Internal solitary waves control offshore extension of mud depocenters on the NW Iberian shelf


Hydrodynamic conditions and near-bottom sediment transport on the NW Iberian shelf associated with a 5-day storm in September 2014 were monitored. During the post-storm relaxation period, active bottom sediment transport by internal solitary waves (ISWs) on a mid-shelf mud depocenter, located in between 110 and 130 m water depth (WD), was observed. To explore the potential of internal waves in sediment transport and its link to development of mid-shelf mud depocenters, we apply a weakly nonlinear model based on the variable-coefficient Gardner equation to estimate the flow fields and bottom shear stress induced by shoaling of mode-1 long internal solitary waves. Shoreward propagation of the ISWs in three representative periods (pre-, intra- and post-storm) is simulated, respectively. Transformation of the internal wave, from a single sech2 shape characterized by negative polarity and small amplitude to a dispersive trailing wave packet with varying amplitude and inverse polarity, are satisfactorily reproduced. Model results indicate enhancement of the maximum orbital velocity of the ISWs during and after the storm on the outer shelf (130–220 m WD) including the seaward margin of the mud depocenter. Bottom shear stress consequently becomes strong enough (≥0.1 Pa) to winnow unconsolidated sediment and constrains the offshore extension of the depocenter. The enhanced bottom orbital velocity and the asymmetry in the excursion direction of mode-1 long ISWs in the post-storm period prove to be efficient in transporting fine-grained sediment across shelf. Our results suggest that mid-shelf mud depocenters are not necessarily areas under permanently calm conditions where fine-grained sediment can settle straightforwardly. They could also result from convergent sediment transport from both onshore and offshore directions, and sediment may go through numerous cycles of resuspension-transport-deposition before its ultimate lasting burial.
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