Linking process and depositional architecture in large rivers: an integrated field and modelling approach

BEST, JAMES; ASHWORTH, PHILIP J.; AMSLER, MARIO L.; HARDY, RICHARD J.; LANE, STUART N.; NICHOLAS, ANDREW P.; ORFEO, OSCAR; PARSONS, DANIELS; REESINK, ARNOLD J.H.; SAMBROOK SMITH, GREGORY H.; SZUPIANY, RICARDO N.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

Little is known about the processes, dynamics and deposits of the World's largest multi-thread rivers and whether they differ from smaller (less than 1 km wide) channels. This paper reports on an integrated field and numerical modelling campaign on the sandy Rio Paraná, Argentina, the World's 6th largest river. Bathymetric and 3D flow data were taken in a 38 km long, 4 km wide reach using single-beam echo sounders, acoustic Doppler current profilers and dGPS. Results demonstrate that the main channels of the Rio Paraná, at low flow, are dominated by dunes up to 3.5 m high and less common unit bars 2-5 m high, with some up to 10 m high. Analysis of satellite images from 1973-present shows that mid-channel bars develop from an amalgamation of unit bars that stall at barheads and wrap around bartails. However, in the Rio Paraná, and unusually for such a large and dynamic river, new mid-channel bar formation is relatively infrequent and is often associated with flow divergence at, or near, established vegetated islands. The deposits of seven, km-scale, mid-channel bars were characterised by ~43 km of Ground Penetrating Radar (GPR), and ground truthed by suction cores. The internal structure of mid-channel bars is dominated by: (i) decimetre to sub-m high, stacked dune sets, (ii) up to 7 m thick, high-angle, bar margin sets, and (iii) decimetre-thick ripple sets, most commonly, but not exclusively found at the bartop. Re-activation surfaces on bar margins are common. The input of a significant finer, suspended sediment load, from the Rio Paraguay, leads to a change in the depositional architecture of the bars immediately downstream, with larger scale cross-stratified sets being less frequent. The fieldwork has been complemented by numerical modelling of flow, sediment transport and morphological change in the 38 km study reach using a combination of 3D Computational Fluid Dynamic (CFD), Reduced Complexity (RC) and Delft3D depth-averaged models. The models show a good match between field and modelled flow distributions. Simulated channel change is characterised by talweg shifting, but does not indicate the significant influence of migrating unit bars. Ongoing modelling is quantifying the impact of unsteady discharge on bar formation and simulating bar evolution in a 10 km sub-reach over ~30 years, in order to test if the process of bar evolution matches the rate and form of change observed in the satellite record.