Effect of river runoff on sea level from in-situ measurements and numerical models in the Bay of Biscay

Daily time series of in-situ tide gauge records and river runoff data were analysed to investigate the contribution of river discharge storm events to sea level in the Bay of Biscay. Three main river systems were considered for this study, representing cases of small (Nervión, 1900 km2), medium (Adour, 16,880 km2) and large (Gironde, 84,811 km2) watershed basins. Typical storms correspond to water discharge rates of 150, 700, and 1100 m3 s−1 from the Nervión, Adour, and Gironde rivers, respectively. The effect of these events on daily mean sea level was evaluated using two different approaches: (1) through the analysis of time series of tide gauges placed within the river mouths; and (2) through numerical simulations using the ROMS model (Regional Ocean Modeling System). The three selected tide gauges are located at Bilbao (Spain) for the Nervión river, Boucau-Bayonne (France) for the Adour river and Port-Bloc (France) for the Gironde river. River runoff extreme events were more notable at the two tide gauges located within the largest rivers, namely, Adour and Gironde, where approximately 13% and 53.6% of the pressure-adjusted sea-level variance was explained by those events. The results obtained from the ROMS simulations suggest that the main effect of river discharge storm events occurs as a response to the input of lower salinity water. This plume of overlaying less dense water would produce a sea-level increase around the river mouth and along the coast, suggesting the generation of a coastal density current through the balance between the Coriolis force and the cross-shore pressure gradient. The main area of influence of the selected river discharges was confined to the river mouth for the Nervión, but extended up to approximately 28 km (33 km) offshore for the Adour (Gironde) river, before turning northward to flow as a density current along the coast.