Reservoir Level Rise under Extreme Driftwood Blockage at Ogee Crest

Abstract

Dams are civil structures essential to modern civilization. However, they can be a threat if not properly designed and operated. A particular risk that potentially can lead to dam failure is the blocking of the spillway inlet with driftwood or debris. This study investigated, on the basis of physical modeling, this blocking as well as the related backwater rise and discharge-capacity reduction. Considerable quantities of driftwood were supplied upstream of an ogee weir with piers, and the subsequent reservoir level rise was measured. Particular focus was placed on extreme events in terms of driftwood occurrence (volume) and discharges (design value). It was found that a gated ogee blocked with driftwood performs with a reduced discharge coefficient as long as no countermeasures are taken, such as pier overhang, the removal of piers, or the installation of a rack. The performance of these countermeasures was studied, and criteria were developed to control the perturbing effect of driftwood.

Blockage Probability Modeling of Large Wood at Reservoir Spillways With Piers

“…Certainly, during a flood, large volumes of wood are transported. However, it must be considered that a single stem can become a key member of a jam, defined as that piece or pieces which initiated the formation of an accumulation of LW (Abbe & Montgomery, 1996; Bénet et al., 2021). Once a key element is blocked at the structure, it can start collecting other elements (Panici & de Almeida, 2018).…”

“…The classification of LW includes stems longer than 1 m and with a diameter larger than 0.10 m (Braudrick et al., 1997; Ruiz‐Villanueva, Piégay, Gurnell, et al., 2016; Wohl et al., 2016). The transport of LW can induce dangerous obstructions at hydraulic infrastructures like bridges, weirs and spillways during floods (Allen et al., 2014; Bénet et al., 2021; De Cicco et al., 2016, 2020; Gschnitzer et al., 2017; Hartford et al., 2016; Hartlieb, 2012; Iroumé et al., 2015; Panici & de Almeida, 2018; Pfister, Capobianco, et al., 2013; Piton et al., 2020; Schalko et al., 2020; Schmocker & Hager, 2011).…”

Influence of submerged woody debris at labyrinth weirs in river applications

“…To illustrate, the U.S. Department of Transportation Federal Highway Administration (FHWA, 2005) and the International Commission on Large Dams (ICOLD, 2018) published reports detailing observed effects of debris accumulation, mechanisms by which woody debris enters the water, transportation of woody debris, and potential solutions to resulting safety issues. Blockage probabilities of driftwood at a gated weir was studied by Johansson and Cederström (1995) and at ogee crests by Bénet, Cesare, and Pfister (2021). Vaughn (2020) studied blockage probability and effects of floating woody debris accumulation on labyrinth weirs (Crookston, Erpicum, Tullis, & Laugier, 2019) in reservoir applications (an example labyrinth shown in Figure 1a).…”

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

“…Flooding and erosion are frequent consequences. Approaches to handle driftwood are the (1) maintenance of the riparian vegetation, (2) construction of in-stream (Schmocker and Weitbrecht 2013) or on-site (Bénet et al 2021b) retention structures where the wood is removed after a flood, or (3) implementation of transfer structures provoking a (partial) passage.…”

“…This means that trunks typically block and provoke a backwater-level rise if the bays are narrower. • Bénet et al (2021b, a) investigated the reservoir-level rise upstream of an ogee crest with piers under an extreme driftwood arrival. For narrow bays (0.40 ≤ b=L M ≤ 0.77) and without countermeasures, the discharge coefficient was reduced to around C d ≈ 0.36, as compared to C dR ≈ 0.49 for free conditions and the design discharge.…”