Coarse deposits on river terraces: Evidence of suspended transport of gravels during high-magnitude floods

Mobilized coarse sediments in gravel-bed rivers are typically transported as bedload. In contrast, finer material (e.g., sands and silt) can be transported in suspension and, during overbank stage of floods, possibly deposited on topographically elevated surfaces such as floodplain or recent terraces.
We studied the rivers’ response induced by a high-magnitude hydrological event (recurrence interval of several hundred years) that affected several catchments in Central Italy in September 2022. Specifically, we focused on the Misa River, an Apennines gravel-bed stream that flows embedded 1.5 to 8 meters into the surrounding alluvial plain in its hilly sector. In the face of rather limited effects in terms of channel widening, we recognized the widespread presence of fresh gravel deposits (D₅₀ ranging from 6 to 19 mm) organized in lobes and fans placed on terraced surfaces at elevations 3-5 meters higher than the channel bed. The aim of this work was to investigate the morpho-sedimentary dynamics and transport mechanisms that may have led to the deposition of such anomalous coarse sedimentary bodies on river terraces.
The field work made it possible to characterize the grain sizes of the deposits, the topography of the river sections, the composition of the banks and the maximum hydrometric levels locally reached during the flood. The streambed slope was also calculated remotely based on available DTM data. The hypothesis we investigated was that the gravel particles may have moved in suspension during the paroxysmal phase of the flood event. We performed hydraulic calculations based on the classical Shields-Parker River Sedimentation Diagram, which considers dimensionless Shields stress and particle Reynolds number (a dimensionless surrogate for grain size) to determine the transport mechanism that affected the clasts (i.e., suspension, bedload, no motion). Despite the uncertainties related to water density and kinematic viscosity for which, in the absence of measured data, we assumed realistic values (e.g., 1078 kg/m³ and 4.6⋅10^-6 m²/s, respectively), the results obtained show that in most cases (10 out 12 anomalous deposits analyzed) the hydraulic conditions at the flood peak were consistent with movement in suspension of the medium and coarse gravels found on river terraces. Following the overflow, the hydrometric level dropped abruptly on terraces, inducing first bedload transport and then deposition of elongated gravelly lobes.
These results suggest that during intense floods, anomalous mobilization of fluvial gravels in suspension is possible, which can reach and reactivate external surfaces lying at considerably higher elevations compared to the active channel. This particular condition of coarse particle mobility is enabled by the enhanced unit stream power during flood events. Specifically, in the analyzed context, the stream power could not be dissipated via lateral erosion and increase of channel width due to the cohesiveness of the banks, the widespread presence of outcropping bedrock and bank protection structures. In light of the above, suspended transport of gravels and subsequent deposition on high surfaces outside the active channel should be considered as an additional morphodynamic process that can occur in gravel-bed rivers during intense flooding.