Abstract
Gravel bars have an important role in the exchange between surface and subsurface waters, in preventing and mitigating riverbank erosion, in allowing the recreational use of rivers, and in preserving fluvial or riparian habitats for species of fishes, invertebrates, plants, and birds. In many cases, gravel bars constitute an important substrate for the establishment and development of ground flora and woody vegetation and guarantee higher plant diversity. A sustainable management of braided rivers should, therefore, ensure their ecological potential and biodiversity by preserving a suitable braiding structure over time. In the present study, we propose an analytical–numerical model for predicting the evolution of gravel bars in conditions of dynamical equilibrium. The model is based on the combination of sediment balance equation and a regression formula relating dimensionless unit bedload rate and stream power. The results highlight the dependence of the evolving sediment particles’ pattern on the ratio of initial macro-bedforms longitudinal dimension to river width, which determines the gradual transition from advective and highly braiding to diffusive transport regime. Specifically, the tendency to maintain braiding and flow bifurcation is associated with equilibrium average bed profiles and, therefore, equilibrium average stream power characterized by the maximum period that does not exceed transverse channel dimension.
Highlights
Braided rivers are typically organized in multithread patterns, with central or alternate bars dividing channels whose reach-scale statistical properties are controlled by discharge, bed slope, and grain size [1,2,3,4,5]
We propose an analytical–numerical methodology for the reconstruction of the equilibrium dynamics of gravel braided beds
The mathematical model is based on the combination of sediment continuity with or without forcing term and the practically quadratic relationship between dimensionless unit bedload rate and dimensionless unit stream power, which was derived by the authors in a previous work by regression, including ad hoc laboratory experimental measurements performed at high grain Reynolds numbers and real river data available in the literature
Summary
Braided rivers are typically organized in multithread patterns, with central or alternate bars dividing channels whose reach-scale statistical properties are controlled by discharge, bed slope, and grain size [1,2,3,4,5]. Bedload transport in gravel-bed rivers is a very intricate phenomenon that is characterized by high variability in time and space [8,9,10,11] Such variability derives from different mechanisms that act at different spatial scales [12,13,14,15], ranging from grain to reach: (1) the movement of single grains (e.g., [16,17,18,19]); (2) the creation of small bed forms, such as sediment waves [6], bed waves [20], and bedload sheets [21,22]; and (3). Channel morphological dynamics are the result of single-particle displacements from erosion sites
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