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.

Geomorphic heterogeneity as a framework for assessing river corridor processes and characteristics

“…Another channel‐focused schema designed to indicate the capacity for wood storage might consist more simply of deep channel (pools and chutes lumped together), shallow channel (glides, bars, runs, and riffles all lumped together), and vegetated bars. The high diversity of this simpler, three‐unit schema would indicate a more even mix of landforms that tend to trap wood and those that tend to transport wood, and thus a higher wood storage capacity (i.e., enough space to get wood to trapping sites balanced with sufficient trapping sites to store that wood; Okitsu, Iwasaki, Kyuka, & Shimizu, 2021; Scott & Wohl, 2018). Both schemas produce diversity values for the channel, and although these values are similar (both close to 88% of maximum diversity), they indicate completely different processes and are not meaningfully comparable.…”

“…An example is the ability of a river corridor to trap wood. Investigations of wood load variability and wood deposition patterns indicate that river corridors with a mix of landforms that transport wood and those that trap wood tend to accumulate the highest wood loads (Okitsu et al, 2021; Ruiz‐Villanueva, Wyżga, Hajdukiewicz, & Stoffel, 2016; Scott & Wohl, 2018). How would one measure the degree to which geomorphic units are mixed together, as well as their spatial arrangement?…”

Refining Geomorphic Heterogeneity as a Framework for Assessing River Corridor Processes and Characteristics

“…Another channel-focused schema designed to indicate the capacity for wood storage might consist more simply of deep channel (pools and chutes lumped together), shallow channel (glides, bars, runs, and riffles all lumped together), and vegetated bars. High diversity of this simpler, three-unit schema would indicate a more even mix of landforms that tend to trap wood and those that tend to transport wood, and thus a higher wood storage capacity (i.e., enough space to get wood to trapping sites balanced with sufficient trapping sites to store that wood; Okitsu et al, 2021;. Both schemas produce diversity values for the channel, but these diversity values indicate completely different processes and are not meaningfully comparable.…”

“…An example is the ability of a river corridor to trap wood. Investigations of wood load variability and wood deposition patterns indicate that river corridors with a mix of landforms that transport wood and those that trap wood tend to accumulate the highest wood loads (Okitsu et al, 2021;Ruiz-Villanueva et al, 2016;. How would one measure the degree to which multiple classes of landforms are mixed together, as well as their spatial arrangement?…”

Effect of Sediment Supply on Morphodynamics of Free Alternate Bars: Insights from Hydrograph Boundary Layer

“…We used the Nays2DH model [23], a two-dimensional morphodynamic model enclosed in the iRIC software [24]. This model has been applied to various morphodynamic phenomena in rivers [25], and the model can simulate the alternate bar morphodynamics focused on in this study [26,27].…”