Storm‐driven bottom sediment transport on a high‐energy narrow shelf (NW Iberia) and development of mud depocenters

Abstract

Bottom sediment transport on the NW Iberian shelf was monitored during a downwelling storm in September 2014. Collected data were analyzed and fed into a 3-D coastal ocean model to understand storm-driven sediment transport on the shelf and its impact on midshelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the prestorm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short-lasting strong bottom current in combination with enhanced wave-current interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from downwelling. An across-shelf horizontal density gradient as high as 0.32 g/m 4 occurred at the interface between the downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward-directed coastal jet. Simulation results suggest that during the storm, local near-bottom sediment suspensions with concentrations on the order of 10 kg/m 3 would be triggered by wave-current interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine-grained sediment.

Bottom Boundary Layer and Particle Dynamics in an Upwelling Affected Continental Margin (NW Iberia)

“…During the highly variable downwelling season, the combined effect of waves and currents achieved maximum levels of shear stress, more than sufficient to promote resuspension and erosion of surface sediments that agree with near‐bottom transport events reported by Zhang et al () for this area. This is clearly shown during the autumn and winter cruises (Figures and ) when turbidity values at the inner continental shelf achieved maximum levels.…”

“…More recently, modeled wave storm conditions support the important role of waves on the bottom dynamics (Oberle, Storlazzi, et al, ) whose interaction with bottom currents was suggested by Zhang et al () to be the key factor to understand the storm‐driven sediment transport. The presence of a thermohaline front and the possible winnowing effects of internal waves on unconsolidated sediments were also suggested to be important factors that can explain the mud depocenters shoreward and seaward limits, respectively (Zhang et al, , ).…”

“…The studies have been developed under the framework of the OMEX II (Van Weering & McCave, , and references therein), EUROSTRATAFORM (Weaver et al, and references therein), and HERMES (Masson & Tyler, ; Weaver & Gunn, , and references therein) projects. North of 41°N on this coastal margin, observational efforts to analyze seafloor dynamics and suspended sediment transport on the continental shelf have been carried out only during the OMEX II project (1997–2000) (Huthnance et al, ; Oliveira et al, ; Vitorino et al, ; Van Weering et al, ) and recently during a short period in 2014 (Zhang et al, ). In the Douro river area, current and wave measurements during 6 months of winter and spring seasons showed the important role played by waves on the BBL dynamics and how its highly energetic levels during winter were the main factor in the formation of nepheloid layers (NLs) through seabed resuspension (Oliveira et al, ; Vitorino et al, ).…”

The Impact of Waves and Tides on Residual Sand Transport on a Sediment‐Poor, Energetic, and Macrotidal Continental Shelf

“…Waves dominate sediment transport in areas of the New Zealand and Australian shelves (Carther & Heath, ; Hale et al, ; Moriarty et al, ; Porter‐Smith et al, ). Wave‐current interactions are significant drivers of sediment resuspension on the Northwest Iberian shelf (Zhang et al, ). The Northwest European shelf experiences high tidal and high wave energy, which is enhanced by wave‐current interactions in areas of strong tidal currents (Hashemi & Neill, ).…”

“…Studies of shelf-scale sediment transport are considering wave-tide coupling more regularly (Dietrich et al, 2011;Moriarty et al, 2014;Xu et al, 2016;Zhang et al, 2016); however, coupled modeling systems are more computationally expensive. A number of studies consider uncoupled tides and waves as an approximation, ignoring wave-tide interactions (e.g., Bricheno et al, 2015;Neill et al, 2010;Porter-Smith et al, 2004;van der Molen, 2002;Xing et al, 2012).…”