Hydrodynamic characterization of the Arcachon Bay, using model-derived descriptors

Abstract

A numerical model (MARS-2D) was developed, with the aim of describing the hydrodynamics that prevail in Arcachon Bay. Direct model results as well as derived mixing and transport time-scales (tidal prism, local and integrated flushing times, age of water masses), were used to understand the behaviour of water masses and exchanges between the Bay and its frontiers. Particular attention was paid to the processes that drive the hydrodynamics (tides, wind and rivers), in order to understand their respective influence. The Arcachon Bay hydrodynamic system appears primarily to be highly influenced by tides; secondarily, by winds. About two third of the lagoon total volume is flushed in and out at each tidal cycle, which represent a mean tidal prism of 384 millions of cubic meters. The percentage of seawater flushed out during the ebb, that returns into the lagoon during the following flood flow is very high (return flow factor=0.95). This pattern leads to calculated integrated flushing times (IFT) ranging from 12.8 to 15.9 days, respectively, for the winter 2001 and summer 2005 simulations (two contrasting climatological situations: in summer, light northwesterly winds and low discharges in the rivers and, in winter, stronger southwesterly winds and higher river flows). Moreover, it has been found that northerly and westerly winds tend to reduce the flushing time, whilst southerly and easterly winds tend to hinder the renewal of the water in the Bay. The behaviour of the waters originating from the two main rivers of the lagoon, was studied also by means of the mean age assessment, under varying conditions of river flow and wind regime.