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  1. Abstract

    Erosive beach scarps influence beach vulnerability, yet their formation remains challenging to predict. In this study, a 1:2.5 scale laboratory experiment was used to study the subsurface hydrodynamics of a beach dune during an erosive event. Pressure and moisture sensors buried within the dune were used both to monitor the water table and to examine vertical pressure gradients in the upper 0.3 m of sand as the slope of the upper beach developed into a scarp. Concurrently, a line‐scan lidar tracked swash bores and monitored erosion and accretion patterns along a single cross‐shore transect throughout the experiment. As wave conditions intensified, a discontinuity in the slope of the dune formed; the discontinuity grew steeper and progressed landward at the same rate as theR2%runup extent until it was a fully formed scarp with a vertical face. Within the upper 0.15 m of the partially saturated sand, upward pore pressure gradients were detected during backwash, influencing the effective weight of sand and potentially contributing to beachface erosion. The magnitude and frequency of the upward pressure gradients increased with deeper swash depths and with frequency of wave interaction, and decreased with depth into the sand. A simple conceptual model for scarp formation is proposed that incorporates observations of upward‐directed pressure gradients from this study while providing a reference for future studies seeking to integrate additional swash zone sediment transport processes that may impact scarp development.

     
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  2. Free, publicly-accessible full text available August 1, 2024
  3. Erosion of coastal dunes during storm events is an increasingly common problem in the face of global climate change and sea-level rise. To investigate the efficacy of bio-mediated ground improvement for reducing the impact of extreme events such as hurricanes, a near-prototype-scale experiment was performed. In the experiment, a model sand dune was constructed in a large wave flume and divided into treated and untreated zones which were instrumented with pressure and moisture sensors. One of the treated sections was subjected to a surface-spray technique to apply bio-cementation. Afterward, the dune was subjected to a discretized severe storm event (a scaled Hurricane Sandy) consisting of 25 trials. Surge runup and drawdown cause surface erosion and also internal instability due to liquefaction. Pore pressure sensors were embedded in different depths of the dune to study the pressure fluctuations during the wave action and the consequent momentary liquefaction phenomenon. Momentary liquefaction leads to detachment of fine sand particles and the initiation of internal erosion and sediment transport. In this project, remote assessment technology (lidar) was used between each trial to evaluate the performance of the dune under the surge flow by detecting the eroded volume of the sand. To better quantify material properties in-situ, a series of triaxial experiments were conducted on bio-cemented cores taken from the formed crust. The strength and stiffness of the cemented sand were measured under different drainage conditions. Element test results indicate a significant increase in critical bed shear stress (τc) due to cementation. 
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  4. null (Ed.)
    Coastal dunes often present the first line of defense for the built environment during extreme wave surge and storm events. In order to protect inland infrastructure, dunes must resist erosion in the face of these incidents. Microbial induced carbonate precipitation (MICP), or more commonly bio-cementation, can be used to increase the critical shear strength of sand and mitigate erosion. To evaluate the performance of bio-cemented dunes, prototypical dunes consisting of clean poorly graded sand collected from the Oregon coast were constructed within the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University. The bio-cementation treatment was sprayed onto the surface of the unsaturated dune. The level of cementation was monitored using shear wave velocity measurements throughout the duration of the treatments. The treated and control dunes were subjected to 19 trials of approximately 300 waves each, with each trial increasing in water depth, wave height, and wave period. The performance of the dune was evaluated using lidar scans between each wave trial. The results indicate that the surface spraying treatment technique produced consistent levels of bio-cementation throughout the treated length of the dune and demonstrated significant resistance to erosion from the wave trails. 
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  5. Rice, J. ; Liu, X. ; Sasanakul, I. ; McIlroy, M. ; Xiao, M. (Ed.)
    Coastal dunes often present the first line of defense for the built environment during extreme wave surge and storm events. In order to protect inland infrastructure, dunes must resist erosion in the face of these incidents. Microbial induced carbonate precipitation (MICP), or more commonly bio-cementation, can be used to increase the critical shear strength of sand and mitigate erosion. To evaluate the performance of bio-cemented dunes, prototypical dunes consisting of clean poorly graded sand collected from the Oregon coast were constructed within the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University. The bio-cementation treatment was sprayed onto the surface of the unsaturated dune. The level of cementation was monitored using shear wave velocity measurements throughout the duration of the treatments. The treated and control dunes were subjected to 19 trials of approximately 300 waves each, with each trial increasing in water depth, wave height, and wave period. The performance of the dune was evaluated using lidar scans between each wave trial. The results indicate that the surface spraying treatment technique produced consistent levels of bio-cementation throughout the treated length of the dune and demonstrated significant resistance to erosion from the wave trails. 
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