Morphodynamics of macrotidal rivers in far Northern Australia

Abstract

This thesis compares the morphodynamic behaviour of the South Alligator, Daly and Adelaide rivers in far north Northern Territory, Australia. These three macrotidal rivers have formed under similar boundary conditions of climate and sea level rise, but exhibit dissimilar plan and cross-sectional morphometries, hydrodynamic behaviour, bed sediment characteristics and sediment transport regimes. The rivers under consideration are large, funnel-shaped, meandering estuaries which are tidal for a distance of between 100 and 130 km. They are characterized by high tidal ranges (up to 6 m). large width/depth ratios up to 350), high current velocities (up to 2.5m/s)and high suspended sediment concentrations (up to 16 g/1). Channel migration rates vary greatly between and along the rivers, ranging from 0.1 m/year to 65 m/year. The monsoonal climate of the region produces a highly seasonal fluvial runoff regime; during the dry season the rivers are strongly tide dominated, while during the wet season, tides are modified by wet season flooding. The degree of tide modification by fluvial floods "depends principally on catchment size, which ranges from ~10000 km² in the South Alligator and Adelaide rivers, to ~50000 km² in the Daly river. Both empirical and deterministic approaches are used in this study to describe interactions between channel form. tidal hydrodynamics, bed sediment texture and sediment movement in these three rivers. At several locations along each river, field measurements were made of channel bathymetry, tidal behaviour, current velocities, suspended sediment concentrations and bed sediment texture. Aerial photo interpretation provided information on channel widths, meander geometries and rates and patterns of channel migration along each of the rivers. Tidal elevation and current velocity data were used to calibrate a one- in the three rivers under a range of hydrodynamic conditions. Tidal flow predictions obtained from the model were then applied to a suite of sediment transport formulae to estimate saltation load transport capacities at different points along each of the rivers. Tidal asymmetry is shown to greatly influence the sediment transport regime in the rivers. Shortened flood tide durations are accompanied by appreciably .higher current velocities than prevalent during the ebb tides. Results show that the resultant of upstream and downstream directed sediment transport, averaged over a year, depends strongly on the degree of tidal asymmetry and the relation between tidal prism and wet season fluvial flood discharge. Special emphasis is placed on describing the evolution, morphometry and hydrodynamic behaviour of cuspate meanders. These meander forms are distinguished from regular sinuous meanders by a cusp-shaped inner bank at the bend apex. Between successive bend apex, broad ovoid pools occur which flood and ebb flows pass on alternate sides of mid-channel shoals. Large horizontal eddies occur within these areas and are shown to focus sediments in shoals. thereby influencing meander bathymetry. Cuspate meanders are shown to evolve from cutoff and/or enlargement of regular sinuous meanders The various tidal hydrodynamic and sediment transport factors influencing the stability of cuspate meanders are discussed . Observations of channel form. hydrodynamic behaviour, and sediment transport are drawn together to illustrate the relative roles of tidal and fluvial forces in controlling channel morphodynamics in the three river systems Results show that they represent a spectrum of tidal river types, ranging from strongly tide dominated in the case of the South Alligator river. to strongly fluvially dominated in the case of the Daly river. The influence of inherited channel morphologies on tidal river morphodynamics is recognized. Show more