Morphodynamics of a macrotidal beach

Abstract

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-2 of 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-1 just 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.