Near-bottom suspended sediment fluxes on the microtidal low-energy Ebro continental shelf (NW Mediterranean)

Near-bottom sediment fluxes were measured at several locations across the Ebro continental shelf during the FANS Project. Moored current meters and transmissometers were installed near the bottom at 12, 60 and 100 m water depth. Measurements of near-bottom currents and water turbidity in combination with wave data provided information about physical forcing and resulting resuspension and sediment fluxes on this Mediterranean continental shelf. Mean near-bottom sediment fluxes decrease sharply (by one to two orders of magnitude) from the inner to the mid-shelf and more gradually from the mid- to the outer shelf. The resulting near-bottom sediment fluxes are towards the SW and the alongshelf fluxes are about one order of magnitude higher than the across-shelf fluxes. Maximum near-bottom sediment fluxes occur by sediment resuspension on the inner shelf during storm events when wave shear stress increases. Wave shear stresses decrease with depth and sediment resuspension by waves occurs quite often (about 30% of the time) on the inner shelf, very sporadically (<0.5% of the time) on the mid-shelf (during the strongest storms) and never on the outer shelf. Near-bottom currents can resuspend bottom sediment only on the inner shelf, although very sporadically (<0.5% of the time). The lowest mean current shear stress is on the mid-shelf, where wave shear stresses are insignificant most of the time and finer sediment accumulates. On the outer shelf, the current shear stress increases, causing the winnowing of fine sediments and preventing the relict transgressive sands from being covered by mud. These conditions differ from those of shelves with strong tides and long period waves, where resuspension by waves reaches the outer shelf and the across- and alongshelf sediment fluxes have the same order of magnitude. (C) 2002 Elsevier Science Ltd. All rights reserved.