Tidal influence on the intertidal bar morphology of two contrasting macrotidal beaches

摘要

Analyses of morphological and hydrodynamic data from two macrotidal beaches (mean spring ranges respectively 5.6 and 6.4 m) with intertidal bars and troughs were carried out in order to detect and explain the relationship between tides and bar morphological attributes (bar crest height and location) and morphological change. The following four aspects are treated in the paper: (1) tidal levels, vertical tidal translation rates and bar morphology, (2) tidal range and morphological relaxation, (3) the neap-spring tidal variation, and (4) tide-generated longshore currents. Bar distribution and bar heights across the profile are not related in a straightforward manner to tidal levels and expected vertical tidal translation rates at these levels, because the wave process domains involved in bar formation and bar morphological change are strongly modulated by wave energy, by tide modulated water levels which vary with the neap-spring cycle, and by the beach morphology. The variability of these conditions may explain the large variability of bar-trough beach morphology. The relatively large tidal ranges and rapidly shifting wave domains across the profile imply long periods of non-exposure of the bars to wave processes during each semi-diurnal tidal cycle, and this tide-generated effect reduces the scope for prolonged wave reworking while contributing to enhancing the long relaxation times and morphological feedback which minimise change on these beaches. The effects of the neap-spring tidal range variation on wave activity on these beaches are subordinate to those due to the random wave energy regime, but this neap-spring variation appears to influence the morphologically important intertidal drainage channels associated with bar-trough topography. Although longshore tidal currents are relatively strong on these beaches, digital elevation models show no significant longshore bar and tidal channel migration over time, due to the absence of sustained strong wind conditions, which considerably reinforce such longshore tidal currents, and which therefore are the dominant control on longshore processes on these beaches. It is also tentatively suggested that there is no established relationship between tidal range and the number of intertidal bars on these beaches.