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Step-pool sequences typically form in steep mountain streams. Single step-pool units exist in steep channels with bed slopes exceeding 4% and a continuous step-pool morphology occurs in channels with bed slopes higher than 6%-7% (Church & Zimmermann, 2007;Recking et al., 2012). The flow regime in a step-pool channel is referred to as tumbling flow as the flow alternates between supercritical flow over the steps and subcritical flow in the pools (Whittaker, 1987). Thus, a large amount of flow resistance is attributed to the spill caused by hydraulic jumps (

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.

Typical features of steep mountain streams are alternating channel-spanning steps and pools (Church & Zimmermann, 2007;Comiti & Mao, 2012;. Isolated step-pool units form in streams with bed slope higher than 3%-4% .Step-pool sequences, that is, consecutive step-pool units, are expected in streams with bed slope exceeding ∼7% (Chin, 1999; Church & Zimmermann, 2007;Comiti & Mao, 2012;Whittaker & Jaeggi, 1982). The prerequisites for step formation are heterogeneous and widely graded bed material being mobile during step-forming conditions, high flood events (i.e., high bed shear stresses), and low sediment supply (

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