Role of morphologic feedback in surf zone sandbar response

Abstract

Abstract. Several aspects of feedback mechanisms associated with surf zone sandbar response have been characterized using bathymetric surveys, sampled approximately monthly over a 16-year period at the Army Corps of Engineers' Field Research Facility (North Carolina). The measured bathymetry was alongshore averaged and modeled by the superposition of two Gaussian-shaped sandbars on an underlying planar slope. A third, half-Gaussian-shaped bar represented steepening at the shoreline. The rms error between the measured bathymetry and the profile model was 0.10 m (estimated over 322 different surveys). The model explained 99% of the profile variance that remained after first removing the linear, cross-shore trend from each observed profile. Bar response, which was extracted from the modeled profiles, was compared to a local hydrodynamic forcing variable F (F was defined as the ratio of the wave height to water depth, evaluated at bar crest locations). At low values of F (i.e., nonbreaking conditions), bars migrated onshore, and their amplitude tended to decay. At high values of F (i.e., breaking conditions), bars migrated offshore, with relatively little change in amplitude. The transition between onshore and offshore migration occurred at a value of F that was consistent with the onset of wave breaking. Bar migration was associated with a stabilizing feedback mechanism, which drove bar crests toward an equilibrium position at the wave breakpoint. However, we observed that the rate of bar response showed no reduction for any nonzero choice of F, indicating that bars never reached equilibrium. Systematic bar amplitude decay was observed under nonbreaking conditions. Bar amplitude decay could drive F farther away from breaking conditions, allowing further bar amplitude decay. This is a destabilizing feedback mechanism, potentially leading to bar destruction.

Dynamics of single-barred embayed beaches

“…Cross-shore bar migration has been described at different time scales. At short time scales, bars undergo offshore migration during high-energy wave conditions, when the wave height-water depth ratio is large and the undertow current (nearbottom, breaking wave-driven steady flow) is dominant (e.g., Plant et al, 2001). Onshore bar migration occurs as the wave height-water depth ratio decreases, during intermediate wave conditions.…”

Nonlinear transfer function modelling of beach morphology at Duck, North Carolina

“…Previous studies have shown that these nearshore bars at Duck interact with driving forces to generate nonlinear feedback mechanisms (Lippmann and Holman, 1990;Lippmann et al, 1993;Plant et al 2001). The stronger nonlinearity identified in this study for periods of increased beach volume is thus inferred to be associated with these nonlinear bar mechanisms.…”

“…storm waves and/or surge effects); more intricate nonlinear feedback mechanisms (e.g. "destabilising" feedback effects that result in the decay of nearshore bars under continuous nonbreaking conditions (Plant et al, 2001)); and limitations imposed by the differences in the sampling frequencies of the beach profile and wave data. It should also be recalled that the state-dependent nonlinearity investigated here is associated only with P (i.e.…”

“…Thus, the response of the beach profile can be nonlinearly related to external forcing conditions (e.g. Lippmann and Holman, 1990;Lippmann et al, 1993;Plant et al, 2001). However, previous studies have shown that the beach profile response can also be linearly related with forcing.…”

“…Changes in the sand bar position or shape will alter wave breaking (and sediment transport) patterns across the profile. Consequently, as the beach profile responds to wave forcing, alterations in the sediment transport patterns may suppress or reinforce further morphological changes (Plant et al, 2001). Thus, the response of the beach profile can be nonlinearly related to external forcing conditions (e.g.…”