Quantifying Bottom Friction over Rippled Beds with High Resolution Observations

Previous investigations of turbulent flow over irregular topography have shown that the physical roughness provided by the presence of ripples decreases as wave forcing increases. Contributions by both wave radiation stress and turbulent Reynolds stress during periods of high waves are counteracted by reduced bedform-induced stresses from the flattened topography. These individual contributions combine to affect the total bottom friction and accompanying drag coefficient.

In this study, laboratory observations of the flow field over active ripple fields (obtained with Particle Imaging Velocimetry techniques) are used to evaluate the stresses induced by regular surface waves propagating over the seabed. The turbulent signal (contributing to the Reynolds stress) is estimated by ensemble averaging over multiple wave periods, and a bedform disturbance signal is defined by spatially averaging over individual ripples. The observations are compared with the Doubly Averaged (in both time and space) Navier Stokes equations discussed by Rodriguez-Abudo and Foster in the companion presentation. The formulation provides an excellent opportunity to evaluate the drag partitioning in active and relic rippled beds. For a range of ripple and regular wave conditions, the relative contributions by bedform induced drag, wave radiation stress, turbulent Reynolds stress, and viscous stresses are examined. In each case, we evaluate the total bottom friction with the acceleration deficit through the boundary layer.