PSI - Issue 84

Maria Giovanna Tanda et al. / Procedia Structural Integrity 84 (2026) 272–279

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stabilizes upon reaching the collar level, consistent with observations for the shorter pier. Subsequently, as the scour hole expands laterally and the collar edge becomes exposed, erosion resumes. By the end of the test, the maximum normalized scour depth reaches approximately y s / b =0.8, corresponding to a reduction of about 55% of the value relative to the no-collar condition. However, since the system had not yet reached an equilibrium state, it is possible that continued testing could result in further scour development approaching values observed in the absence of a collar. Nonetheless, the previously discussed considerations regarding the equivalent duration of the prototype event remain applicable in this context.

Figure 4 - Perspective view of the bottom configuration at the end of the test with round-nosed piers with L / b =3a) without collar; b) with collar (removed) at position d / b = − 0.66.

Even the test with the collar positioned 2 cm below the bed surface (d/ b =0.66, CS-2) shows some differences compared to the corresponding configuration with the shorter round-nosed rectangular pier. Once the scour depth reaches the collar level ( y s / b =0.66), vertical erosion does not fully cease, although the subsequent development is relatively limited. By the end of the test—which does not appear to have reached a fully developed equilibrium—the maximum normalized scour depth is approximately y s / b =0.9. This value represents a reduction of about 50% compared to the no-collar case under otherwise identical conditions, though it is slightly higher than that observed for the collar embedded at 1 cm below the bed ( d / b =0.33).

Figure 5 - Runs of the B2 series (round-nosed rectangular pier with L / b =4): scour evolution in time. NC no collar; CS-0 collar at the bed level; CS-1 collar 1 cm below the bed; CS-2 collar 2 cm below the bed. 4. Conclusions The experimental findings demonstrate that the presence of a collar can markedly retard scour progression and, in specific configurations, effectively reduce the maximum scour depth achieved. In the most effective scenarios − such