Browsing by Subject "Earth sciences"

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    A comparison of the planforms of meandering tidal and fluvial channels on the Ganges-Brahmaputra-Jamuna Delta, Bangladesh
    (2014-08) Bain, Rachel Louise
    The planform geometry of tidal channels is controlled by bidirectional flow, a fundamentally different process from that controlling the shape of meandering river channels. However, prior studies do not reach any consensus on how channel planforms differ in these two environments. We use Google Earth imagery to digitize 130 meandering channels on the Ganges-Brahmaputra-Jamuna Delta in Bangladesh, from which we produce a data set of meander and channel width statistics for tidal and fluvial channels. Our data indicate that the tidal meanders are more symmetrical than the river meanders, which are both downstream and upstream skewed. The fluvial meanders are also more sinuous than the tidal meanders. We observe some evidence of longer intrinsic and Cartesian meander wavelengths in the tidal environment, although this could be a consequence of anthropogenic modifications to the fluvial channels. Despite these differences, we find that the relative strength of the first through third curvature harmonics is the same for the tidal and fluvial meanders. Our data set also provides insight into the funnel-shaped geometry of tidal channel mouths; we find that there is a positive correlation between channel mouth width and e-folding length. For those channels with a fluvial connection upstream, there is also a positive correlation between mouth width and upstream width. We propose several causes for the observed trends in our data, but future study will be necessary to test these hypotheses.
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    Stress-driven melt redistribution in partially molten rocks deformed in torsion: from pressure shadows to base-state segregation
    (2014-10) Qi, Chao
    The redistribution of melt in partially molten rocks during deformation plays an important role in the evolution and dynamics of Earth's mantle. Previous studies discovered different scales of melt redistribution: melt alignment and melt segregation to form melt-enriched bands , both of which have demonstrated their importance to the deformation of the mantle. In this dissertation, two new forms of stress-driven melt redistribution in deformed partially molten rocks are produced: a formation of pressure shadows around rigid particles and a large-scale, base-state melt segregation. For pressure shadows, observations on the microstructure around the rigid particles revealed the melt distribution and solid flow field, which will provide a constraint on the bulk viscosity of the partially molten rock, if associated with theoretical studies. The presence of base-state melt segregation validated a hypothesis of viscous anisotropy, which provides explanations for melt segregation processes and will cause a significant impact to the dynamic of the mantle. Therefore, the studies of stress-driven melt redistribution in this dissertation are of great significance that will influence the future studies of Earth's mantle.