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ABSTRACT

An experiment was conducted to study megaripple morpho dynamics on a sandy intertidal shoal in a mesotidal mangrove creek (Gordon Creek, Townsville, Australia). Tidal current velocity and depth were recorded with S4 current meters over a period of 35 tides. The tidal megaripples were 0.06–0.2 m in height and 1–2 m in wavelength, and their movement was monitored by (1) electromagnetic bed-elevation probes (which automatically recorded bed level every 2 min at three positions along the survey transect) and (2) daily surveying for 8 days around spring tidals.

The tidal currents in Gordon Creek are ebb-dominated, with maximum depth-mean current velocities for the flood and ebb tides of 0.62 and 0.98 m s−1 respectively. Significant bedload transport occurs only during spring tides, and only on the larger of the unequal semi-diurnal tides. Bedload transport is overwhelmingly in the ebb direction. Megaripple migration rates reach 5.6 m per tide in the ebb direction and up to 0.1 m min−1 within individual tides. Within-tide ‘bedform transport rates’are up to 0.29 kg m−1 s−1.

The results suggest that for reconstruction of palaeoflows from deposits of preserved fine- to medium-grained sandy tidal megaripples, it is valid to use a depth-averaged velocity of 0.5–0.6 m s−1 as the migration threshold. Velocity thresholds associated with partial or complete reversal of megaripple asymmetry are invalid.