Browsing by Subject "Channel morphology"
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Item Sediment transport dynamics in the lower Mississippi River : non-uniform flow and its effects on river-channel morphology(2010-12) Nittrouer, Jeffrey Albert; Mohrig, DavidThis dissertation examines the dynamics of sediment transport and channel morphology in the lower Mississippi River. The area of research includes the portion of the river where reach-averaged downstream flow velocity responds to the boundary condition imposed by the relatively uniform water-surface elevation of the receiving basin. Observational studies provided data that are used to identify channel-bed sediment composition, and measure bed-material sediment flux and the properties of the fluid-flow field over a variety of water-discharge conditions. The analyses demonstrate that a significant portion of the channel bed of the final 165 kilometers of the Mississippi River consists of exposed and eroding underlying relict sedimentary strata that qualify as surrogate bedrock. The exposed bedrock is confined to the channel thalweg, particularly in river-bend segments, and actively mobile bed-material sediments are positioned on subaqueous bars fixed by river planform. The analyses for sediment flux provides insight to the nature of sediment transport: during low- and moderate-water discharge, bed-material movement occurs primarily as minimal bedform flux, and so bed materials are not transferred between alluvial bars. During high-water discharge, bed-material transport increases one-hundred fold, and sands move as a part of both suspended and bedform transport. Physical models are used to show that skin-friction shear stress increases by a factor of ten for the measured water-discharge range. This change is not possible given conditions of uniform water flow, and therefore non-uniform flow in response to the Mississippi River approaching its outlet has a significant impact on the timing and magnitude of sediment flux through the lower river. In order to estimate the dynamics of bed material movement from the uniform to non-uniform segment of the river (lower 800 km), data for channel morphology are used to construct a model that predicts spatial changes in water-flow velocity and bed-material flux over a range of water-discharge conditions. The model demonstrates that non-uniform flow tends to produce a region of net channel-bed aggradation between 200-700 kilometers above the outlet, and a region of channel-bed degradation for the final 200. The implication for these results for the spatial variability of channel morphology and kinematics is explored.Item Urbanization and its effects on channel morphology(2013-12) McCann, Cody James; Maidment, David R.A focus on river and stream morphology with a specific emphasis on how urbanization and human impacts affect river channels. In the study of rivers, specifically looking at how the channel geometry changes with time, there are five main physical factors described that affect the channel morphology: (1) bank and bar stability; (2) sediment size distribution; (3) sediment supply; (4) flow variability; and (5) downstream slope, width and height. Understanding how these five factors affect channel form is vital in constructing realistic and accurate models of rivers and how they change over time. It is also important to understand some of the limitations of the combined modeling of all these factors together for a general stream. Research studies are presented in order to further understand what knowledge has been acquired, and what areas are lacking in adequate understanding. Examples of cases where urbanization and land use change have a large impact and almost no impact are examined. It is important to understand what the limiting factors are in such cases, and whether it is possible to mitigate the effects or urbanization by any means other than natural channel phenomenon. A two-dimensional hydrodynamic and sediment transport model is thoroughly described. The model is evaluated and verified, and potential problems and limitations are then discussed. Then a one-dimensional sediment transport and bed variation model is examined and tested using parameter controlled cases. Urbanization increase near rivers and streams reduces the time frame over which certain natural events would have occurred in those channels. The affects of urbanization include but are not limited to changes in streamflow, sediment transport and deposition, channel bank stability, and increased channel widening. The magnitude of these affects will increase over time if careful steps are not taken to minimize the human influence within channels.