Sedimentology and heterogeneity of braiding rivers

Group Leader: Peter Ashmore

The geometry, sedimentary architecture and grain size sorting characteristics of braiding rivers are a direct reflection of their controlling hydraulic processes and morphodynamics. While this is understood, relatively little attention has been given to how the characteristics of braiding channels, bars and floodplains are reflected in the subsurface record. Specifically, there is no general consensus  on the geometry and heterogeneity of braided river deposits and their relationship to channel morphodynamics across a range of boundary conditions, such as absolute channel size, grain size, slope and the influence of bank strength/cohesitivity.

This component of the workshop will focus on several key questions and themes concerning the sedimentology of modern braided rivers:

  1. Characterization of the three-dimensional geometry of braided river deposits and their internal structure (bedding, shapes of depositional bodies) and variations among river types from ‘labile’ sand bed rivers to shallow, coarse-grained channels. This theme will exploit recent advances in methods and instruments for investigation of river deposits and their temporal development in field and laboratory settings, as well as approaches using numerical modelling.
  2. The relationship of deposit geometry/characteristics and grain size sorting to the morphodynamic features and sedimentary processes of channel change (e.g. lateral migration, bifurcation, local scour, avulsion, bar evolution, bedforms). This includes: deposit thickness, geometry of upper and lower bounding surfaces, depth of scour/re-working of deposits (morphological active layer) and development of descriptors of these characteristics. Bar dynamics in braided rivers and the sedimentology of braided rivers in channels that have a differing dominance of unit bars.
  3. Grain size sorting (3D) mechanisms and characteristics and their relationship to river morphology, features and processes. Methods for measuring, characterizing and predicting size sorting patterns and patchiness in relation to bed elevation, and the implications for models of sediment transport in braiding rivers.
  4. Effects of bar and island vegetation on deposit characteristics, deposition of fines in gravel-bed rivers and feedbacks to vegetation dynamics. Partitioning of bed and suspended load sediment transport and its links to morphodynamics, bank erosion and channel stability.
  5. The role of event magnitude in modifying or ‘re-setting’ channel geometry, and the consequences for recognising such events in the ancient sedimentary record, for deposit geometry and sediment size sorting.
  6. The links between the morphodynamic evolution of braided rivers and their deposits and the subsurface movement of fluids within such deposits, including their modelling within both industrial (e.g. hydrocarbon and aquifer flow) and environmental (i.e. groundwater pollutant and organic material accumulation) applications within braided river alluvium.

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