Observations of bed form evolution with field-scale oscillatory hydrodynamic forcing

The evolution of rippled beds in response to irregular waves was examined over short temporal scales. Observations of natural irregularly rippled beds and the oscillatory, two‐dimensional, time‐varying velocity field were collected using a submersible particle image velocimetry (PIV) system. Bed form evolution regimes characterized by the ripple radius of curvature were examined relative to measures of the nondimensional bed stress (the grain roughness Shields parameter), the nondimensional pressure gradient (the Sleath parameter), and the water column coherent structure formation from the ripples (the swirling strength). Bed forms were found to respond to individual waves by modulating amplitude as wave groups passed. The bed form approached its initial geometry following the passage of a group even when the critical value of the Shields parameter for sheet flow was exceeded. However, when the Sleath parameter exceeded the critical value for plug flow, the bed form profile flattened, requiring significant time after the wave group passed to rebuild the ripples. These observations suggest that the ability to predict bed form geometry with respect to wave forcing is possible but requires knowledge of the initial bed form geometry, as well as the flow field evolution.