Gravel barrier resilience in a changing climate (#gravelbeach)

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Beach and barrier systems with a substantial gravel fraction, including 'pure', 'composite' and 'mixed sand-gravel', are common globally at mid to high latitudes. These geomorphological features, collectively referred to herein as 'gravel barriers', represent up to 20% of the wave-exposed coastline of Great Britain. They provide natural protection against coastal flooding and erosion, and support unique ecosystems. However, much of our ability to predict their morphodynamics comes from research on sandy coasts, which is flawed in translation. Specifically, our understanding and capability to model the morphodynamics of barriers comprising sand-gravel mixtures is significantly lacking.Fundamental differences in sand and gravel sediment transport processes and morphological response preclude direct application of sandy coastal models to gravel barriers. In mixed sand-gravel systems, the presence of one grain size fraction affects the transport of another through selective entrainment and cascading disturbances. Our understanding of gravel barrier behaviour, especially their response to sea-level rise, storms, and changes in wave conditions related to climate variability, is thus mainly qualitative. We understand, for example, that landward barrier migration occurs due to storm-induced overwash, but the constraints on the migration rate are poorly understood. Whilst migration is likely to increase with sea-level rise, the influence of different sediment grades and accommodation space is unclear. Likewise, changes in the predominant wave direction induce both alongshore and cross-shore response through beach rotation and sediment sorting. This, in turn, changes shoreline position and can create erosion hotspots, but the ability to robustly predict these processes for different climate change scenarios is deficient.Gravel barriers have an important role in providing coastal protection. Traditionally they have been maintained through sediment recycling and beach nourishment. However, evidence indicates that some of these practices negatively impact barrier stability making it unsustainable, particularly under rising sea levels. Thus, this project (#gravelbeach) aims to develop, reliable, consistent and appropriate approaches for working with, and making space for, these natural features, to enable more sustainable and adaptive national-scale management practices.