CSDMS 2015 annual meeting poster AngelMonsalve
Formation and establishment of forced sediment patches in high gradient channels
Elowyn Yager, University of Idaho, Idaho, United States.
Riverbeds in high gradient channels are often composed of sediment patches, which consist of distinct areas of the bed with relatively narrow grain size distributions and greater sorting compared to the reach. Field, laboratory and theoretical modeling have shown that the presence of sediment patches affects the dynamic of the sediment fluxes, for example patches generate large spatial and temporal variations in bed load composition and transport rates, which then induces changes in the flow and boundary shear stress field. This continuous feedback partially governs the bed surface evolution and can also have biological implications (i.e. salmon spawning). Although sediment-water interactions are affected by sediment patches, rarely are they explicitly included in bed load transport calculations, this is partly due to the fact that their formation and evolution are controlled by mechanisms that are highly temporal and spatially variable, such as shear stress field, hydrograph, turbulence, local grain sorting, and hiding effects. Little is known about how water and sediment adjust to form and establish patches of sediment and what conditions are needed to sustain them. It has been hypothesized, in particular for alternate bars, that shear stress divergence induced by topographic controls is the main driver for the existence of sediment patches. As a consequence of this divergence, the boundary shear stress field is matched by divergences in the sediment transport field, with a cross‐stream sediment flux that is size‐selective. However, the magnitude of the divergence that initiates the formation of sediment patches is unknown, as is the bed response to different shear stress divergences. We conducted a series of laboratory experiments to study the mechanisms that control the formation of forced patches. Starting from an identical condition, we provide different shear stress divergences (while maintaining the channel’s averaged shear stress) to a bed that had a staggered configuration of immobile grains. Patches of sediment formed around these immobile grains and patch grain size distribution, elevation and area responded to the local shear stress field and immobile grain configuration. Patches formed at the upstream face of the immobile grains were coarser than the bed surface texture, while downstream patches were significantly finer. Here we present an analysis and results from a 3D numerical model comparing the flow and sediment transport field around a single immobile grain before and after the establishment of sediment patches. Comparisons between local surface grain size distributions are also presented for a range of shear stress divergences and immobile grain configurations.
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