Water and sediment fluxes on the Ebro Delta shoreface: On the role of low frequency currents

Abstract

Field measurements of water and sediment fluxes were acquired on the Ebro Delta shoreface at depths of 8.5 and 12.5 m during winter. This period was characterised by high river discharges, a dominant NW wind and the presence of six wave storms. The survey can be considered as being typical of high-energy conditions. Water fluxes on the shoreface were jointly dominated by mesoscale shelf processes (anticyclonic gyre on the Ebro shelf) inducing a dominant northward flow and by local meteorological forcing (N–NE winds), inducing southward current reversals, which were `coupled' with the presence of E wave storms. The concentration of suspended sediment in the water column was associated with wave activity, i.e. wave-induced near-bottom velocity. Moreover, a lag of 6 h was found between the concentration time series at both locations, which was associated with a slow seaward advection of fine sediment. As a result of this advection, an increase of 30% in the content of fines of the bottom sediment at the deepest location was detected during the survey period. Measured sediment fluxes presented a pulsating structure related to mean current fluctuations and fluctuations in sediment concentration induced by waves. Longshore transport rates were dominant over cross-shore rates. When time integrated, the net cross-shore fluxes were offshore at the outer site and onshore at the inner site, due to differences in current direction, since it shifted towards the coast at the shallower location. Net longshore transport was almost nil when current reversals were present, and when disappeared, transport at the inner shelf was northward.