PSI - Issue 62

Tommaso Lazzarin et al. / Procedia Structural Integrity 62 (2024) 625–632 Lazzarin et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results The pressure-flow ( PF ) regime occurs when the water elevation reaches the bridge deck. For higher water levels, part of the flow may also overtop the bridge deck. Describing the complex three-dimensional structure of the flow in case of a PF regime is not straightforward, especially in complex cases where the bathymetry is irregular and the realistic structure of the bridge is considered. In the present case, the deck is submerged; a discharge of 215 m³/s flows over the deck, and 14 ’270 m³/s flow below it. The submerged deck induces an additional resistance for the flow, thus water levels upstream the deck increase (backwater effect). Immediately upstream of the bridge, water is conveyed downwards and accelerates beneath the deck, similar to the orifice flow condition forming beneath a sluice gate (see the two longitudinal sections in Fig. 2). The formation of a region with high velocity close to the bed is particularly critical for the stability of the bed material and, in turn, for the bridge stability, since it increases both the bed shear stress and the entrainment of bed particles. In case of free-surface ( FS ) regime, relatively small flow accelerations are observed, mainly at the side of piers; in the PF simulation the velocity in the streamwise direction is approximately twice as high as in the corresponding FS simulation (compare Fig. 3a,c). Another consequence of the PF regime is the formation of recirculation regions with low average velocity below and just downstream of the deck. Upstream of the bridge, the downward velocity is expected to induce the formation of bed scour. At the back of the piers, well-developed horseshoe vortices have been observed to form especially at the piers P1 and P2, where scour holes are already present in the bathymetric relief. Horseshoe vortices form in both the FS and the PF simulations, but their strength (quantified in terms of circulation) and their shape differ in the two cases. The downward flux in the PF simulation limits their height, and yet induces a stronger circulation, which is associated with a higher potential to entrain bed sediments.

Fig. 2. 3D views of the bridge in the PF regime simulation from upstream (a) and downstream (b). Streamlines and velocity color maps are illustrated in the y = 0 m and y = 105 m longitudinal sections.