Jennifer L. Best

Mechanisms of anabranch avulsion within gravel-bed braided rivers: Observations from a scaled physical model

Abstract The physical modelling of gravel-bed braided rivers, using a 1:20 Froude scale model, permits analysis of the types and relative occurrence of different avulsion mechanisms. Anabranch avulsion within braided rivers involves three main mechanisms: choking avulsion caused by blockage of one channel by a sediment lobe, constriction avulsion produced by deflection, confinement and subsequent diversion of the flow by a barform and apex avulsion following erosion at the outside of sinuous thalwegs and confined meander bends. Each avulsion mechanism is described and analysis of the abundance of each type illustrates choking avulsion to be predominant without braided rivers. Three factors are found to control anabranch avulsions, namely flow discharge, sediment flux and floodplain topography. These are briefly discussed together with the implications of avulsion type for the alluvial architecture of gravelly braided alluvium.

Use of GPR in developing a facies model for a large sandy braided river, Brahmaputra River, Bangladesh

A large (2 km X 1 km) sand bar has been surveyed using ground penetrating radar (GPR), augmented by vibracoring and trenching, to investigate the sedimentary structures and three-dimensional alluvial architecture. Around 8 Km of GPR profiles were collected with nine survey lines running transversely across the bar at 250 m intervals with a profile along the bar. Profiles image structures beneath the water table to depths of up to 15 m. Reflectors from GPR profiles are interpreted as sets of cross-stratification, channel erosion surfaces, bar slipface accretion, mud-drapes, upstream accretion surfaces and the water table. Elements of upstream, downstream, lateral and cross-channel accretion are identified. Apart from the preservation of a range of dunes, the dominant style of deposition within the bar is oblique downstream and cross-channel bar margin accretion with large low angle and angle-of-repose foresets. These GPR facies characteristics are used to construct a three-dimensional model of facies architecture which may be applied in the interpretation of ancient braided alluvium. Bounding surfaces on the GPR can be tied to earlier bathymetric profiles, yielding a unique insight into the chronology of sedimentation and bar evolution. Trenching and coring of the bar top confirms that dunes and bar-margin slipfaces are the predominant facies units.