The lower shoreface: Morphodynamics and sediment connectivity with the upper shoreface and beach

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.