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

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, f w , from 0.017 to 0.02. The nearbed turbulence intensities and consequently the estimated values of u 2 * and f w 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.