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Formation of Ripple Bands in a Wave-Convergence Zone
K. P. Black (2), T. R. Healy
Sidescan sonographs of the Whangarei Harbor ebb-tidal delta in northeastern New Zealand revealed distinct bands of ripples of short wavelength lying in bands on otherwise undistinguished flat sand. The ripple bands occurred in a confined and patchworked region across the delta extending some 4 km offshore and 1.0 km wide. Combined surface-gravity wave-refraction studies, numerical hydrodynamic modeling, and field measurements indicated that both the ripples and the bands were wave induced. The ripples were found to have symmetrical profiles and wavelengths increasing shorewards, both indicative of a wave-generation mechanism. The bands were aligned with the dominant surface-wave orientation, and typical bed-orbital velocities exceeded the sediment threshold speed, while tidal currents were perpendicular to the crests, and velocities were below the sediment threshold. The wave-refraction study showed that the ebb-tidal delta bathymetry concentrates surface-gravity-wave energy along a distended zone across the delta, and this distinctive band of wave-height reinforcement corresponded with the location of the bands of ripples. Numerical hydrodynamic modeling demonstrated that the shoaling waves induce a net shoreward circulation capable of transporting winnowed fines away from the ripple bands. The regional wave-height reinforcement and the associated shoreward current can explain the zonal nature of the bands, the measured coarseness of the ripple sediments compared with neighboring sites, and the presence of the bands in topographic lows.
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