Abstract
Abstract. A physical scale model of a gravel-bed braided river was used to measure vertical grain size sorting in the morphological active layer aggregated over the width of the river. This vertical sorting is important for analyzing braided river sedimentology, for numerical modeling of braided river morphodynamics, and for measuring and predicting bedload transport rate. We define the morphological active layer as the bed material between the maximum and minimum bed elevations at a point over extended time periods sufficient for braiding processes to rework the river bed. The vertical extent of the active layer was measured using 40 hourly high-resolution DEMs (digital elevation models) of the model river bed. An image texture algorithm was used to map bed material grain size of each DEM. Analysis of the 40 DEMs and texture maps provides data on the geometry of the morphological active layer and variation in grain size in three dimensions. By normalizing active layer thickness and dividing into 10 sublayers, we show that all grain sizes occur with almost equal frequency in all sublayers. Occurrence of patches and strings of coarser (or finer) material relates to preservation of particular morpho-textural features within the active layer. For numerical modeling and bedload prediction, a morphological active layer that is fully mixed with respect to grain size is a reliable approximation.
Highlights
Information about the distribution of grain sizes within a volume of an alluvial gravel river bed is sought for a variety of reasons
The goal of the analysis described in this paper is to characterize the grain size sorting of the morphological active layer aggregated over an area of the full width of a gravel-bed braided river in a nonaggrading or degrading state
The surface and near-surface sorting of grain sizes associated with the active morphological processes of braided rivers results in patterns of grain size related to local bed elevation and flow structure
Summary
Information about the distribution of grain sizes within a volume of an alluvial gravel river bed is sought for a variety of reasons. Analyses of the statistics of vertical tracer particle exchange for different size fractions have been used in developing observations and theories of particle kinetics for bedload prediction based on long-term mixing and burial/exhumation within the particle exchange layer (Haschenburger, 2011) This relates partly to the development of bed surface armor and the size of material available for transport at different phases of particle mobility. In braided rivers, bed topography can change rapidly during normal channel-forming events due to local bed scour, deposition and channel avulsion so that exchange depths are likely to extend through the entire range of bed elevation (of the order of 10 × D90) rather than the relatively thin grain exchange layer of more stable river beds This is a primary reason to define a morphological active layer that is distinct from the grain exchange active layer related to flood events on a stable gravel bed with limited topographic amplitude. Laser scanning and other technologies may yield equivalent data in the field but would still potentially require years of data acquisition, and periodically dry river bed, to reproduce what is possible in tens of hours in a physical model
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