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

Field measurements of water and sediment fluxes were acquired on the Ebro D elta shoreface at depths of 8.5 and 12.5 m during winter. This period was c haracterised 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-e nergy conditions. Water fluxes on the shoreface were jointly dominated by m esoscale shelf processes (anticyclonic gyre on the Ebro shelf) inducing a d ominant northward flow and by local meteorological forcing (N-NE winds), in ducing southward current reversals, which were 'coupled' with the presence of E wave storms, The concentration of suspended sediment in the water colu mn was associated with wave activity, i.e. wave-induced near-bottom velocit y. Moreover, a lag of 6 h was found between the concentration time series a t both locations, which was associated with a slow seaward advection of fin e sediment. As a result of this advection, an increase of 30% in the conten t of fines of the bottom sediment at the deepest location was detected duri ng the survey period. Measured sediment fluxes presented a pulsating struct ure related to mean current fluctuations and fluctuations in sediment conce ntration induced by waves. Longshore transport rates were dominant over cro ss-shore rates. When time integrated, the net cross-shore fluxes were offsh ore 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 loca tion. Net longshore transport was almost nil when current reversals were pr esent, and when disappeared, transport at the inner shelf was northward. (C ) 1999 Elsevier Science B.V. All rights reserved.