Onshore scour characteristics around submerged vertical and semicircular breakwaters

A laboratory study is presented herein investigating the two-dimensional onshore scour along the base of submerged vertical and semicircular breakwaters. Experiments were conducted with normally incident monochromatic waves breaking at the breakwater on both sloping and horizontal sandy bottoms. A principle conclusion of this investigation is that the characteristics of onshore breakwater scour are independent of submerged breakwater shape/type. It is also concluded that the onshore scour patterns can be divided into two distinct regimes that solely rely on the Keulegan–Carpenter number (KC=HiπWbw; Wbw — breakwater crest width, Hi — incident wave height). For KC values larger than π, the scour forms “detached” from the breakwater while for KC values smaller than or equal to π, the scour occurs “attached” to the onshore breakwater face. Three important scour characteristics are investigated: maximum scour depth (Smax), scour length (Ls), and the distance of Smax location from the onshore breakwater face (Ds). Smax value is observed to be regime independent and rely on both KC and the mobility number (ψ=(HiπTsinh(kh))2g∗d; g⁎ — reduced gravitational acceleration, d — median diameter of the sediment, k — wave number, h — still water depth, T — wave period) while Ls and Ds are observed to be regime dependent and rely only on KC. Semi-empirical parameterizations to predict Smax, Ls, and Ds values for onshore breakwater scour are proposed.