Megaripple dynamics and sediment transport in a mesotidal mangrove creek: implications for palaeoflow reconstructions

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.