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 the R_2% 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.

中文翻译：

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海滩和沙丘地下水动力及其对沙丘陡坡形成的影响


侵蚀性海滩陡坡影响海滩的脆弱性，但其形成仍然难以预测。在本研究中，采用 1:2.5 比例的实验室实验来研究侵蚀事件期间海滩沙丘的地下流体动力学。埋在沙丘内的压力和湿度传感器用于监测地下水位，并检查上部海滩的斜坡发展成陡坡时沙子上部 0.3 m 的垂直压力梯度。同时，在整个实验过程中，线扫描激光雷达跟踪斜孔并监测沿单个跨岸横断面的侵蚀和增生模式。随着波浪条件的加剧，沙丘的坡度形成了不连续性；不连续性变得更加陡峭，并以与 R _2% 爬升范围相同的速度向陆地推进，直到完全形成具有垂直面的陡坡。在部分饱和沙子的上部 0.15 m 内，反冲洗期间检测到向上的孔隙压力梯度，影响沙子的有效重量并可能导致滩面侵蚀。向上压力梯度的大小和频率随着冲刷深度的加深和波浪相互作用的频率而增加，并随着进入沙子的深度而减小。提出了一个简单的陡坡形成概念模型，该模型结合了本研究中对向上压力梯度的观察，同时为未来寻求整合可能影响陡坡发展的额外冲沙带沉积物输送过程的研究提供了参考。