Wave-driven porewater and solute circulation through rippled elastic sediment under highly transient forcing

Waves induce porewater flow and solute transport through permeable marine sediment. However, past studies have ignored high-frequency pressure pulses, under the assumption that the porewater flow field is adequately represented by a time-averaged one or that the saturated sediment is incompressible. We modeled porewater flow and solute transport inside ripples, forced by instantaneous pressure profiles along the sediment-water interface (SWI) with 0.1-s temporal resolution. The transient pressure profiles were taken from a field data–driven large-eddy simulation model of wave-driven oscillatory flow. The simulations suggest that in elastic, permeable, and saturated sediment, a time-averaged representation of the flow field may be inadequate and that this also leads to shortcomings in how transport is modeled. Bursts in fluid flushing occur when high-frequency pressure fluctuations were considered, leading to larger long-term average fluid fluxes compared to a steady flow field driven by a time-averaged pressure profile. The pressure perturbations along the SWI propagate within a few milliseconds to meter depths within the sediment leading to strongly transient porewater velocity fields. This leads to enhanced dispersion of solutes and larger time-averaged solute fluxes. However, enhanced solute flux across the SWI diminished through time with increasing permeability. The high-frequency transient pressures and sediment elastic properties we considered have been largely ignored and unrecognized. Future observational and modeling studies should consider these processes, especially since they mediate timing-sensitive biogeochemical reactions.