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.
Highlights
Potential hazards associated with intense bedload transport in steep mountain streams have traditionally been managed by building check dams and sediment trap structures [1]
Flume experiments were conducted to study the effect of large wood (LW) accumulations at an inclined bar screen on both backwater rise and bedload transport continuity with a special focus on varying LW characteristics
The results demonstrated that the inclined bar screen does not affect bedload transport prior to the addition of LW with a bedload reduction coefficient of ξ ≈ 1
Summary
Potential hazards associated with intense bedload transport in steep mountain streams have traditionally been managed by building check dams and sediment trap structures [1]. It is important to design retention structures that are effective in retaining sediment during potentially hazardous events (high flows), but are permeable during ordinary flows With this aim, a novel design of semipermeable check dams has recently been proposed by combining a hydraulic with a mechanical barrier and a guiding channel [8,9,10,11]. A novel rack design for low-gradient (small Froude number) river sections is the bypass retention system This rack is installed at an outer river bend parallel to the flow and enables the separation of bedload and LW due to the secondary currents induced by the river curvature, thereby allowing the structure to trap LW without limiting the bedload transport continuity [17]. First experiments at an inclined bar screen with a lower gap demonstrated that LW volume blocking 20% of the flow cross section is sufficient to reduce bedload transport by 50% [22] and the effect of LW on bedload transport should, not be overlooked
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