Role of morphologic feedback in surf zone sandbar response

Several aspects of feedback mechanisms associated with surf zone sandbar re sponse have been characterized using bathymetric surveys, sampled approxima tely monthly over a 16-year period at the Army Corps of Engineers' Field Re search Facility (North Carolina). The measured bathymetry was alongshore av eraged and modeled by the superposition of two Gaussian-shaped sandbars on an underlying planar slope. A third, half-Gaussian-shaped bar represented s teepening at the shoreline. The rms error between the measured bathymetry a nd the profile model was 0.10 m (estimated over 322 different surveys). The model explained 99% of the profile variance that remained after first remo ving the linear, cross-shore trend from each observed profile. Bar response , which was extracted from the modeled profiles, was compared to a local hy drodynamic forcing variable Gamma (Gamma was defined as the ratio of the wa ve height to water depth, evaluated at bar crest locations). At low values of Gamma (i.e., nonbreaking conditions), bars migrated onshore, and their a mplitude tended to decay. At high values of Gamma (i.e., breaking condition s), bars migrated offshore, with relatively little change in amplitude. The transition between onshore and offshore migration occurred at a value of G amma that was consistent with the onset of wave breaking. Bar migration was associated with a stabilizing feedback mechanism, which drove bar crests t oward an equilibrium position at the wave breakpoint. However, we observed that the rate of bar response showed no reduction for any nonzero choice of Gamma, indicating that bars,never reached equilibrium. Systematic bar ampl itude decay was observed under nonbreaking conditions. Bar amplitude decay could drive Gamma farther away from breaking conditions, allowing further b ar amplitude decay. This is a destabilizing feedback mechanism, potentially leading to bar destruction.