Formation of fine sediment deposit from a flash flood river in the Mediterranean Sea

Abstract

We identify the mechanisms controlling fine deposits on the inner-shelf in front of the Besos River, in the northwestern Mediterranean Sea. This river is characterized by a flash flood regime discharging large amounts of water (more than 20 times the mean water discharge) and sediment in very short periods lasting from hours to few days. Numerical model output was compared with bottom sediment observations and used to characterize the multiple spatial and temporal scales involved in offshore sediment deposit formation. A high-resolution (50 m grid size) coupled hydrodynamic-wave-sediment transport model was applied to the initial stages of the sediment dispersal after a storm-related flood event. After the flood, sediment accumulation was predominantly confined to an area near the coastline as a result of preferential deposition during the final stage of the storm. Subsequent reworking occurred due to wave-induced bottom shear stress that resuspended fine materials, with seaward flow exporting them toward the midshelf. Wave characteristics, sediment availability, and shelf circulation determined the transport after the reworking and the final sediment deposition location. One year simulations of the regional area revealed a prevalent southwestward average flow with increased intensity downstream. The circulation pattern was consistent with the observed fine deposit depocenter being shifted southward from the river mouth. At the southern edge, bathymetry controlled the fine deposition by inducing near-bottom flow convergence enhancing bottom shear stress. According to the shortterm and long-term analyses, a seasonal pattern in the fine deposit formation is expected.

Sediment Transport and Dispersal in the Nearshore of “Flash-Flood” Rivers.

“…Topographic effects results in an energetic area in front of the south mouth of the Barcelona harbor (see Figure 3). This is consistent with an area without significant deposition due to an increase of the bottom stress (Grifoll et al 2014). …”

“…Silt fractions from the buoyant plume were transported and deposited offshore due to near bottom fluxes rather than fluxes in the surface plume (Grifoll et al 2014). Sediment data reveals preferential mud deposition between 20 m and 50 m water depth ( Figure 3) consistent with seismic data presented by Liquete et al (2007).…”

“…Sediment grain size used to show a seward fining trend on the inner-shelf (Galparsoro et al 2010;Grifoll et al 2014;Guillèn and Jimenez 2009). Sediment transport numerical experiments were conducted without considering the effect of the river load.…”

“…Preliminary computations have been performed in order to identify the cross-shelf sorting of sediment grain-size observed in the D50 measurements (Grifoll et al 2014). In this case the model was initialized with 4 homogenous sediment classes (D50 in mm): 0.125, 0.063, 0.06 and 0.015mm.…”

“…Wave information at numerical boundaries was obtained from buoy measurements in the study area (see Figure 1.b). Hydrodynamics (currents, waves and sea-level) and sediment (suspended sediment concentrations) variables were recorded during two specific field campaigns and used to assess the skill of the code (see details in Grifoll et al 2013b, Grifoll et al 2014 in comparison to observations (Grifoll et al 2013b, Grifoll et al 2014. Results obtained from numerical experiments reveal that the water circulation in the inner-shelf is controlled by a prevalent along-shelf direction current forced by local wind on short time scales and by remote pressure gradients on synoptic time scales (Grifoll et al 2013b).…”