Morphodynamics of a macrotidal beach

  • Wright L
  • Nielsen P
  • Short A
 et al. 
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An intensive field investigation of hydrodynamic processes, processes of sediment entrainment and suspension, and morphologic change was carried out on an unprotected macrotidal beach near Broome in northwestern Australia. The spring-tide range was 9.5 m; waves had heights of 0.5-1.2 m and periods of 9-13 s. The beach had an overall concave-upward profile with low-gradient and dissipative subtidal and low-tidal zones, and steeper more reflective mid-tidal and high-tidal zones. Direct measurement of energy-flux dissipation over the intertidal profile showed dissipation rates on the order of 1-2 W m-2of bed and indicated an approximate balance between shoaling and dissipation of unbroken waves so as to maintain a constant wave height. Time-averaged predictive estimates of wave work over the lunar half cycle for different points on the intertidal profile show similar dissipation rates and reveal a relatively uniform distribution of work over most of the profile but with maxima in the middle of the low-tidal zone and over the lower part of the high-tidal zone. Most of the work over the low- and mid-tidal zones was performed by unbroken shoaling waves rather than by surf-zone processes; surf-zone processes only dominate over the high-tidal zone. The nature of the surf-zone processes varied across the profile as local gradient and degree of reflectivity changed with changing tide level. The growth of standing waves and infragravity ("surf-beat") oscillations, as identified from spectra and cross spectra of surface elevation, η, and currents, u and v, was inhibited over most of the profile. However, well-developed secondary standing wave energy, particularly at infragravity frequencies, was observed over the high-tidal zone at spring high tide and over the mid-tidal zone at neap high tide. Over the low-tidal and subtidal zones, strong shore-parallel tidal currents were subordinate only to the orbital velocities of unbroken incident waves. Over the subtidal zone asymmetrical tidal currents, skewed toward the north, attained maximum speeds of 0.5 m s-1just after high water. Field measurements of suspended-sediment concentration profiles under broken and unbroken waves showed very good fit to a diffusion model for wave-induced sediment suspension and suggested that sediment suspension was probably attributable largely to waves. Northerly advection of wave-suspended sediment by asymmetrical tidal currents over the subtidal and low-tidal zones accounted for a net northerly longshore transport. The greatest morphologic mobility of the intertidal profile occurred over the lower high-tidal and upper mid-tidal zones corresponding to the position of the coarsest material and secondary maximum of time averaged wave work and to a beach state intermediate between the reflective and dissipative extremes. Temporally, the greatest mobility of the profile as a whole was observed on the short, within-tidal-cycle time scale. Net changes over the longest time scale of a lunar half cycle were negligible. © 1982.

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  • L. D. Wright

  • P. Nielsen

  • A. D. Short

  • M. O. Green

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