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The controls of morphodynamics in steep, aggrading channels : a flume investigation

This thesis presents the results of ten experiments conducted to examine the role grain size distributions play in the behaviour and evolution of one dimensional alluvial fans using a novel flume setup. The two grain size distributions share the same median grain size (D50 = 1.42 mm), one (GSD1) is composed of log normally distributed material from 0.25 to 5.6 mm and the other (GSD2) is only composed of two classes: 1.4 and 2.0 mm. Images of the deposit profile and surface were used to monitor the evolution of the deposit and allow for the quantitative and qualitative assessment of behaviour, respectively. A detailed comparison of the aggrading phase of one run condition (Q = 0.1 l s−1, Qb = 1 g s −1) demonstrates a difference in the morphology that must originate from the grain size distribution. The addition of coarser grains in GSD1 creates patches of immobility and allows the formation of bars more resistant to flow than in GSD2. This reduction in transport efficiency of the material results in a higher slope and more stable bed configuration for GSD1. More widely, this pair of experiments shows that differences in mobility may still manifest in aggrading environments. For all five pairs of experiments this difference is present. Overall, GSD1 exhibits more stable behaviour: lower transport rates, higher slopes and later transport than GSD2. However, the difference between the two grain size distributions decreases as discharge increases. That is, the difference is most pronounced at the lowest discharge where grains are less mobile due to the lower maximum stress and lower frequency of it occurring. This reduced mobility is due to a threshold of entrainment present in GSD1 that is less commonly exceeded at low discharges, but that is not present to the same extent in GSD2. Therefore, the proportion of immobile grains controls the behaviour and stability of an aggrading channel.