PSI - Issue 84

Francesco Ballio et al. / Procedia Structural Integrity 84 (2026) 151–158

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1. Introduction Driftwood can be transported by rivers in large quantities, particularly during high-flow events, and accumulate at bridges (e.g., Erpicum et al., 2024). This represents a hazard for river bridges because the floating material, after accumulating against a structure, increases the latter’s apparent size and reduces the sectional area available for flowing Nomenclature A Da projected accumulation area downstream of the bridge measured from above ( cm 2 ) A Ds projected accumulation area downstream of the bridge measured from the side ( cm 2 ) A Ua projected accumulation area upstream of the bridge measured from above ( cm 2 ) A Us projected accumulation area upstream of the bridge measured from the side ( cm 2 ) F hydrodynamic load ( kg ) F 0 hydrodynamic load without driftwood accumulation ( cm ) Fr Froude number (-) h water depth ( m ) p 1 proportion of long sticks in the driftwood mixture (-) Q s wood feeding rate ( m 3 /s ) q s wood feeding rate per unit width ( m 2 /s ) t time ( s ) V water velocity ( m/s ) W cont accumulation volume = W in -W out ( cm 3 ) W in cumulative wood volume joining the accumulation upstream of the bridge ( cm 3 ) W out cumulative wood volume leaving the accumulation downstream of the bridge ( cm 3 )  h backwater rise ( cm )  h 0 backwater rise without driftwood accumulation ( cm ) water. The debris accumulation modifies the flow field around the bridge, and its hazardous consequences include (i) the increase of the backwater rise upstream (e.g., Schalko et al., 2018), (ii) the increase of the hydrodynamic load acting onto the structure due to the load transmitted by the driftwood (e.g., Ballio et al., 2024), (iii) the possible enhancement of local scour processes, among others (e.g., Melville and Dongol, 1992). As a matter of fact, driftwood accumulation is an important process to be considered while preparing flood hazard maps (Mazzorana et al., 2012). The scientific literature provides examples of studies of driftwood accumulation at bridge piers (e.g., Panici and de Almeida, 2018; De Cicco et al., 2020) and decks (Schmocker and Hager, 2011; De Becker et al., 2025), finalized at determining the probability of driftwood entrapment, the expected size of the accumulation, and the resulting backwater rise. However, in spite of the recognized importance of the issue, the amount of existing research is still relatively limited. The present manuscript contributes to the ongoing research on the topic by presenting the results of some laboratory experiments for driftwood accumulation at a rectangular bridge deck, specifically considering (i) the temporal evolution of the debris accumulation size due to continuous arrival of driftwood, (ii) the backwater rise upstream of the bridge, and (iii) the hydrodynamic load exerted on the structure. In the following, we first describe the experimental facility and measurement methods, then delve into the experimental findings. 2. Materials and methods 2.1. Laboratory facility The experiments presented in this manuscript were performed at the Hydaulics Laboratory “G. Fantoli” of the Politecnico di Milano. We used an open-channel flume with a length of 7 m, width of 0.5 m, and height of 0.6 m. All the flume walls, made of plastics, are transparent to ensure good observation of the ongoing processes. The flow rate in the flume was measured with an electro-magnetic flowmeter, while a number of piezometric probes returned the