**5. Conclusions**

Scour around single cylindrical piers and the effects of anti-scour collars were investigated by carrying out laboratory tests. The scour mechanism and the time-varied development of scour depth were studied. Experimental investigations regarding local scour using anti-scour collars on cylindrical piers were carried out. The protective efficiency of anti-scour collars with different design parameters, including collar installation height, collar external diameter and collar protection range, were investigated and discussed respectively. The main findings of this study can be summarized as follows:

(1) In the scour test of a cylindrical pier without protection, the maximum scour depth first appears in the front side of the pier, then moves to the front of the pier, while the minimum scour depth appears in the downstream side of the pier. The shape of the local scour hole around the pier is approximately symmetric with the flow direction.


It should be noted that there are still many limitations in this study. The parametric study of anti-scour collars here is carried out only under one thickness of anti-scour collars and one constant current condition. The change of both flow velocity and water depth and the type of sand are not considered. The application of two or more collars is not considered as well. These limitations can be further addressed by experimental or numerical simulation study. Notwithstanding the shortcomings listed above, the experimental results of anti-scour collars are of grea<sup>t</sup> practical guiding significance to the optimal design of anti-scour collars and can provide a useful testing reference for further improvement of active scour countermeasures for bridges.

**Author Contributions:** S.W., K.W., Z.S., Q.X. designed and supervised the experiments; S.W. conducted the analysis on the experimental data; K.W. provided motivation and contributed interpretation of results; S.W. and K.W. wrote the paper; and all authors participated in final review and editing of the paper.

**Funding:** This research was funded by National Natural Science Foundation of China (Grants No. 51708455), the Fundamental Research Fund for Central Universities (A1920502051907-2-001) and Key Research and Development Project of Sichuan Province (2019YFG0460).

**Acknowledgments:** The authors sincerely thank the anonymous reviewers for their valuable suggestions in improving the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
