Hydrodynamics and sediment transport within the inner surf zone of a lacustrine multiple-barred nearshore

Abstract

It has become increasingly recognized that the typically ‘unexpected’ behavior of the nearshore profile over a sequence of storm events is a response to strong feedback between the antecedent morphology and the incident wave field. This paper systematically examines the suspended sediment transport and morphodynamics within the surf zone of a lacustrine multiple-barred upper shoreface. A range of significant wave heights ( H s) and local relative wave heights ( H s h −1; h = water depth) are considered in order to describe and identify the limits to the feedback mechanisms that can dominate the nearshore response. Data from collocated concentration and velocity sensors indicate that the direction and magnitude of suspended sediment transport varies with the degree of local wave energy dissipation and the cross-shore variation in these relationships depends partly on the local bed slope. This dependence indicates the presence of a strong “self-organization” at the scale of the individual bar. Within the inner surf zone, these bar-scale relationships are constrained by the shoaling transformations, which occur over the outermost bar. For ranges of incident wave height defined locally (e.g. intermediate waves H s = 0.4 to 0.9 m; storm waves = 0.9 to 1.4 m) the innermost and middle bars remain in a state of self-organized equilibrium. For intermediate range waves, this equilibrium is a consequence of wave height reduction over the outer bar through breaking and the generation of secondary (harmonic) waves. For the storm wave range, the equilibrium is maintained through a resonant amplification of discrete edge wave modes, with the bars forced to a position coincident with the surface elevation antinodes. These upper shoreface scale feedback mechanisms are weakened (particularly for intermediate wave heights) following threshold storms capable of moving the outermost bar offshore and lower on the profile. This allows the middle and inner bars to be forced directly by variations in the offshore incident waves. Thus, the bars respond in a cyclic manner, the bar cycle.