River mouths are highly dynamic environments responding very rapidly to changes in wave energy or river floods. While the morphological response during floods or during marine storm events has been widely documented in the literature, little is known about the mechanisms acting during the co-occurrence of fluvial and marine hazards. This concomitance of river flood and marine storm is quite common in the western Mediterranean Sea, and was the case for the Gloria event, considered to be the most extreme event in recent decades. During this event, monitoring of hydrodynamics and morphological evolution was implemented, making it possible to better understand the impact of concomitant marine storm and fluvial flood during an extreme meteorological event on spit breaching of a small Mediterranean river mouth. Monitoring using a combination of high-resolution hydrodynamic measurements, topographic and bathymetric surveys, and sediment cores was used before, during, and after the storm “Gloria”. The results suggest an amplification of the morphological impact of the events and a different morphogenic response than if each of the events had acted independently on the system. The marine storm, occurring first, weakened the spit and initiated its breaching, which was continued by the extreme fluvial flood, thus leading to the complete destruction of the mouth. The destruction of the spit acted as a sediment source for subaqueous large delta deposition amounting to 50% of the total volume. The contribution of the river, estimated at 30%, was quite low for an exceptional event, showing the importance of locating rainfall in a catchment area controlled by a dam. For this event, extreme morphological evolution was observed, as well as the importance of water levels in the river mouth, which probably increased flood hazards, demonstrating the importance of including the compounding effect of extreme coastal water levels in river flood risk management.

Small mountainous rivers provide an important part of the sedimentary inputs to the oceans. The particularity of theses rivers comes from the fact that their inputs take place mostly during brief and intense flood events. While the quantification of fine sediment flux is fairly well known, sandy inputs are very poorly known and field measurements are scarce. River mouth sand discharge is a key variable in the coastal sediment budget as it participates to the coastline evolution and its protection against marine events. Coarse sediments are mainly transported as bedload, making it difficult to estimate with traditional methods such as traps. In this study, a fixed Acoustic Doppler Current Profiler (ADCP Nortek Aquapro 1 MHz) was deployed on a bottom frame, near the mouth of the Têt river (SE France) to estimate near-bottom sediment transport during flood events. In addition, cross sections have been undertaken with a Sontek Hydroboard equipped with a Sontek M9 ADP at different water discharge during several flood events. A calibration of the backscatter index was carried out using gravimetric measurements and granulometric analysis of water samples to estimate the sediment flux and the sand proportion. Sediment fluxes were then compared with the altimetric variations observed from bathymetric surveys. Results allowed to characterize the variability of the boundary layer thickness and the sediment concentrations during flood events. Those results give useful information to estimate sand fluxes from mountainous rivers to the coastal area in a context where these are considered to be low or non-existent compared to large coastal rivers.