The role of large-scale bedforms in driftwood storage mechanism in rivers

Abstract

The quantification of driftwood deposition in rivers is important for understanding the total budget of driftwood at the watershed scale; however, it remains unclear how such driftwood storage in rivers contributes to the overall system because of the difficulties in undertaking field measurements. Herein, we perform numerical simulations of driftwood deposition within an idealized river reach with a sand-bed, to describe the role of large-scale bedforms, more specifically, alternate bars, multiple bars, and braiding, in driftwood storage in rivers. The numerical model we propose here is a coupling model involving a Lagrangian-type driftwood model and an Eulerian two-dimensional morphodynamic model for simulating large-scale bedforms (i.e., bars and braiding). The results show that the channel with a braiding pattern provides a wide area with enhanced capacity for deposition of driftwood, characterized by exposed mid-channel or in-channel bars, leading to high driftwood storage. The alternate bar is also a large bedform representing a sediment depositional element in rivers; however, because of the narrow exposed bar area and its downstream- migrating feature during floods, the alternate bars seem to contribute less to driftwood deposition in rivers. This suggests that the role of multiple bars and braiding is critically important for the driftwood deposition in rivers.