Fractal properties of shoreline changes on a storm-exposed island

Extreme storm events and their consequent shoreline changes are of great importance for understanding coastal evolution and assessing storm hazards. This work investigates the fractal properties of the spatial distributions of shoreline changes caused by storms. Wavelet analysis and upper-truncated power law (UTPL) fitting are used to study the power spectra of shoreline changes and to evaluate the upper limits of the cross-shore erosion and accretion. During a period affected by storms, the alongshore shoreline change patterns are strong on the 15 km scale but are weak with lower spectral power on the 20 km scale. The areas adjacent to the eroded shoreline are usually accrete, and the cross-shore extent of erosion is larger than that of accretion when the coast is affected by storms. The fractal properties of shoreline changes due to storms are found to be temporally continuous: the effects of later storms build on the preceding shoreline conditions, including both the effects of previous storms and the subsequent shoreline recoveries. This work provides a new perspective on the various scales of the spatial variations of the morphodynamics of storm-affected shorelines.

Instability and finite-amplitude self-organization of large-scale coastline shapes

Recent research addresses the formation of patterns on sandy coastlines on alongshore scales that are large compared with the cross-shore extent of active sediment transport. A simple morphodynamic instability arises from the feedback between wave-driven alongshore sediment flux and coastline shape. Coastline segments with different orientations experience different alongshore sediment fluxes, so that curvatures in coastline shape drive gradients in sediment flux, which can augment the shoreline curvatures. In a simple numerical model, this instability, and subsequent finite-amplitude interactions between pattern elements, lead to a wide range of different rhythmic shapes and behaviours-ranging from symmetric cuspate capes and bays to alongshore migrating 'flying spits'-depending on the characteristics of the input wave forcing. The scale of the pattern coarsens in some cases because of the merger of migrating coastline features, and in other cases because of non-local screening interactions between coastline protrusions, which affect the waves reaching other parts of the coastline. Features growing on opposite sides of an enclosed water body mutually affect the waves reaching each other in ways that lead to the segmentation of elongated water bodies. Initial tests of model predictions and comparison with observations suggest that modes of pattern formation in the model are relevant in nature.