The Role of Dynamic Seepage Response in Sediment Transport and Tsunami-Induced Scour

Abstract

Tsunamis have long been recognized to destabilize the seabed by causing severe erosion and potential liquefaction. However, the effect of the dynamic seepage response induced by tsunami loading on sediment transport remains elusive. Here, we explicitly quantify the role and mechanics of seepage response in field-scale tsunami-induced bed mobility and scour through theoretical analyses and fully coupled hydrodynamic and morphological simulations. The increased hydraulic gradient can lower the onset threshold of the sediment motion, thus facilitating sediment transport. In the meantime, it can also curtail the fluid–sediment momentum transfer, consequently weakening sediment transport. The competing effects of seepage response on the onset threshold and fluid agitation are such that the seepage response during the depression wave does not necessarily increase bed mobility. The suspended load transport can dominate the near-field scour processes, as demonstrated with the scour beneath a submarine pipeline. The seabed suction response to the elevation wave shows insignificant effects on the continuous exchange between the suspended load and bed load, although it inhibits the near-bed sediment concentration. The seabed injection response to the depression wave induces more bed load particles to be entrained into the water column, contributing to the increased concentration. This results in increased sediment transport and exacerbated scour, especially for the bed liquefaction scenario. The seepage response plays a critical role in the spatiotemporal variations of the seabed morphology and the sediment suspension. The outcomes significantly update the knowledge about the role of seepage in the progress of tsunami-induced sediment transport and scour.