Experiments with non-uniform sediment in case of bed-load transport

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Summary

An

experimental study was carried out ~n the framework of a research project concerning the development of a mathematical model for morpho-logical computations in rivers in case of non-uniform sediment. The study consists of a series of laboratory experiments in a straight flume under steady, uniform (equilibrium) conditions with a restriction to bed-load transport and dune regime. The flume was fed upstream by different mixtures of two very narrow sieved size fractions. During one experiment the total amount and composition of the input mixture, the water discharge and the downstream water level were kept constant. When equilibrium was reached besides regular registrations of water and bed level the dunes were extensively sampled. The latter occurred in such a way that vertical probability distributions of the size fractions could be determined. The main results of the experiments are: (i) Vertical sorting of the size fractions occurred in all experiments:

at the steep lee side of the dunes the coarse size fraction is generally deposited at a lower level than the fine size fraction. Differences in volume concentration per size fraction until 30% occur between upper and lower layers.

(ii) A transition layer was found which is generally below the propa-gating dunes; it has a relatively coarse composition (vertical sorting:) and has a thickness of 0.1 - 0.5 H (H

=

average dune height). Exchange of size fractions between this layer and the upper bed layer occurs at a time scale much larger than the dune period. (iii) Because of the phenomena described above several assumptions in

a mathematical model for non-uniform sediment (Ribberink, 1980) concerning the transport layer and the deposition~erosionof

s~ze fractions t~from non-moving bed are generally not fulfilled. (iv) Data are obtained for the verification and development of semi

empi-rical components in the mathematical model (i.e. transportformula per size fraction, predictors for dune height and bed roughness).

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The theory of Egiazaroff (1965) concerning the critical bed

shear stress per size fraction seems to be useful in a bed-load for-mula per size fraction of the type of Meyer-Peter&Mueller (1948).

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A bed sampling technique was developed and suggestions are made concerning the conditions of a non -equilibrium experiment which has the aim to verify the above-mentioned mathematical model for non-uniform sediment.

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