Numerical modeling of instream wood transport, deposition, and accumulation in braided morphologies under unsteady conditions: sensitivity and high-resolution quantitative model validation

Abstract

Large wood promotes fundamental changes in river hydraulics and morphology, playing a relevant role in river ecology but also in flood hazard. Accurate predictions of large wood dynamics in terms of deposition patterns and travel distance are still lacking and only recently have numerical models been developed to this end. In this work we enhance the capabilities of the numerical model Iber-Wood in reproducing large wood dynamics in shallow braided rivers and validate it by comparing simulations with the results of previous laboratory experiments. The flume experiments provide high-resolution observations of wood travel distances and depositional patterns of wood. The comparison proves useful to improve the numerical simulation of i) the interactions between wood pieces and the river bed (e.g., when wood pieces are transported by dragging); ii) wood pieces with roots, and iii) the formation of wood jams (i.e., accumulations of > 3 wood pieces). A sensitivity analysis reveals the crucial role of bed topography, with limited effect played by drag and restitution coefficients. Taking advantage of a controlled environment with similar simplifying hypotheses, we combine the strengths of both physical and numerical modeling to explore the parameters that are most effective in controlling wood dynamics. We use the numerical model to explore the effect of unsteady flow conditions, with different wood supply input. The resulting wood depositional patterns, jam formation and travel distances during floods may improve our understanding of some of the controls on bio-geomorphic evolutionary trajectories of braided rivers.