PSI - Issue 84

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

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vertical erosion. However, as the scour hole widens and the collar becomes fully exposed, vertical erosion resumes. By the end of the test, the normalized scour depth reaches y s /b =1.4, which is nearly identical to that recorded for the collar placed at the bed level ( d / b =0), and only marginally lower than the no-collar case. The temporal development of local scour in the test with the collar positioned 2 cm below the sand bed ( d / b = 0.66) is particularly noteworthy. In this configuration, the scour hole rapidly developed to the collar elevation (y s / b = 0.66) and remained stable at this depth for roughly the first half of the test duration (12 hours). Thereafter, the scour starts to increase again, albeit at an extremely slow rate, reaching y s / b = 0.77 by the end of the test (26hh22mm). This represents a substantial reduction compared to the no-collar scenario, where scour depths reached y s / b = 1.6  1.7, amounting to an approximate 50% decrease in maximum scour depth relative to the reference case. Hence, this collar configuration demonstrated good efficiency, attributable to both its position under the sandy bottom and its lateral extent. Moreover, the extremely slow progression in time of the scour suggests that − even with more prolonged test duration representative of extended flood events under prototype conditions − the additional increase in scour depth would likely remain very limited.

Figure 3 - Dimensionless scour evolution versus dimensionless time: a) Runs of the A series circular pier; b) Runs of the B1 series (round-nosed rectangular pier with L / b =3) (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. CH0 hooked collar at the bed level; CH-1 hooked collar 1 cm below the bed; CH-2 hooked collar 2 cm below the bed). Photogrammetry enabled the complete 3D reconstruction of the bed surface, providing elevation models that can be directly compared across tests, with higher accuracy than manual measurements (Morgan et al., 2017). Figure 4 present the perspective views of the sandy bed morphology at the conclusion of the tests involving round-nosed rectangular piers with an aspect ratio L / b =3, comparing scenarios without collars and with collars positioned at d / b = − 0.66. In the collar test, the device was carefully removed to reveal the underlying bed. The figures clearly illustrate that the scour depth immediately upstream of the pier is significantly reduced, resulting in a correspondingly narrower scour hole when a collar is present. It is noteworthy, although of limited impact on pier stability, that the bedforms (dunes) developing downstream exhibit morphological differences between the two cases; specifically, the presence of the collar correlates with dunes of reduced wavelength and amplitude. This variation is consistent with expectations, given that the experiments were conducted under clear-water conditions, wherein sediment mobilization originates solely from localized scour processes around the pier, which differ markedly between the collared and non-collared configurations. Figure 5 presents the results of the experiments conducted with round-nosed rectangular piers having a shape ratio of L / b =4. Consistent with the findings for previous pier geometries, the two tests without a collar demonstrate good repeatability, with final normalized scour depths reaching y s / b =1.63  1.80, slightly higher than those observed for the shorter round-nosed rectangular piers ( L / b =3). In contrast, the temporal scour evolution in the presence of a collar exhibits a markedly different behavior. For the case d / b =0 (CS-0), where the collar is positioned flush with the sand bed, an initial phase of relatively rapid scour development is followed by a period of temporary stabilization. However, the scour depth resumes increasing later in the test, eventually approaching values only marginally lower than those recorded in the no-collar configuration. This behavior appears anomalous and may be attributed to potential test related issues, such as an undetected decrease in flow rate or velocity. For the configuration with the collar embedded 1 cm below the sand bed surface ( d / b =0.33, CS-1), scour depth initially