Wood Retention at Inclined Bar Screens: Effect of Wood Characteristics on Backwater Rise and Bedload Transport

Abstract

In forested mountain catchment areas, both bedload and large wood (LW) can be transported during ordinary flows. Retention structures such as sediment traps or racks are built to mitigate potential hazards downstream. Up to now, the design of these retention structures focuses on either LW or bedload. In addition, the majority of LW retention racks tend to retain both LW and bedload, while bedload transport continuity during ordinary flows is an important aspect to be considered in the design. Therefore, a series of flume experiments was conducted to study the effect of LW accumulations at an inclined bar screen with a bottom clearance on backwater rise and bedload transport. The main focus was put on testing different LW characteristics such as LW size, density, fine material, and shape (branches and rootwads), as well as a sequenced flood. The results demonstrated that a few logs (wood volume of ≈ 7 m3 prototype scale with a model scale factor of 30) are sufficient to reduce the bedload transport capacity to below 75% compared to the condition without LW. Fine material and smaller wood sizes further reduced bedload transport and increased backwater rise. In contrast, LW density and LW shape had a negligible effect. The test focusing on a sequenced flood highlighted the need for maintenance measures to avoid self-flushing of the bed material. The results of this study further indicate that an inclined bar screen may need to be adapted by considering LW characteristics in the design of the bottom clearance to enable bedload continuity during ordinary flows.

Collapse dynamics of two-dimensional dry and immersed granular columns of elongated grains

“…For larger grains, the features specific to elongated grains are further amplified when surrounded by a fluid [26], but the extent to which they affect the mobility of granular flows is yet poorly understood, especially in the immersed case. The latter is, for example, of great interest in understanding the jamming of driftwood debris [46,54,55] or in the stability of layered cliffs which can result from debris or pyroclastic flows [42,73], where the collective orientation of its constituent grains plays a dominant role in the process.…”

The Effect of Large Wood Accumulations With Rootwads on Local Geomorphic Changes

“…Schalko et al. (2021) highlighted the negligible importance of LW shape (e.g., rootwads and branches) on both backwater rise and bedload retention. Welber et al.…”

“…Among the few studies that have been conducted so far, Schmocker and Hager (2011) demonstrated that, under steady flow conditions, the blocking probability for both simple logs and logs with rootwads decreases with an increasing Froude number. Schalko et al (2021) highlighted the negligible importance of LW shape (e.g., rootwads and branches) on both backwater rise and bedload retention. Welber et al (2013) explored the characterization of LW deposition patterns, linked to LW characteristics (e.g., rootwads), flow rate, and bed morphology.…”

Impact of Large Wood on River Ecosystems

“…Schalko et al [21] present flume experiments to study the effect of LW accumulations at an inclined bar screen with a bottom clearance on backwater rise and bedload transport. The main focus was put on testing different LW characteristics.…”