The Beginning of Bedload Movement of Mixtures Investigated as Natural Armoring in Channels
Creators: name: Gessler, Johannes
Publisher: name: California Institute of Technology
Year: 1968

description: Beginning of bedload transport in channels, whose beds are formed by gravel-sand mixtures, is theoretically and experimentally investigated. 

In order to make a theoretically approachable treatment to the problem two assumptions are made: 

1. the turbulent fluctuations of the bottom shear stress are statistically describable by a Gaussian distribution; 

2. a grain starts in motion when the effective (instantaneous) eroding bottom shear stress on a grain exceeds a critical value, which is a function of the grain size and Reynolds number of the grain. 

On the basis of these assumptions the probability of remaining still (or being eroded) for a certain grain under given hydraulic conditions is calculated (equation 8; this probability is independent of the grain size distribution of the gravel-sand mixture). During the work a feasible way was found to determine the critical shear stress by a basically new method: the average bottom shear stress was defined as equal to the control shear stress, when for the grain in question, the probability for remaining still and being eroded are equal. 

To supplement and verify the theory, natural armoring of channel bottoms consisting of gravel-sand-mixtures was investigated in the laboratory. On the basis of these experiments the dimension- less critical shear stress can be determined as a function of the Reynolds number of the grains (Fig. 8), and the distribution function of the fluctuation of the bottom shear stress (Fig. 9); in doing so it was confirmed that the distribution function can be approximated by the Gaussian distribution with a standard deviation of σ = 0.57. 

The laboratory experiments were supplemented by observations in the field.
description: Report No. 69 of the Laboratory of Hydraulic Research and Soil Mechanics of the Swiss Federal Institute of Technology in Zurich under the direction of Prof. G. Schnitter. 

Translated with permission of the Author and The Swiss Federal Institute of Technology by Edmund A. Prych. 

Reproduction of this material was sponsored under a grant from the Bechtel Corp. 

Forward

In this paper the beginning of bedload movement of mixtures is investigated.

I wish to express my sincere thanks to Prof. G. Schnitter, my advisor, for the expert guidance and valuable suggestions which contributed to the success of this work. I thank Prof. Th. Dracos for reviewing and approving the work, in this way he assisted in this project. I thank Engineer J. Zeller, Chief of the Department of Hydraulics in the Laboratories of Hydraulic Research and Soil Mechanics of the Swiss Federal Institute of Technology, for the great interest and the assistance that he provided for this work at many times. To Mr. J. Zeller, draftsman, I give my thanks for the careful preparation of all drawings and writing the equations. Furthermore, I wish to thank every employee of the Laboratories of Hydraulic Research and Soil Mechanics, that assisted me many times with advice and actions, especially while conducting the experiments.

Johannes Gessler

Itschnach-Kusnacht, December 1965