Des courants de turbidité hyperpycnaux dans la tête du canyon du Var ? Données hydrologiques et observations de terrain

The Var River is 120 km long upon entry into the western Mediterranean Sea and drains a 2820 km(2) basin (Fig. 1). A steep submarine canyon connects directly to the river mouth (Fig. 2). The submarine canyon is sinuous and shows typical badland features such as high slopes resulting from erosion (Fig. 3). The average water discharge 152-53 m(3) s(-1); Fig. 4) can be multiplied by tens during spring or fall ''flash floods'', when suspended sediment concentration can reach many tells of kg m(-3). The rating coefficient b corresponding to instantaneous discharges is 1.534 (Eq. (1) and Fig. 5, curve 1), according to data published by Laurent (1971), but might be larger if suspended sediment concentration related to floods occurring after a dry period (Fig. 5, curve 2) is taken into account. Nevertheless, concentrations predicted by (2) using a larger value of b (1.65 b less than or equal to 1.7) are not consistent with data published by Laurent (1971). For this reason, we used relationship (1) in the paper that follows, and our results can thus be considered as the minimum estimate of hyperpycnal plume production. A definitive choice between relationships (1) and (2) in Figure 5 requires additional measurements of suspended sediment lend, especially during peak flood discharges. The b value corresponding to monthly discharges is estimated to be 1.7 to 1.75 (which leads to mean annual suspended sediment concentrations in the range 0.68-0.83 kg m(-3) Relationship (1) suggests that the critical discharge needed to produce a turbid hyperpycnal plume during floods is of the order of 1250 m(3) s(-1), depending on the salinity and temperature of the sea water near the river mouth (Fig. 6). A hyperpycnal plume is a turbidity current generated at a river mouth when thedensity of the river plume exceeds the density of ambient sea water due to sediment in suspension. The plume plunges and becomes an auto-maintained turbidity current. The threshold is between 620 and 750 m(3) s(-1) using relationship (2) in Figure (5). Using relationship (1), short duration ( 10,000 m(3) s(-1). Nevertheless, this hypothesis remains speculative, since no real sedimentological evidence exists to differentiate turbidites due to hyperpycnal plumes from those generated by ignitive failures. A final remark relates to the event that occurred in 1979. Since discharge values as high as 1200 m(3) s(-1) were measured at the river mouth during the period preceding the triggering of the slide and since the event involved a fall flood occurring after a dry summer, a hyperpycnal plume might have been triggered and have played a role in the sedimentary mass wasting processes that took place later. For the Var River, hyperpycnal plumes are probably one of the most important transport mechanisms according to the volume of sediment transferred, as shown by the 1994 flood. This Study confirms that the Var River is one of the best sites in the world for the study of slope sediment transfer processes, including: (1) - hyperpycnal plumes with a direct sediment transport from the continent to the basin; (2) - high-frequency surficial failures in underconsolidated sediments due to excess pore pressure; and (3) - lower-frequency failures affecting a large volume of sediment and generated by external forcing. Although mechanisms (1) and (2) include unitary events involving relatively low volumes of sediment, they may represent a large contribution to the construction of the Var delta because of their higher frequency in comparison of externally-forced failure events.