The armouring of alluvial channel beds and the evaluation of the hydraulic characteristics of the armour coat

Saad, Mohamed Bahaa El-Din Ahmed Mohamed (1986) The armouring of alluvial channel beds and the evaluation of the hydraulic characteristics of the armour coat. University of Southampton, Doctoral Thesis.

Abstract

This thesis is divided into two parts, the first part deals with the phenomena of the natural armouring of the alluvial sediment beds and the second part explains the changes in the hydraulic characteristics of a sediment bed after it has been armoured. In the first part the phenomenon of the natural armouring of channel beds has been investigated experimentally as follows: (a) the development of the bed armouring and the prediction of the grain size distribution curves of the armour coat and eroded material with respect to the flow and initial bed conditions, (b) under these flow conditions the extent to which the sediment bed becomes stable by armouring. Six different types of grading of sediment having a log-normal distribution with geometric mean diameters and standard deviations, namely 1.94 mm, 1.51; 1.96 mm; 2.91; 4.01 mm, 3.03; 4.0 mm, 2.0; 1.8 mm, 3.31 and 1.0 mm, 1.85, respectively, were used in the armouring experiments. These six sand-gravel mixtures were each placed in a flume bed and exposed to different shear stresses exerted by the flow discharges. The flow parameters-depth, discharge, slope and temperature - were kept constant throughout the duration of the experiment. The armour coat was then developed gradually by degrading finer particles and accumulating the coarser ones on the bed surface. The experiment was brought to an end when there was no further degradation observed and the movement of eroded particles had or had nearly ceased. The eroded material collected in a stilling basin placed at the downstream flume end was removed, dried, weighed and sieved to determine its grain size distribution curve; also a sample from the armour coat was removed for the same purpose. Empirical equations based on the observed measurements were developed to predict the grading distribution curves for both armour coat and eroded material and to answer the question whether, for a given logarithmic normal sediment distribution and hydraulic flow properties, the sediment bed would armour. In the second part, three types of bed material, namely uniform, non-uniform and armour material were used to determine the differences in the hydraulic properties of each type of bed material and to describe the effects of the armouring on the hydraulic performance of the sediment bed. In general the bed roughness, as a result of armouring, was found to become greater with increasing bed shear stress. Empirical equations were developed to determine for each bed material type the hydraulic characteristics, which include equivalent and roughness height, Manning's roughness coefficient, Chezy's coefficient and Darcy-Weisbach friction factor. The effect of the geometric standard deviation of a sediment mixture on its hydraulic properties was found vital. Shear velocity was obtained from the direct measurements and deduced from the velocity profiles; a comparison between the measured and deduced values disclosed a good agreement in spite of the presence of some minor differences. (D71905/87)

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