Near-bed turbulence and relict waveformed sand ripples: Observations from the inner shelf

[1] Results are presented from a field investigation of near-bed turbulence above degrading waveformed sand ripples in 17-m water depth on the inner shelf. The heights of the 50-cm wavelength primary ripples were about 5 cm at the start of the observation period, and decreased by a factor of 2 within 15 days. The principal degradation mechanism involved fish making pits in the seafloor. Near-bed turbulent kinetic energy dissipation rates are estimated both from the energy spectrum and from the vertical structure function within the inertial subrange, and ranged from 0.1 10 6 to 3 10 6 W/kg. The friction velocity, u*, at the bed ranged from 0.3 to 0.5 cm/s, and the wave friction factor, fw, from 0.017 to 0.02. The nearbed turbulence intensities and consequently the estimated values of u* and fw are likely too small by a factor of 2, partly to satisfy the smooth-wall constraint, and partly to account for the effects of small-scale turbulence within the finite-volume range cells of the coherent Doppler system used to make the turbulence estimates. Finally, the results indicate that the hydraulic roughness of relict ripples is likely a function of both ripple height and steepness, and that the relative roughness should also depend on the near-bed wave orbital excursion. For modeling purposes, Nielsen’s ripple roughness formula is recommended, with a reduced proportionality constant to account for the effects of irregular wave forcing and non-equilibrium ripple history.