Scour at breakwaters under combined waves and current

Stefan Carstensen, B. Mutlu Sumer

Tuesday 30 june 2015

15:05 - 15:20h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 6A. Sediment - Erosion

Scour at breakwaters have been studied for the two fundamental cases: current alone (Fredsøe et. al 2001, Petersen et. al 2012) and waves alone (Xie 1981, Sumer and Fredsøe 2000), see Sumer and Fredsøe (2002) "The Mechanics of Scour in the Marine Environment" for a review. In comparison there is little information for the situation experienced in practice: orthogonal combined wave and current. Results from a recent experimental investigation on scour at rubble mound breakwaters have shown that the combined effect of orthogonal combined wave and current may cause larger scour than the two fundamental cases alone. The experiments were carried out in a 24m times 12m current and wave basin. A breakwater was installed in the basin at the edge of the sand pit. The slope of the breakwater was 1:1.2. Current parallel with the breakwater was established, while regular waves were directed perpendicular to the breakwater. The mean current speed was maintained throughout the test at U = 0.30m/s. The current was high enough to ensure live bed conditions. A number of wave conditions was chosen covering a water depth to wave length ratio of 0.1 < h/L < 0.2. Scour at the toe of the breakwater as well as in the far field was continuously monitored using video, while scour in the equilibrium stage was measured with a set of laser distance meters. Bed profiles perpendicular to breakwater have been established for orthogonal combined wave and current scour. The bed profiles show the distinct features of the wave scour forming due to the presence of recirculating cells, namely deposition/scour at the antinodes/nodes depending on whether the sediment transport falls in the no-suspension or the suspension mode. Also, the current scour formed by the presence of secondary flow as a result of the corner flow and the change in roughness. The mechanism in both fundamental cases is that the sediment is mobilised by the primary flow while the secondary flow/recirculating cells cause a mean transport of sediment, i.e. scour. For the orthogonal combined wave and current the mechanism appears to be similar.