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Stratigraphic feedbacks on alternate bar morphology




Brown, Ryan A., author
Nelson, Peter A., advisor
Morrison, Ryan R., committee member
Rathburn, Sara L., committee member

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As gravel bed rivers aggrade, they can develop subsurface stratigraphy consisting of heterogeneous grain-size distributions in the downstream, cross-stream, and vertical directions. During subsequent periods of degradation, this heterogeneous stratigraphy can be exhumed and may feedback on the processes that drive morphodynamic evolution. However, these surface-stratigraphy feedbacks are poorly understood and difficult to predict. These feedbacks are investigated by implementing the ability to store, track, and access bed stratigraphy in the 2-dimensional mixed-grain-size morphodynamic model FaSTMECH. The stratigraphy framework consists of a 3-dimensional grid of subsurface layers containing grain size fractions. The active layer (surface) is then allowed to exchange sediment with bedload as well as the stratigraphy layers. During aggradation, size fractions of sediment in the active layer and bedload are mixed with the highest stratigraphy layer size fractions. During degradation, the active layer takes on the sediment properties stored in the stratigraphy. This model is used to investigate stratigraphic feedbacks on the coevolution of surface patchiness and alternate bar morphology. When alternate bars are forced by an obstruction, differences between model simulations with and without stratigraphy enabled are minimal because bars quickly stabilize and become fixed. With no obstruction, however, migrating alternate bars formed with stratigraphy enabled are wider and display stronger sorting patterns than those formed without accounting for stratigraphy. The repeated aggradation and degradation associated with bar migration results in frequent interaction between subsurface and surface material. The repeated access of material finer than the initial bulk material during degradation allows for greater degrees of surface sorting where coarse bar tops become coarser and fine pools become finer. Changes in sorting patterns are shown to increase bar width and increase bar celerity by 1 cm/min. This suggests that surface/subsurface interactions play an important role in setting bed morphology, sorting patterns, and bedform dynamics in gravel bed rivers.


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