Megaripple dynamics and sediment transport in a mesotidal mangrove creek: implications for palaeoflow reconstructions |
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| Authors: | P LARCOMBE P V RIDD† |
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| Institution: | Marine Geophysical Laboratory Earth Sciences Department, fames Cook University, Townsville 4811, Australia;Marine Geophysical Laboratory Physics Department, fames Cook University, Townsville 4811, Australia |
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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. |
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