Author(s): Isabella Schalko; Volker Weitbrecht

Linked Author(s): Isabella Schalko, Volker Weitbrecht

Keywords: BBedload transport; Flood hazard assessment; Large wood; Retention racks; River engineering

Abstract: Large wood (LW) can be transported in rivers during both small and large floods. To manage LW transport in Alpine rivers, racks have been installed to retain LW upstream of settlements or infrastructures. Such racks are commonly designed to span the river and consist of vertical poles. Due to this setup, they effectively retain LW, however, they also inhibit bedload transport and may disrupt bedload continuity. Therefore, novel rack designs are needed that retain LW, while enabling bedload transport. Flume experiments were conducted to study LW retention and bedload transport for different inclined racks to the horizontal without and with bottom clearance, various initial flow and bedload transport conditions, and LW volumes. Given an inclined rack without bottom clearance, LW predominantly accumulated at the upper part of the rack, leading to an open flow cross section below the accumulation. Therefore, backwater rise, and local scour decreased with decreasing rack angle. The effect of the rack angle has been included in an existing design equation for backwater rise. The flume experiments with an inclined rack with bottom clearance were conducted with sediment addition to evaluate bedload continuity. The results showed that a LW volume blocking 50% of the flow cross section, corresponding to 10 to 20 logs, is sufficient to reduce bedload transport by 25% during ordinary flows. Fine material and smaller log diameters further increased backwater rise and reduced bedload transport capacity.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022287

Year: 2022