Morphodynamics of meso- and macrotidal beaches: examples from central Queensland, Australia

Abstract

Detailed measurements of the morphology, waves, and longshore and cross-shore currents were conducted on two beaches on the low- to medium-energy, macrotidal coastline of central Queensland (Australia). Nine Mile Beach is characterised by a relatively steep upper profile, a low-gradient intertidal zone with one or two swash bars, transverse bar/rip morphology at low-tide level and a very low-gradient, dissipative subtidal zone. The beach is composed of fine to medium sand and experiences a modal wave height of 0.75 m and a mean spring-tide range of 3.6 m. Lambert's Beach possesses a steep, coarse-grained, reflective upper profile, and a low-gradient dissipative low tide terrace composed of fine to medium sand. Subdued rip channels intersect the low tide terrace and the beach is subject to a modal wave height of 0.6 m and a mean spring-tide range of 4.6 m. The two beaches are classified as Low Tide Bar/Rip and Low Tide Terrace beaches, respectively, following the model of Masselink and Short (1993). During the present investigation, the Low Tide Bar/Rip beach (Nine Mile Beach) exhibited a larger temporal variability than the Low Tide Terrace beach (Lambert's Beach), despite similar hydrodynamic conditions over the survey period. This difference is principally attributed to the differences in the beach morphologies and the types of morphological changes that occurred. On Nine Mile Beach, morphologic changes were primarily associated with secondary features (swash bars and low tide bar/rip system), whereas on Lambert's Beach the changes in morphology were largely the result of adjustments of the overall beach profile. A number of surf zone processes were investigated, including incident-wave attenuation, infragravity-wave energy, bed return flow and longshore currents. The influence of the tide on surf zone dynamics was found to be insignificant, except in the case of longshore currents. On Nine Mile Beach, the longshore current associated with the nearshore cell circulation was stronger during low tide. On Lambert's Beach, the shore-parallel current on the low tide terrace was largely driven by the tide. A beach profile zonation was defined for Nine Mile Beach and Lambert's Beach based on an examination of the varying influence of swash, surf zone and shoaling wave processes. The four-part tidal zonation consists of: (1) an upper intertidal zone across which swash and aeolian processes are the major influence; (2) a mid-intertidal zone dominated by swash and surf zone processes; (3) a lower intertidal zone controlled by surf zone and shoaling wave processes; and (4) a subtidal zone largely influenced by shoaling wave processes.