MEGARIPPLE DYNAMICS AND SEDIMENT TRANSPORT IN A MESOTIDAL MANGROVE CREEK - IMPLICATIONS FOR PALAEOFLOW RECONSTRUCTIONS

An experiment was conducted to study megaripple morphodynamics on a sa ndy intertidal shoal in a mesotidal mangrove creek (Gordon Creek, Town sville, Australia). Tidal current velocity and depth were recorded wit h S4 current meters over a period of 35 tides. The tidal megaripples w ere 0 . 06-0 . 2 m in height and 1-2 m in wavelength, and their moveme nt was monitored by (1) electromagnetic bed-elevation probes (which au tomatically recorded bed level every 2 min at three positions along th e survey transect) and (2) daily surveying for 8 days around spring ti dals. The tidal currents in Gordon Creek are ebb-dominated, with maxim um 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 directio n. Megaripple migration rates reach 5 . 6 m per tide in the ebb direct ion and up to 0 . 1 m min(-1) within individual tides. Within-tide 'be dform transport rates' are up to 0 . 29 kg m(-1) s(-1). The results su ggest that for reconstruction of palaeoflows from deposits of preserve d 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 thre shold. Velocity thresholds associated with partial or complete reversa l of megaripple asymmetry are invalid.