The relative importance of landward-directed tidal sediment transport versus freshwater flood events in the Normanby River estuary, Cape York Peninsula, Australia

Abstract

Variation in freshwater discharge into the tropical estuaries of northern Australia is extreme between wet and dry seasons, and the resultant effect upon sedimentary processes in relation to the geomorphological evolution of the coastal plain is largely undocumented. Observations of tidal height, currents and suspended sediment concentrations were taken over a 3-week period in July 1994 (dry season) in the mesotidal Normanby River estuary, northeastern Australia. The estuary and adjacent inner shelf were surveyed using a 3.5 kHz seismic system and an echo-sounder, and sampled with a total of 175 grab samples and 17 vibrocores. Sediment sampling occurred in July 1994, December 1994 and March 1996, the latter following the 1995/96 wet season. Flood tidal currents dominate the estuary at spring tides with peak velocities of 0.85 m s −1 recorded at the mouth. Measured tidally driven sediment transport is landward at the mid-estuary site, and extrapolation gives estimates of 15,000–30,000 tonnes of bedload and ca. 50,000 tonnes of suspended load moved landward per year. The surficial sediments in the main estuarine channel are inferred to be Holocene, and are: (1) delta-front sediments (which extend to at least 6 km seawards of the mouth); (2) lower estuarine silts (located at 2–11 km landwards of the mouth); and (3) channel dune sands (13 to >50 km, i.e. nearly to the tidal limit). All dunes surveyed on the river bed during the dry season showed a strong asymmetry, with lee faces consistently facing landward, whether surveyed at high or low water, indicating landward bedload sediment transport in the dry season. An underlying hard regional surface is exposed at 11–13 km landward of the estuary mouth. In the absence of direct measurements we have estimated sediment transport during wet season freshwater flood events. A major freshwater flood event may transport only 6000–32,000 tonnes of bedload seawards, and modern wet season events appear incapable of supplying sand in significant quantities to the inner shelf. We infer that for the late Holocene, the frequency, magnitude and duration of wet season events has been insufficient to reverse the landward bedload transport driven by tidal currents in the dry season.