The interaction between channel geometry, water flow, sediment transport and deposition in braided rivers

Abstract

Models of braided-river deposition must be detailed, fully 3D, and preferably quantitative to be of use in understanding and predicting the nature of ancient deposits. In order to construct and validate adequate predictive models it is necessary to have information on: (1) variation and interaction of channel geometry, water flow and sediment transport in time and space in modern channel belts, as these control erosion and deposition, the formation and migration of channels and bars, and channel abandonment and filling; (2) 3D variation of bed geometry, texture, sedimentary structures and paleocurrents throughout modern channel-belt deposits, including the age and spatial arrangement of preserved parts of bars and channel fills; (3) long-term (more than hundreds of years) trends in channel and floodplain geometry, flow and sedimentary processes in order to understand channel-belt movements such as avulsions, and the spatial arrangement of channel-belt deposits relative to overbank deposits. Such information is rare because: (1) it is difficult to study modern braided-river geometry, flow and sedimentary processes throughout a range of the all-important high discharges; (2) detailed reconstructions of braided channel and bar geometry and movement are only available for the past half-century and cannot readily be linked to causative mechanisms; (3) 3D documentation of modern deposits below the water table (especially large scale features like lateral-accretion bedding) requires extensive coring and dating of the deposits, and geophysical profiling. As a result of this lack of information, and because of the quality of analysis and presentation of the information available, existing braided-river facies models are virtually useless as interpretive and predictive tools. The nature of the information available is critically reviewed. Using information from recent detailed field and laboratory studies of the geometry, flow and sedimentary processes in braided rivers of simple geometry, in single river bends, in channel confluences, and using some theoretical reasoning, it has been possible to construct fully 3D qualitative and quantitative models of braided river deposits. These models can be used to provide sophisticated quantitative interpretations of palaeochannel geometry, hydraulics and migration, as illustrated by comparison with some particularly well described examples of ancient braided river deposits.

Field investigation of three‐dimensional flow structure at stream confluences: 1. Thermal mixing and time‐averaged velocities

“…These zones of maximum velocity represent the separate thalwegs of the two incoming tributary flows. This basic pattern is consistent with conceptual models of flow through confluences [Bridge, 1993], with previous field studies of asymmetrical Kenworthy, 1995, 1998;Rhoads, 1996 The results also support the contention that helical motion can be, but is not necessarily, a characteristic feature of confluence flow structure. Pronounced helical motion occurs only at the Kaskaskia River-Copper Slough confluence, one of three confluences examined in this study.…”

Morphological Response of the Brahmaputra–Padma–Lower Meghna River System to the Assam Earthquake of 1950

“…It is apparent from Equation 3.6 and also from the findings of other researchers such as van den Berg (1995) and Bridge (1993) that, for given slope and discharge, braiding becomes less likely as the size of the bed material increases. This finding is consistent with the earlier work of Chang (1986), and Bettess and White (1983), who also investigated the role of sediment concentration in determining the planform pattern.…”

“…Braiding intensity is strongly stage dependent (Bridge, 1993;and Nanson and Knighton, 1996) as is the size of the bars. As defined by Bristow (1987) (1987) and Nanson and Knighton (1996) 1.5 1970 1975 1980 1985 1990 1995 2000 2005 Year Reach 1 ----e-Reach 2~Reach 3 -*-Reach 4~Reach 5…”

Network response to disturbances in large sand-bed braided rivers

“…The dynamics within large braiding rivers include the interplay among bars, branches, islands, and floodplains (Bridge, 1993;Ashworth et al, 2000). A major role is played by channel bar bifurcations that distribute discharge and sediment through the braided channel network (Bolla Pittaluga et al, 2003).…”