuss the spacial and temporal structure of the movable bed configuration in a lower flow regime from the standpoint that the structure has a close connection with the turbulence structure in open channels. The authors measured the bed configuration along the longitudinal lines located at different distances from the side wall at various stages of growth of the bed forms. The measurments were conducted in two flumes with different widths in order to clarify the effect of the width on the bed configuration. The turbulence structures were also measured in the flumes with fixed and movable beds at the various stages of growth of the bed forms by using a small propeller-type current meter. Comparison between the spectra of bed configuration and those of turbulence reveal intereresting characteristics as follows: (1) The structure of the bed configuration in a lower flow regime may be classified by three different origins in turbulence structure, which are horizontal turbulence characterized by a scale of width, vertical turbulence characterized by water depth, and longitudinal streaking. (2) The range of the wave length of the alternating bar which may correspond to horizontal turbulence is about 2～10 times the width B, and the eminent wave length is 4B. (3) The range of the wave length of dunes which corresponds to vertical turbulence spreads to about 10 times the depth h, and its statistical structure composes some "inertia subrange" in time and space, which has a similarity independent of external conditions. The wave length of dunes accompanied with surface water wave, which occurs when F, is nearly 1, is about 4～5 h. (4) Another bed configuration is longitudinal streaking, the interval of which is 2h laterally. (5) The above three components are able to coexist. (6) The variation of turbulence structure has a close connection with the process of bed configuration. The energy power density of turbulence decreases relatively in the range of which the spectrum of bed congiguration shows the maximum increase, as the bed configuration comes to reach its equilibrium stale.