Sediment dynamics during wet and dry storm events on the Têt inner shelf (SW Gulf of Lions)

Abstract

The importance of short-term processes, such as floods and storms, on sediment delivery and reworking on the Têt inner shelf was investigated. The Têt inner shelf is a small, event-dominated system located in the south-western part of the Gulf of Lions. The expected sedimentary scenario in this environment is that fluvial sediment should be deposited on the prodelta and later dispersed around the shelf and slope by wave and current reworking during storms. This paper investigates differences in inner shelf sediment dynamics between storm events occurring during usual river discharges and those occurring during river floods. Waves, current velocities, near-bottom water turbidity, bottom sediment grain-size and sea-floor erosion/accretion were concurrently measured at 28-m water depth on the Têt inner shelf from November 2003 to March 2004. Two major storms took place on 4 December 2003 and 21 February 2004, and two moderate storms occurred on 8 December 2003 and 14 March 2004. The two major storms displayed similar wave characteristics: maximum significant wave height (Hs)>7 m, peak period (Tp)>12 s and wave direction around 90°. The main environmental differences during the two major storm events correspond to the amount of sediment discharged from the Têt River. About 2 × 104 t of sediment was delivered by the river during the 4 December flood (wet storm) and less than 5 × 102 t during the 21 February storm (dry storm). Sediment dynamics were quite similar during the storm events: resuspension caused by waves and sediment advection towards the southeast due to near-bottom shelf currents were the dominant sedimentary processes. The result was a bottom sediment erosion of several centimetres at the study site during both the wet and dry events. The main differences between the wet and dry events arose after the storm. Immediately after the peak of the wet storm, sediment supplied by the Têt River (and probably from other rivers) was deposited around the river mouth. A few days later, during a moderate storm, this unconsolidated sediment was resuspended, transported offshore and deposited on the inner shelf. In contrast, a moderate storm which occurred some days after the dry February storm caused bottom erosion on the inner shelf, because no fresh sediment was available on the shallower area. The results from this study indicate that the Têt inner shelf at 28-m water depth is mainly a bypass zone for sediment that is transported S-SE towards the middle-outer shelf and slope, although ephemeral sediment deposits may be favoured by the sediment supplied from flood events. Sediment transport across the shelf, from the river to the slope, follows a complex, multi-step pattern that needs to be addressed using a multi-event approach.