CECOAL   02625
CENTRO DE ECOLOGIA APLICADA DEL LITORAL
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
The four-dimensional sedimentology of sandy braided rivers: linking process to product, assessing the impact of flood magnitude and questions of scale invariance
Autor/es:
BEST, J.; ASHWORTH, P.; BRIDGE, J.; LANE, S.; LUNT, I.; ORFEO, O.; PARSONS, D.; SMITH G, SAMBROOK; SIMPSON C,
Lugar:
San Carlos de Bariloche
Reunión:
Congreso; IV Congreso Latinoamericano de Sedimentología - XI Reunión Argentina de Sedimentología; 2006
Institución organizadora:
Asociación Argentina de Sedimentología
Resumen:
Sandy braided rivers form some of the most active and geomorphologically important alluvial channels in the world, ranging in scale from small braided rivers in upland valleys, to spectacular large braided channels such as the Paraná River that runs the length of Argentina. The alluvium of braided rivers also forms important economic repositories for water, oil, gas and heavy minerals and has resulted in the sedimentological architecture of braided rivers being of key importance in exploitation of these reserves, as well as in our attempts to reconstruct Earth history from interpretations of past sedimentary environments. What we require ideally is a process-based knowledge of how braided rivers behave and how their surface dynamics are represented by their deposits. We also need to understand if, and how, braided river processes and deposits may change as the scale of the river changes.....are large braided rivers such as the Paraná similar or different to smaller sandy braided rivers? During the past six years, an interdisciplinary research team from the UK, USA, Argentina and Canada, have been conducting research aimed to address some of these questions. This presentation will provide a summary of this work and highlight some common themes that are emerging. The talk will address three main areas: 1) Study of the South Saskatchewan River, Canada, using combined aerial photogrammetry, ground penetrating radar (GPR) and coring, to assess the relationship between surface process and subsurface product; 2) use of this methodology in the South Saskatchewan to assess the sedimentological and geomorphological impact of a large, 1 in 50 year, flood event on the river, and, 3) use of ground penetrating radar to examine the sedimentary architecture of a large braid bar in the Paraná River, which allows an opportunity to address the question of scaling in large rivers. Research examining the sedimentology of the South Saskatchewan River, Canada, was inspired by the seminal work of Cant & Walker (1978), who proposed a facies model for this sandy braided river that has since been adopted as the standard model from which to reconstruct the sedimentology of such rivers. We sought to re-examine the model of Cant and Walker and assess its three-dimensional applicability through use of a twofold approach: 1) repeated aerial photographic surveys of the river. A major advantage of studying the South Saskatchewan is that, since the river is impounded upstream, the suspended sediment load is negligible and the water is clear, even at the highest river discharges. Consequently, aerial photography reveals great detail of the entire river channel (Figure 1). These images, combined with novel photogrammetric techniques, have allowed us to produce the first, high resolution Digital Elevation Models (DEMs; Figure 1) of bars and channels within a sandy braided river. DEMs and maps of channel change will be presented; 2) ground penetrating radar has been used repeatedly over this time period to examine the evolving subsurface sedimentology of several sites (penetration was down to ~5m) that encompass: i) newly formed braid bars; ii) braid bars that have existed for a number of years and been modified by successive flood events, iii) areas of channel fill and channel erosion, and, iv) areas of stable channel associated with vegetated islands. GPR surveys, often in high-resolution grids, have allowed us to reconstruct the three-dimensional architecture of the South Saskatchewan River, and crucially tie this subsurface product to the temporal picture of surface geomorphological change (i.e. the fourth dimension) identified by the aerial photogrammetry. Details of the principal facies will be presented (Sambrook Smith et al., 2006), together with illustrations of the power of this research approach for detailing the nature of bar evolution and channel fill. et al., 2006), together with illustrations of the power of this research approach for detailing the nature of bar evolution and channel fill.