Abstract The lower shoreface provides the connection between the continental shelf and the shoreline via its onshore transition called the upper shoreface. Lower shorefaces are diverse, and range from bedrock-controlled, through sediment-starved to sediment-rich, siliciclastic, carbonate, low to high wave-energy, microtidal to macrotidal, and are variably affected by storm and wind-driven flows. The lower shoreface can be a repository for deposits of terrestrial origin, and a zone of active carbonate production. It can therefore be an important source of sediment for beaches, dunes, estuaries, and tidal basins. There has been progress in the ability to predict suspended sediment transport under non-breaking and shoaling waves across the lower shoreface. However, high-resolution measurement of sediment transport over unknown seabed configurations with unpredictable bed-level changes under hydrodynamic conditions that are unknown at the outset, and especially involving bedload transport, is still faced with significant challenges. Non-linear interactions between processes contributing to sediment transport render calculations and modelling of transport directions and magnitudes uncertain, and the spatial and temporal scales of transport are much larger than those of the upper shoreface. On the other hand, transport rates and morphological change may be much smaller on the lower shoreface compared to the upper shoreface. Another challenge is the upscaling of short-term measurements to explain the long-term morphological evolution of the lower shoreface. This limited understanding implies that current paradigms of lower shoreface dynamics based on morphological equilibrium and disequilibrium relative to the ocean-forcing conditions may be too simplistic, though possibly appropriate over long timescales (decades to millennia), and modelling work and prediction of change no more than exploratory. Over such long timescales, boundary conditions (sea level, wave climate) are likely to change. Making way forward on these issues is important for understanding the connectivity between the lower shoreface and beach recovery after major storm erosion, and for estimating coastal sediment budgets, short- to long-term coastal change and response to natural and anthropogenic perturbations. At geological timescales, the lower shoreface is a central element in tracking shoreline changes. Progress is needed in measuring sediment transport and upscaling to timescales compatible with lower shoreface change. It is also important to take advantage of on-going rapid progress in seabed and shallow stratigraphic mapping, bed-level changes, including remote-sensing approaches, for a better understanding of lower shoreface morphodynamics and sediment connectivity with the coast. This includes the now routine identification of large subaqueous bedforms, possibly ubiquitous features on the world's continental shelves, their mobility over time, and their potential link with the shoreline. The common relationship between fine sand, dissipative beaches and large aeolian dunes also poses the question of how fine sand is abundantly supplied from the lower shoreface, given the common perception that it is readily swept offshore on beaches. These multi-theme challenges need to be addressed in order to advance our understanding of the lower shoreface and its connectivity with the upper shoreface and beach.

中文翻译：

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下岸面：与上岸面和海滩的形态动力学和沉积物连通性

摘要 下岸面通过称为上岸面的陆上过渡提供大陆架和海岸线之间的连接。下岸面多种多样，从受基岩控制的、缺乏沉积物的到富含沉积物、硅质碎屑、碳酸盐、低到高波能、小潮到大潮，并且受到风暴和风驱动流的不同影响。下岸面可以是陆源沉积物的储藏库，也是碳酸盐活跃生产区。因此，它可能是海滩、沙丘、河口和潮汐盆地的重要沉积物来源。预测在非破碎波和浅滩波下的悬浮泥沙输运能力方面取得了进展。然而，在起初未知的水动力条件下，在未知海床构造下具有不可预测的床位变化的沉积物运输的高分辨率测量，特别是涉及床载运输，仍然面临重大挑战。沉积物输运过程之间的非线性相互作用使得输运方向和幅度的计算和建模不确定，并且输运的时空尺度远大于上岸面的尺度。另一方面，与上岸面相比，下岸面的运输速率和形态变化可能要小得多。另一个挑战是扩大短期测量以解释下岸面的长期形态演变。这种有限的理解意味着当前基于相对于海洋强迫条件的形态平衡和不平衡的低岸面动力学范式可能过于简单化，尽管可能适用于很长的时间尺度（几十年到几千年），并且建模工作和变化预测不再适用比探索性。在如此长的时间尺度上，边界条件（海平面、波浪气候）可能会发生变化。在这些问题上取得进展对于理解大风暴侵蚀后下岸面和海滩恢复之间的联系，以及估计沿海沉积物预算、短期到长期沿海变化以及对自然和人为扰动的响应非常重要。在地质时间尺度上，下岸面是跟踪海岸线变化的核心要素。在测量沉积物输送和升级到与较低的岸面变化兼容的时间尺度方面需要取得进展。利用海床和浅层地层绘图、海床水平变化（包括遥感方法）的持续快速进展，更好地了解低岸面形态动力学和沉积物与海岸的连通性也很重要。这包括现在对大型水下床形态的常规识别、世界大陆架上可能无处不在的特征、它们随时间的移动性以及它们与海岸线的潜在联系。细沙、消散海滩和大型风成沙丘之间的共同关系也提出了一个问题，即细沙是如何从低岸面大量供应的，因为人们普遍认为细沙很容易在海滩上扫到离岸。