The impact of stress history on non cohesive sediment bed stability and bed structure.
PhD thesis, University of Glasgow.
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Historically the inter-flood period has been disregarded from investigations as it was deemed that the stability of non cohesive beds could only be altered by above threshold flows capable of sediment transport. However, this is at odds with more recent ‘stress history’ data which provides unequivocal evidence that entrainment thresholds can be delayed to higher shear stresses after being subjected to longer periods of sub threshold flows. The magnitude of this effect appears related to the surface grain size distribution and relative grain size effects, whilst the specific mechanics associated to generating a more resistant bed under sub-threshold flows are merely speculated upon. The aim of the present thesis is therefore to provide a comprehensive and quantitative data set on stress history that specifically address comparative grade effects and provides a detailed mechanistic understanding of the processes responsible for generating a more resistant bed configuration under sub threshold flows.
Using a range of grain size distributions, a series of flume based experiments assess two main aspects of the stress history process. Firstly the effects of grain size distribution on the relationship between stress history duration and entrainment threshold is quantified. This is split into two sets of experiments based on the duration of the applied sub threshold antecedent flow, prescribed as 50% of the critical shear stress ( ) of the median grain size (D50). The antecedent durations of first set of experiment ranged from 0 to 60 minutes, whilst the antecedent duration of the second set of experiments ranged from 0 to 960 minutes. To ascertain the effect of the antecedent period on critical entrainment threshold and transported bedload, each experiment is concluded with a stability test composed of incrementally increased flow discharges until critical threshold conditions were reached. Secondly, aspect of stress history investigated uses high resolution laser scanning to assess bed topography and particle repositioning in order to ascertain the granular mechanics underpinning the stability process. The bed is scanned before and after the application of the applied antecedent flow with changes to bed surface structure described using Digital Elevation Models (DEM’s), statistical analysis and 1D and 2D semi-variograms to analyse scaling behaviour.
In all experiments, increasing the antecedent flow duration significantly increases river bed stability in that the critical shear stress increases by up to 25% where uniform beds are more responsive to antecedency than bimodal beds. Laser based analysis reveals that vertical settlement, localised changes to bed roughness, pockets of more pronounced development of hiding effects, and particle repositioning are all mechanisms by which the bed reorganises under an applied sub threshold flow. However, the different bed grain size distributions cause significant differences in the importance of each mechanism in determining the magnitude of stress history induced bed stability.
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