Evidence of Seabed Fluidization during Solitary Waves

Sediment transport in coastal environments can result in the migration of bedforms such as ripples. In some instances, the transport can be so severe that the morphologic roughness can change state (e.g. rippled to flat). These changes of states, can, at times, occur rapidly and seemingly within a single wave period. This type of extreme change can be observed when forcing mechanisms due to shear stress and pressure gradients reach significant magnitude and duration. This research utilizes a full scale wave laboratory environment (O.H. Hinsdale Large Wave Flume at Oregon State University) over a sandy substrate to closely investigate bottom boundary layer dynamics coupled with observations of extreme morphologic change from a rippled to a flat bed. The observational array includes two millimeter scale resolution profiling ADVs (Acoustic Doppler Velocimeter), a PIV (Particle Image Velocimetry) used to estimate velocity fields as well as morphologic evolution, porewater pressure sensors, and multiple single point ADVs and wave gages. The focus of this effort is to investigate the effects of solitary waves (i.e. tsunamis) upon events of extreme morphologic change, both isolated as well as introduced into bimodal wave groups.

Bed geometry evaluation was compared against estimates of the applied shear stress and the wave induced horizontal pressure gradient. Under purely short wave conditions with moderate height and periods, the pressure gradient was low relative to the vertical gradient of the shear stress at the bed and the ripple maintained its shape, migrating in the onshore direction. However, when combined with a solitary wave, the wave induced pressure gradient was relatively large and significant ripple transformation (both flattening and high asymmetry) occurred. Ripple transformation is concomitant with momentary positive vertical pressure gradients in the bed during the passing of the solitary waves, as well as large negative near bed vertical velocities. These results suggest significant bed fluidization may occur during the passing of a tsunami and would suggest that tsunami focused sediment transformation should include the combined effects of the shear stress and pressure gradient.