Suspended transport of gravel in rivers: Empirical evidence from the 2022 flood in the Misa River (Eastern Apennines, Italy)

In September 2022, an exceptional flood in the Misa River basin (Eastern Apennines, Italy) resulted in the unusual deposition of gravelly lobes on terraces up to 6 m higher than the riverbed. These deposits suggest that coarse bed sediments were transported in suspension rather than as bedload, as typically occurs under competent flow conditions. To verify this hypothesis, we combined field evidence-obtained from geomorphological and sedimentological surveys-with theoretical insights based on sediment transport theory. Our findings indicate that medium-sized gravels, which are part of the riverbed material, were transported in suspension within the water column. This phenomenon required specific conditions to generate the necessary shear stress and energy, including (i) a high-magnitude flood enriched with fine sediments, which increased the fluid density and viscosity, and (ii) an entrenched channel with stable banks that limited channel widening during the flood event. When these processes coincided with alluvial plain inundation by overbank flows, gravel transported in suspension was able to reach and settle on elevated surfaces, such as terraces far above the active channel. These observations highlight the potential for gravel-bed rivers to support the transport of coarse sediment in suspension under extreme flood conditions and specific geomorphological constraints on the active river channels. By demonstrating the role of sediment concentration, channel morphology and flood dynamics, our research provides new insights into sediment transport mechanisms and contributes to a broader understanding of the morphodynamic processes governing gravel-bed rivers under such exceptional conditions, with broad implications for refining flood hazard models and improving sediment transport predictions in fluvial systems.