Bedform Migration and Sediment Dynamics in the Nearshore of a Low-energy Sandy Beach in Southwestern Australia

Jeffrey S. Doucette

Abstract


This paper relates nearshore ripple behaviour to cyclic changes in hydrodynamic conditions due to a sea breeze. Simultaneous time series measurements of waves, currents, sediment suspension and ripple crest positions were obtained from the nearshore of a micro-tidal low energy beach in southwestern Australia during two diurnal sea breeze cycles. Large parallel ripples with wavelengths of 0.3m-1.2m and heights of 0.05m-0.15m were present in coarse sand for the duration of observations. Mean ripple wavelengths and migration rates were measured at half hour intervals. During the sea breeze, wave height increased and wave period decreased with the addition of locally generated, short period wind waves on the incident swell waves. As a result, mean offshore flow (undertow) increased and cross-shore asymmetry decreased. Ripple wavelength increased during the sea breeze in proportion to the nearbed orbital diameter. Ripple wavelength remained relatively constant, except for a slight decreasing trend, with respect to the varying near-bed orbital diameter during the swell-dominated periods before and after the sea breeze suggesting the presence of suborbital ripples. Ripples migrated onshore during the swell-dominated period find offshore during the sea breeze. It is proposed that the ripple migration direction is a function of the balance between onshore cross-shore velocity asymmetry and offshore mean flow. Increased sediment suspension concentration during the sea breeze resulted in an increase in the amount of sand available for transport by the larger mean offshore flow providing a mechanism for the offshore migration of ripples.


Keywords


Ripple migration; sediment resuspension; cross-shore flow; ripple wavelength

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