BAR TROUGH GENERATION ON A NATURAL BEACH

Mechanisms for bar/trough generation are examined using velocities mea sured in the field applied to the Bowen [1980]/Bailard [1981] energeti cs-based sediment transport model. Measurements consist of a cross-sho re array of nine electromagnetic current meters spanning the surf zone and daily bathymetric surveys during a IO-day period during which two storms occurred, when the bathymetry evolved from a three-dimensional terrace to a well-developed linear bar. The model predicts bed and su spended load transport separately based on various velocity moments. T he velocities are partitioned into mean currents, low-frequency infrag ravity and shear instabilities (<0.05 Hz), and high-frequency short wa ves and turbulence (>0.05 Wt) to determine the relative importance of various mechanisms to the total transport. Velocity moments are comput ed over 90-min intervals to resolve tidal fluctuations. Tidal signatur es were apparent in all modes of transport. Predicted transport rates are integrated and compared with daily cross-shore bathymetric profile s (averaged over a 400-m length of beach). The suspended load terms we re an order of magnitude greater than bed load terms owing to the low fall velocity of the fine-grain sand within the surf zone. Model resul ts for this experiment indicate the dominant mechanism for bar develop ment was sediments mobilized by the strong longshore current and incid ent short waves within the surf zone and transported offshore by the m ean undertow and shoreward transport onshore due to short wave velocit y skewness. Using standard coefficients, the model correctly predicted the first-order movement of the bar during storms, but underpredicted trough development, and did not always perform well during mild wave conditions.