Partial Driftwood Rack at Gated Ogee Crest: Trapping Rate and Discharge Efficiency

Abstract

Driftwood belongs to natural rivers just like water and sediment do. A sound ecosystem requires driftwood, although it might jam at civil structures, altering the flow section and rise the backwater. Safety considerations suggest removing wood from rivers, whereas ecological experience asks for its presence. The situation might become critical if spillways clog during floods, so that their discharge capacity reduces. For narrow bays, full racks mounted upstream of the weir or overhanging piers trap the driftwood distant from the flow control section. The hydraulic capacity is then maintained, but the driftwood has to be removed after the event. We thus investigated herein with a physical model a novel partial rack, motivating the driftwood for uncongested appearance to partially pivot and pass, but ensuring a high discharge capacity under hypercongested appearance. The partial rack configuration was specified, together with the related trapping rate and discharge efficiency.

Effect of blocked driftwood on the hydraulic performance of a gated standard weir

“…Accordingly, we conducted physical model tests on a standard weir equipped with piers and an upstream reservoir to evaluate the effect of blocked driftwood on the rating curve, as well as countermeasures to handle the driftwood such that the driftwood does not significantly reduce the discharge capacity. The detailed outcomes are described in Bénet et al (2021a, b) and Bénet et al (2022).…”

“…Similar to overhanging piers, a rack installed in front of the weir retains the driftwood at a certain distance upstream, and accordingly also distant from the critical flow section. We have conducted several model tests with two rack configurations installed at various positions (30 supplementary tests by Bénet et al 2021a, and 35 supplementary tests by Bénet et al 2022). Relative bay widths of b/L M =0.40, 0.60 and 0.77 were tested, the pier overhang was p=0 m, and relative heads between χ R =0.17 and 1.02 were provided.…”

“…Piers typically represent such obstacles. The driftwood could alternatively be partially oriented and then rather pass between the piers (Bénet et al 2022). This alternative is unfortunately less efficient in terms of blockage as well as level rise.…”

“…Several studies investigated the behavior of driftwood at weirs or spillways inlets. An overview of literature in this context is given in Bénet et al (2021Bénet et al ( , 2022. All references indicate that a blocked structure imposes significantly higher heads, as compared to the free rating curve.…”

Small-Scale Modeling of Flexible Barriers. II: Interactions with Large Wood

“…When reaching an urbanized area, LW tends to obstruct bridges and other hydraulic structures, and sediment to fill channel bed, both aggravating flood hazards (Badoux et al 2014;Ruiz-Villanueva et al 2014;Mazzorana et al 2018;De Cicco et al 2018;Friedrich et al 2022). Relevant strategies to prevent LW stopping and sediment deposition in critical areas is usually (1) to adapt the bottleneck sections, e.g., by removing piers of bridges or of dam spillways and by increasing their section (Schmocker and Hager 2011;Gschnitzer et al 2017;Bénet et al 2021), and/or (2) to trap the solid transport at dedicated structures, such as the debris basins, open check dams, racks, or flexible barriers (Comiti et al 2016;Piton and Recking 2016a, b;Mazzorana et al 2018;Wohl et al 2019;Bénet et al 2021Bénet et al , 2022.…”