Bays and estuaries, located at the land-sea interface, are key areas for sediment transfer between continental shelves, littoral and continental zones. They are of high recreational, commercial and ecological value and, as such, a better understanding of the sediment dynamics processes and morphological evolution of these coastal systems is needed to improve their management. Since the last Holocene transgression, they have been undergoing infilling; however, their trajectory could evolve under the influence of sea-level rise and intensifying storms.
The Somme Bay, located along the northern French coastline, in the Eastern English Channel, is a macrotidal (8.5 m in mean spring tides) estuarine system actively infilling with marine sands under tides and swells influence. The very low river discharges (mean annual 40 m³.s^-1), combined with human works in the inner bay, greatly limit fluvial hydrodynamics into the bay resulting in only minor inputs of fine particles.
This study aims to better understand the morphodynamic evolution of the Somme Bay over the medium term (2011-2024), particularly under storm influence, based on 13 airborne topographic LiDAR surveys. An automatic classification method was developed to identify the main bay morphosedimentary units, allowing consistent mapping from one survey to another. The evolution of the different morpho-sedimentary units was quantified in terms of surface, barycenter location, fragmentation and sediment budgets. Storm regimes were characterized for each inter-survey period using offshore wave time series (Delft3D), enabling correlation analyses between storm characteristics and morphodynamics parameters. At the scale of the storm event, the analysis of high-frequency hydro-sedimentary and topographic provided insights into sedimentary fluxes involved during these energetic events.
Results confirm that the bay still undergoes infilling (4.9 cm.yr^-1), which is accelerating compared with previous estimates. Accretion is systematic in the inner bay, while some central and outer zones can experience erosion and accretion alternatively. Inner-bay units (e.g., salt marshes), nearly filled, show weak accretion (1 cm.yr^-1) compared with adjacent areas (e.g., hydraulic dunes) still actively infilling (10 cm.yr^-1). Correlation analyses reveal that bay units are disturbed by storm energy and duration and are especially sensitive to storm clusters that reverse storm-free accretional trends into erosional ones. Results on the sedimentary fluxes at the scale of the storm event will be presented also.
In the context of climate change, the bay trajectory could move toward a more unstable morphodynamic evolution: increasing storminess and sea-level rise may intensify erosion in the outer and central sectors of the bay while limiting sediment transfer and accretion in the inner intertidal zones.

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