Detached Breakwaters

Abstract

In this paper, the hydraulic functions and stability of detached breakwaters and submerged breakwaters with wide crown widths are discussed. The following subjects with regard to detached and submerged breakwaters are discussed: - Function of detached breakwaters in controlling waves and wave-induced currents - Function of detached breakwaters in the control of sediment movement - Stability of detached breakwaters The functions of detached breakwaters and submerged breakwaters in controlling waves, wave-induced currents and sediment movement were discussed. As has already been reported by many researchers, detached breakwaters are very effective in controlling incident waves and salient or tombolo formation due to deposition of sediment sometime result in sever erosion of downstream coasts. On the other hand, submerged break waters have a relatively mild but steady effect in retaining shore-side sediment and have little effect on the surrounding coasts. Therefore, submerged breakwaters are recently replacing detached breakwaters in Japan. However, there are some issues related to submerged breakwaters: 1) They may become fatal obstacles for fishing boats or small pleasure boats; 2)The effect of a submerged breakwater on a coast with a wide tidal range is not obvious because the hydraulic function of the submerged breakwater depends on the water depth at the crown. In determining the stable weight of rubble stones for a submerged breakwater, it is important to consider the effects of irregularity and grouping of incident waves. Naturally, the wave period should also be taken into consideration, as was pointed out by Bruun and the author. From this point of view, the formula for determining the stable weight of rubble for submerged breakwaters as proposed by the Ministry of Construction of Japan needs to be further studied.