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.