Bar/trough generation on a natural beach

Abstract

Mechanisms for bar/trough generation are examined using velocities measured in the field applied to theBowen[1980]/Bailard[1981] energetics‐based sediment transport model. Measurements consist of a cross‐shore array of nine electromagnetic current meters spanning the surf zone and daily bathymetric surveys during a 10‐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 suspended load transport separately based on various velocity moments. The velocities are partitioned into mean currents, low‐frequency infragravity and shear instabilities (<0.05 Hz), and high‐frequency short waves and turbulence (>0.05 Hz) to determine the relative importance of various mechanisms to the total transport. Velocity moments are computed over 90‐min intervals to resolve tidal fluctuations. Tidal signatures were apparent in all modes of transport. Predicted transport rates are integrated and compared with daily cross‐shore bathymetric profiles (averaged over a 400‐m length of beach). The suspended load terms were 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 results for this experiment indicate the dominant mechanism for bar development was sediments mobilized by the strong longshore current and incident short waves within the surf zone and transported offshore by the mean undertow and shoreward transport onshore due to short wave velocity 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.