linking 3D particle-scale structures to the evolution of gravel bed morphology

Background

Fig. 1: Interactions that produce and maintain river bed morphology. Processes that incorporate sediment structure are highlighted in bold italic and are additional to the original diagram of Ashworth & Ferguson (1986).

Understanding river bed stability and development is critical to earth science, hydrology, managing flood and erosion-risks and aquatic ecology. Figure 1 outlines the dynamics acting on a river bed and demonstrates the complexities involved. Recent advances in measurement techniques and 3D flow modelling has greatly improved our comprehension of flow structure at the river bed. Our knowledge of other factors in figure 1, namely bedload transport and sediment structure, is still limited. Estimates of bedload transport typically rely on spatially averaged grain-size measurements and shear stress values, and are generally only accurate to within an order of magnitude.

Critical shear stress for sediment entrainment is a key factor for predicting bedload transport but many factors important to bedload movement such as sediment structure and the role of fine sediment mortaring the gravel is often overlooked. The complexity of sediment structure is acknowledged to be important but has rarely been demonstrated (Kirchner et al. 1990; Jerolmack, 2005). Some research has demonstrated that disrupting the natural sediment structures in a river bed can increase sediment transport during the next flood, and that variations in critical shear stress caused by sediment structure are more important than difference in shear stress within the flow itself.

Quantification of the impact of sediment structure on critical shear stress is just beginning. Hodge et al. (2013) have demonstrated that significant spatial variation in critical shear stress would produce spatially variable sediment transport and thus affect channel-scale morphological development. The role of fine sediment is also poorly understood but has been shown to significantly increase values of critical shear stress. The subsurface sediment structure, which influences surface structure via packing aroud surface particles, is very rarely measured.

linking 3D particle-scale structures to the evolution of gravel bed morphology

Background

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