Hydrological functioning and nutrients dynamic of an intermittent Mediterranean river, during low river flows and floods. Spatial and temporal analysis.

This thesis aims at identifying critical periods influencing water quality (nitrogen and phosphorus) in intermittent rivers as part of the tempQsim European project. It was carried out on the Vene River, a tributary of the Thau lagoon. Four main compartments contribute to the catchment's runoff: urban areas, farmed areas, two karstic springs and Waste Water Treatment Plant (WWTP).<br />Four hydrological modes were identified (1) during the recession period, the river dries up and WWTP effluents are the only contribution to stream flow; (2) When flow restarts, flash floods occur due to either runoff on urban or farmed areas or karstic springs; (4) Recession starts when the most upstream spring dries up. The modes' duration and characteristics are variable yearly. A conceptual hydrologic model was used for flash flood events. The model provided a correct representation of hydrologic response of early autumn floods.<br />The river has low contamination levels and nutrient fluxes' variations are linked to the floods. Three modes have been identified for nutrients dynamic: (1) during accumulation periods, phosphorus discharged by the WWTP is precipitated and stored in the riverbed; nitrogen undergoes denitrification; (2) during early autumn floods, nutrients, especially phosphorous, are remobilised in the riverbed; (3) during high flow floods, the nutrients originate from the catchment and karstic springs.<br />This study shows how the nutrient dynamic depends on the hydrological behaviour: recession periods are critical from a contaminant and nutrient accumulation perspective and flash floods induce their remobilisation.