Characterizing beach changes using high-frequency Sentinel-2 derived shorelines on the Valencian coast (Spanish Mediterranean)

Shoreline position can be efficiently extracted with subpixel accuracy from mid-resolution satellite imagery using tools as SHOREX. However, it is necessary to develop procedures for deriving descriptors of the beach morphology and its changes in order to become truly useful data for characterizing the coastal dynamism. A new approach is proposed based on a spatiotemporal model of the beach widths. Divided into 80m analysis segments, it offers a robust and detailed characterization of the beach state along large micro-tidal regions, with continuous information through time and space. Geographical and temporal differences can be recognized and measured, making it possible to study the beach response both to general factors (as wave conditions) and to punctual anthropic actions (as small sand nourishments). Widths were defined throughout two and a half years from 60 shorelines (3.04m RMSE) covering 50km of the Gulf of Valencia. Important width contrasts appeared along the study site associated with sediment imbalances motivated by sediment traps and other anthropic actions. Segments too narrow for maintaining the recreational function were located and mapped (16% narrower than 30m). Short-term width changes appeared linked to storm events, with fast retreatments and slow recoveries. Punctually, even small-magnitude nourishments created perceptible changes in width (12,830m3 were associated with a 4m increase). This novel description of the beach state and its changes from Satellite-Derived Shorelines is useful for coastal management, especially considering the global coverage of these free satellite images. It may improve the comprehension of coastal processes as well as monitor human interventions on the coast, helping in the decision-making. [Display omitted] •Automatic, massive and efficient shoreline extraction from Sentinel-2 images•Shoreline data allow deriving beach width as an indicator of beach morphology•Spatiotemporal model offers broad perspective of coastal dynamics and erosion•Identification of narrow segments problematic for maintaining recreational functions•Monitoring short-term response to storms, sand extractions and nourishments