Predicting sediment transport at sandy beaches is a significant challenge in civil engineering owing to the variability in hydrodynamic, morphological, and geotechnical properties within a site and across multiple sites. Additionally, there are difficulties in measuring in-situ properties, and challenges in identifying and quantifying the different relevant driving and resisting forces. These challenges are further exacerbated in the intertidal zone where the addition of infiltration-exfiltration, wave run-up and run-down, bore collapse, cyclic emergence and submergence of sediments, interactions between standing waves and incident bores, and other processes must be considered. Among these many processes, pore pressure gradients within sandy beach sediments affect sediment transport by reducing the sediment's effective stress to zero (this process is called liquefaction). Despite the known importance of these pressure gradients with respect to sediment transport, there has been little field evidence of the role that these pore pressure gradients have on sediment transport, how they relate to the hydrodynamic properties, and their inclusion into predictive sediment transport equations. This study is based on field measurements of hydrodynamic and geotechnical properties, as well as pore pressure gradients during storm and non-storm conditions at sandy beaches in the intertidal zone. From the analysis of these field measurements, it was found that (1) liquefying pressure gradients are likely to develop in sediments that are rapidly inundated during storm conditions; (2) the magnitude of pore pressure gradients is related to the asymmetry of the pressure gradient and can occur with shoreward-directed near bed velocities; and (3) during non-storm conditions, pressure gradients that often do not exceed liquefaction criteria occurred more (less) frequently during a time period where erosion occurred in large (small) quantities, indicating that small non-liquefying pore pressure gradients may facilitate sediment transport. The results of this study demonstrate that current methods of scour calculations must include effects of pore pressure gradients to reduce error. Additionally, from this work it was found that sediment transport can be directed shoreward under momentary liquefaction. Finally, the results of this study show that sediment pore pressure gradients are related to wave skewness, spatial group steepness, and temporal group steepness which may aid modelling of pore pressure gradients. / Doctor of Philosophy / The transport of sediment particles (in this case, sand grains at beaches) is difficult to predict because of the many different governing processes that can be hard to measure, may be hard to relate to erosion or sediment accumulation specifically, and the variability in sediment and flow properties (grain size, fluid velocity, and others) at a specific location and across different locations. Storms, like hurricanes, tropical storms, and tsunamis, can drastically change the expected water properties (like water depth, wave height, and wave period), and the effects of water pressure within the sand bed. When a wave moves across the sand it causes a change in the water pressure that is within the sand. This water pressure is not the same throughout the sand with depth. When the gradient, or the difference between the water pressure at two different vertical locations, is large enough, the sand behaves like a fluid (like quicksand) and becomes easier to move, this process is called liquefaction. Even though previous work has shown that these pressure gradients (and the resulting liquefaction) is important for sediment transport, there have been few field measurements demonstrating their impact on sediment transport and how these gradients (and the resulting liquefaction) relate to wave and sand properties. This study presents field measurements of pressure gradients, wave and sediment properties, and sediment transport events during both storm and non-storm conditions. From these field measurements, it was shown that (1) during an extreme storm event, pressure gradients that liquefy the sediment are likely to occur on sediments that are not normally subjected to waves; (2) liquefying pressure gradients can occur when waves arrive at the beach, which may cause sediment to be moved shoreward; and (3) during non-storm conditions, pressure gradients that do not liquefy the sand occurred frequently during a sediment transport event, suggesting that these smaller pressure gradients may contribute to sediment transport by reducing the effective weight of the sediment. This work can be used to further understand the behavior of sediment pore pressure gradients, their relation to hydrodynamic properties, and how they influence sediment transport allowing for better predictions of sediment transport, beach nourishment calculations, and the design of coastal structures.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110506 |
| Date | 08 June 2022 |
| Creators | Florence, Matthew Benedict Skaanning |
| Contributors | Civil and Environmental Engineering, Stark, Nina, Mulligan, Ryan Patrick, Irish, Jennifer L., Strom, Kyle Brent, Raubenheimer, Britt |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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