Relative significance of shear stress and horizontal pressure gradients on sediment mobility in the inner surf and swash zone
Date
2021
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Publisher
University of Delaware
Abstract
The swash zone and inner surf zones are often the most dynamic regions of the beach in terms of sediment transport processes that govern the morphological change of the shoreline. Yet, there exists a rather weak understanding in the contributing roles of the driving forces that govern sediment mobility and how these relationships are affected by different hydrodynamic forcing conditions, local morphology, phase of flow cycle, and spatial dependence across the swash and inner surf zones. Comprehensive understanding of the variation between horizontal pressure gradients and shear stress influences will greatly aid in the improvement of modelling skill to predict sediment transport. However, existing horizontal pressure gradient datasets are coarse and too limited to draw direct correlations. Modern advancements in sensor technology have enhanced the ability to collect high resolution measurements and further research the relative time-dependent significance of both shear stress and horizontal pressure gradients throughout the flow cycle. A novel low-profile Pressure Sensor Array (PSA) consisting of seven high resolution pressure transducers was developed to measure the horizontal change in pressure. The low-profile design allows for data collection in shallow water applications and reduces flow disruption. ☐ Observations of shear stress and horizontal pressure gradients were collected in a large-scale wave flume during the Shaping The Beach (STB) experiments, conducted in Barcelona, Spain, during September – October 2019, and February – March 2020. Measurements were collected in the transitional region of the lower swash and inner surf zones under varying wave conditions that were tested on two planar-sloped mobile profiles. Relative time lags between shear stress and bed level fluctuations as well as pressure gradients and bed level fluctuations were examined to determine a temporal correlation between the disruptive forces and bed response. ☐ Shear stresses and horizontal pressure gradients were parameterized as the dimensionless Shields parameter and Sleath number, respectively, to allow for a more generalized parametric comparison. Bed response was identified as the maximum change in bed level elevation exceeding 0.008 m for a given wave, and the corresponding maximum Shields parameter, Sleath number, and associated time lags were compared.
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Keywords
Shoreline morphology, Hydrodynamics, Flow cycle, Spatial dependence, Sediment transport, Pressure Sensor Array