Bedform Growth Beneath Periodic Internal Solitary Waves Propagating Through a Tidal Flow

Internal solitary waves (ISWs), ubiquitous to lakes and coastal oceans, have often been proposed as a mechanism for generating subaqueous sandwaves. Yet, the physical mechanisms underlying ISW-driven bedform growth remain poorly understood. We conducted laboratory experiments to investigate bedform generation by periodic ISWs propagating through a background tidal flow. The regularly-spaced bursts of resuspended material (wisps), that occurred behind the progressive ISW coincided with increased fluctuations in the 3-D velocity field and bed stress. We attributed these to vortices generated by the amplification of the most unstable mode of the separated bottom boundary layer (BBL). We propose a feedback mechanism, between the BBL instability and bedforms, that leads to bedform generation: the BBL instability from a leading ISW creates spatially-periodic bed defects that act as a source of perturbations to the BBL, amplifying the instability and subsequent bedform growth upon passage of trailing ISWs. The spacing between wisps and the bedform wavelength matched the wavelength of the most unstable mode of the BBL, as predicted by a linear stability analysis. The ISW-induced BBL flow drives sediment resuspension and bedform generation, but these processes are modulated by the background tide. During ebb tide, the BBL currents beneath the ISW trough are reinforced by the tidal flow, increasing the local Reynolds number imposed on the BBL and the instability growth rate. The opposite interaction occurs during flood tide. The proposed feedback mechanism provides a mechanistic explanation for bedforms generated beneath periodic ISWs in a laboratory setting.