Feedback mechanisms in the morphodynamics of multiple barred nearshores.

摘要

It is increasingly recognised that the equilibrium model of nearshore morphodynamics is too simplistic and that nearshore morphology reflects a more complex dynamic related to the cumulative effect of a sequence of storms. The purpose of this study is to identify the feedback mechanisms between the pre-existing morphology and the hydrodynamics and sediment transport within the inner surf zone, and to describe the consequence of those feedbacks to the behavior of the nearshore environment over a sequence of storm events. Data relating to suspended sediment transport and to wave transformation were collected from the inner surf zone of a marine tidal and a lacustrine multiple-barred nearshore; the results from the latter site were integrated with a morphometric analysis of seasonal profile variability. Results suggest that the direction and magnitude of suspended sediment transport varies with the magnitude of local wave dissipation; a strong feedback at the individual bar scale. Within the outer surf zone, suspended sediment transport and bar response tend to converge at a condition of Hrms h-1 ≈ 0.3, which is maintained over a locally defined range of incident wave heights due to the ability of the outer bar to filter the incident wave field through breaking and the generation of secondary (harmonic) waves. Further wave breaking offshore also leads to both a weakening of the undertow current within the intertidal zone and to a bar-scale relationship in which suspended sediment transport and bar response are divergent relative to H s h-1 ≈ 0.6; potentially the source of much of the unexpected behavior in nearshore morphodynamics. The feedback relationships at the bar-scale are further influenced by shoreward transport resulting from a modulation of the incident wave field at infragravity frequencies that both balances the offshore transport by undertow currents and forces the inner bars to a position coincident with the antinodes of the standing wave structure. These results, when combined with the morphometric analysis, suggest that the expression of a bar cycle over a sequence of storm events is not affected by storm chronology, although the timing and frequency of that cycle depends on the sequence of storm events.