River plume dispersion in response to flash flood events. Application to the Catalan shelf

The dispersal of freshwater inflow from a flood event is studied using a three dimensional flow model (ROMS). The model domain includes a small part of the Catalan shelf where the combination of local land topography with torrential rainfall can cause high local runoff during a short period but with a large impact on the receiving coastal waters. Both steady, low river discharge, typical of normal (low-discharge) conditions and a high discharge representative of post-rain conditions are considered. Simulated salinity profiles on the shelf near the river mouth are compared with records from CTD measurements with quite good correspondence. A strong correlation between local wind and plume response was observed. Local winds affect the trajectory of the freshwater plume that enters the Catalan shelf waters. During post-rain conditions, northerly and westerly winds exported the plume further away from the coast, whereas southerly and easterly winds confine the plume closer to the coast. During low discharge conditions, the plume remained closer to the coast due to the weak wind stresses. Results show that freshwater spread, shape and dilution are mainly controlled by local wind forcing at relatively short time scales.