Local scour at roundhead and along the trunk of low crested structures

Abstract This paper summarizes the results of an experimental study on scour around submerged breakwaters. The objective of the study is to make a systematic study of scour around low-crested structures/submerged breakwaters. Both the trunk scour and the roundhead scour have been investigated. The breakwater models have a side slope of 1 : 1.5. To substantiate scour measurements, velocity measurements also were made. The latter data were used to obtain steady streaming in front of the breakwater. Regarding the trunk-scour study, the experiments indicated that substantial scour may occur at the toe of the structure. This is irrespective of whether the breakwater is impermeable or porous. The scour data (although very limited) indicated that the scour depth is in the same order of magnitude as in the case of emerged breakwaters. The trunk experiment further showed that scour/deposition bathymetry does not exhibit the pattern experienced in the case of emerged breakwaters where the scour and deposition areas are “correlated” with the nodal and antinodal points of the standing wave in front of the structure. Furthermore, it was found that scour occurs not only at the offshore side of the breakwater but also at the onshore side. As for the roundhead-scour study, it was found that severe scour can be experienced at the roundhead. The scour can occur both at the offshore side of the roundhead and at the back. The one at the offshore side is caused by the combined effect of severe waves and the steady streaming and that at the back is caused by wave breaking/wave overtopping. It was found that the streaming-induced scour is governed by the free-board-to-wave-height ratio, F / H, and the Keulegan–Carpenter number, KC, while the plunging-breaker-induced scour is governed by the free-board-to-water-depth ratio, F / h, and the plunger parameter, T w g ⁢ H / h (Tw being the wave period). The maximum scour depth at the toe of the structure and the plan-view extent of the scour hole are given in terms of the governing parameters. The results from the present laboratory tests (for the roundhead scour) are compared with prototype observations undertaken in the present study. Recommendations are made for toe protection for both the trunk scour and the roundhead scour.

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