Dune erosion models and swash zone kinematics from remote video observations Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tx31qp19m

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  • The subaerial beach, composed of sand dunes and the foreshore, provides a natural buffer zone between vulnerable land and the dissipation of storm wave energy due to wave breaking. The natural beauty of this region is attractive to people, and as a result, significant investment has been placed in this relatively unstable strip between land and water. During storms, when water levels and waves exceed the base of the dune and the dunes are vulnerable to erosion, development and ecosystems landward of the dune are at risk. Ideally, predictive models would forecast potential dune erosion, allowing appropriate management response. One class of existing dune erosion models is based on assumed avalanching once foreshore slopes exceed a user defined maximum value, although vertical or even overhanging dunes frequently occur in nature, suggesting that a maximum slope is not a universal parameterization. Another approach relates the volume of eroded sediment to the normal force of impact via an empirical coefficient. However, neither of these approaches addresses the fundamental physics controlling dune erosion. The objective of this dissertation is two-fold. The primary objective is to improve our understanding of the physics driving dune erosion and develop new predictive models. The secondary objective is to develop innovative new methods for studying the dune and foreshore using remotely sensed observations that can provide the data needed to improve our understanding of the processes. The first section of this dissertation focuses on developing a stereo video method for making quantitative observations of dune erosion at higher spatial and temporal resolution than traditional measurements allow. Resolution of the technique is 0.1 m in the horizontal and 0.04 m in the vertical. Errors were on the order of 0.02 m to 0.08 m (1 to 2 pixels) when compared with in situ surveys. Newly developed confidence intervals accurately quantified observed scatter from the stereo technique. The method was implemented in a large-scale wave flume experiment designed to reproduce a storm hydrograph. The new observations of dune morphology were used to improve an existing dune erosion model, accounting for the interaction between fluid and sediment by relating the momentum flux from waves onto the dune directly to eroded volume. We improved parameterizations for offshore wave forcing in the model based on an assumed normal distribution of swash on dunes. The model reproduced 64% of the observed variance in observations given known forcing at the dune and 55% of observed variance based on the new parameterizations of offshore forcing. The second section describes the development of a new dune erosion model based on observations from the dune erosion experiment. In the new model, the dune slumps when the weight of the dune plus the weight of water infiltrated from swash exceeds the resisting strength of the sediment. Eroded volume of sediment is then equal to the infiltrated volume of sediment. Infiltration was modeled using Darcy’s Law substituted into the continuity equation. The resulting model explains 72% of the observed variance in eroded volume. The final section of this dissertation describes a method for observing swash kinematics by tracking foam on the swash. This technique is useful for making observations on the foreshore where in situ instrumentation is difficult to maintain and significantly alters the flow to be measured. The method was compared with in situ observations collecting using an acoustic Doppler velocimeter. Coherence squared between observations and model was between 0.8 and 0.9 over the energetic frequencies, suggesting that this method would be useful for studying hydrodynamic forcing of sediment transport in the foreshore region.
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