Karst and urban flood-induced solid discharges in Mediterranean coastal rivers: The case study of Las River (SE France)

Abstract Rivers solid discharge represents a substantial environmental issue, especially for the coastal marine environment. Unlike continental climate rivers, Mediterranean rivers show large discharge variability linked to rainfall, runoff and groundwater discharge, and can be temporary dry. Solid yields are difficult to predict due to variable source of floodwater. This paper assesses the suspended solid discharge of the Las River (SE France), a small stream in karst and urban environments, and its proportion due to flood events. Floods characteristics are analyzed to explore the variability of the solid yield and the influence of urban runoff and karst springs discharge, based on in situ data. The 35 floods events monitored during a 17 months survey largely contributed to the annual yield, with a proportion of 47% of the total water, and 69% of the total solid yielded in 11% of the time. The total rainfall and the total water discharge, related to the water levels in karst springs, drive the total solid yield, mostly composed of mud (90%). Urban runoff induces higher suspended solid concentration than karst flood, but generated a lower total solid yield. Karst springs, by expanding the catchment area of the stream, largely contribute to the total volume of water discharged to the sea and enhance the sand proportion due to their influence on the maximal water discharge. These results reveal the strong influence of the karst spring hydrodynamic functioning on the sediment yield to Mediterranean coastal environments. This study also highlights the efficiency of electric conductivity data to provide valuable insights about floodwater sources and sediment transport processes.