Browsing by Subject "Bedforms"
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Item Bedform interaction and preservation(2015-08) Swanson, Travis Eric; Mohrig, David; Kocurek, GaryThe shape and motion of bedforms are envisioned to arise as a consequence of a hierarchical cascade of interactions that occur within a bedform field. Interactions within the field are driven by conditions external to the bedform field, called boundary conditions. One such boundary condition is transport regime, the annual cycle of sediment transporting winds that drive uneven movement and deformation of bedforms. Paradoxically, uneven bedform movement results in geometric organization, as bedform crests align to wind regime. High resolution mapping of bedform crest movement reveals stochastic crest motion that acts to maintain the geometric organization of the bedform field. Stochastic motion of bedforms is driven by the interaction of bedform topography with sediment transporting flows. One such interaction occurs when oblique bedform crests create a wake containing trailing helical vortices. Each trailing helical vortex may preferentially route sediment to or away from a downstream bedform. This routing causes deformation or ablation of the downstream bedform. Either way, a section of the downstream bedform may deform to an oblique orientation. The oblique section may also trail a helical wake and further propagate stochastic bedform motion downstream. Bedform field boundary conditions provide an external influence on stochastic bedform motion. A numerical model of dynamic bedform topography is fueled by four sets of boundary conditions: a uni-modal and bi-modal wind regime each investigated with and without the restriction of a linear bedform source area. The chosen boundary conditions are demonstrated to influence the shape and stochastic motion of bedforms within the field the rate at which the bedforms grow and mature. Fields of bedforms, driven by boundary conditions may leave a stratigraphic record of their stochastic motion. Given sufficient time, boundary conditions are transient. A numerical experiment is performed to investigate the preservation of signals from both stochastic dune motion and a transient boundary condition, the strengthening and weakening of sediment transport with time. The numerical experiment demonstrates that stochastic dune motion and transient boundary conditions influence the preservation of bedform topography in the resulting stratigraphic record.Item An experimental assessment of the influence of bedforms on coupled hyporheic flow and heat transport(2013-05) Norman, Francis Alexander, IV; Cardenas, Meinhard Bayani, 1977-Hyporheic flow influences both biogeochemical cycling in streambeds as well as streambed ecology. Some biogeochemical processes may be temperature dependent; therefore, heat transport associated with hyporheic flow may be an important influence on such cycles. I separately and experimentally assessed the effects of hyporheic flow due to bed topography on thermal dynamics in the sediment using a custom, tilting flume with temperature controls. Diel temperature cycles of 6° C were imposed in the flume and propagation of temperature signals into the sediment was examined for different bed morphologies (plane bed, pool-riffle-pool, and rippled bed), channel flow rates, and sediment grain sizes. Temperature fields in the sediment were monitored using an array of embedded thermistors, and this data was used to identify zones of upwelling and downwelling within the hyporheic zone. Results suggest that bedforms do induce substantially deeper downwelling upstream and downstream of the bedforms, with upwelling near the crest. This in turn leads to substantial advective heat transport and distinct thermal patterns in the sediment. Variation in permeability and channel flow rates further affects the magnitude of this advective transport. These results corroborate existing theoretical models of coupled hyporheic exchange and heat transport under bedforms. Hyporheic flow therefore affects thermal patchiness in sediment, which may in turn exert a control on biogeochemical reaction rates, and form thermal refugia for fauna.Item Morphodynamics and geometry of channels, turbidites and bedforms(2011-12) Peyret, Aymeric-Pierre Bernard; Mohrig, David; Kocurek, Gary; Kim, Wonsuck; Lake, Larry W.; Fulthorpe, CraigThe evolution of landscapes and seascapes in time is the result of the constant interaction between flows and topography. Flows change topography, which in turn change the flow. This feedback causes evolution processes to be highly non-linear and complex. When full analytical derivations of the co-evolution of topography and flow are not possible without oversimplifications, as is the case in river bends, recent large topographical datasets and modern computers allow for correlations between horizontal (planview) and cross-sectional geometry of channels. Numerical analysis in the Mississippi and Trinity rivers indicate that the type of correlation between river radius of curvature and bankfull channel width depends on the migration behavior of the river. In other cases, channel topography may only have a second-order effect on its own evolution, as is the case for fully depositional turbidity currents, and the evolution of aeolian field topography may only be a function of this topography. I show that in these situations, changes in topography may be decoupled from details of the flow field and modeled very easily with a good accuracy.