PSI - Issue 84

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

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2.3. Experimental runs The properties of the experiments carried out during the present work (runs 1 and 2) are listed in Table 1, that also provides the characteristics of two prior experiments (2021-1 and 2025-2, originally reported by Ballio and Radice (2025) and Radice et al. (2025). The common property of the experiments was the Froude number within a range of 0.27 to 0.29. The prior experiments were carried out with the bridge anchored to the flume walls, so did not return the hydrodynamic load onto the structure. Furthermore, they were performed with a different arrangement of the cameras. Thus, for these experiments, the PIV algorithm and a user recognition of the incoming driftwood were applied to obtain values of q s and p 1 , more reliable than the nominal ones, for the wood feeding; furthermore, it was not possible to measure the amount of wood leaving the accumulation at the downstream, so the estimation of the accumulation volume by continuity principle was also impossible. Table 1. Properties of the runs considered in the manuscript. The color depicted in the last column is valid for all the plots. Run h ( m ) V( m/s ) Fr Q s ( m 3 /s ) q s /(Vh) p 1 Color in plots 1 0.171 0.35 0.27 4.6×10 -6 1.5×10 -4 1 2 0.173 0.35 0.27 1.3×10 -5 4.3×10 -4 1 2025-1 0.164 0.37 0.29 4.7×10 -6 1.6×10 -4 0.6 2025-2 0.164 0.37 0.29 9.4×10 -6 3.1×10 -4 0.6 The temporal evolution of wood volumes is depicted in Fig. 2 for runs 1 and 2. Panel (a) shows the cumulative volume of wood joining (W in ) and leaving (W out ) the accumulation at the bridge. The lines for W in are almost straight, since the experimental protocol involved the supply of driftwood into the flow at a constant rate. The lines for W out , instead, reflect the dynamics of release of material from the downstream part of the accumulation. Radice et al. (2025) already discussed that the amount of material that can be trapped downstream of the bridge is limited by the wake developing in the recirculation region just downstream of the deck. The difference between W in and W out returns the temporal evolution of the total volume of the driftwood accumulated around the bridge, as shown in Fig. 2(b). In order to distinguish the driftwood accumulation upstream and downstream of the bridge, one can refer to the projected areas (from above and from the side) of the accumulation, as depicted in Figs. 3 and 4 for both the current experiments and the prior ones. The comparison between panels (a) and their (b) counterparts clarifies that the accumulation area was larger upstream of the bridge than downstream. Furthermore, the lines for the upstream accumulation areas did not reach a plateau, while those for the downstream areas did, again because the latter depended on the size of the flow recirculation. The intensity of driftwood supply is, expectedly, a promoting factor of accumulation, as one can realize comparing the results for runs 1 and 2 that had same p 1 but different q s /(Vh), see Figs. 2(b), 3(a), 4(a); the same was obtained for runs 2025-1 and 2025-2. Furthermore, the driftwood length also had an effect on the accumulation: runs 1 and 2025 1 had very similar driftwood supply rate, but in run 1 the mean driftwood length was larger than in run 2025-1 due to a higher value of p 1 ; this resulted in larger accumulation for run 1 compared to run 2025-1, see Figs. 3(a) and 4(a). 3. Results 3.1. Temporal evolution of the accumulation size