Drawing up a business continuity plan (BCP) for a harbor, it may be important to estimate a remained performance of a breakwater which is sunk completely by an earthquake and/or a tsunami attack. In the other view point, an impermeable submerged breakwater may be expected to use as a new countermeasure to control the waves which act structures and are transmitted in a harbor, avoiding influence of wave power near the water surface. In concrete, a period of swell which can strongly affect oscillation of mooring vessels and wave overtopping volume on seawalls would be shortened by the submerged breakwaters. However, the relation between the transmitted wave periods and the conditions of both specific incident waves and structures may be still unknown.
In this study, a series of model experiments in a flume is conducted to realize characteristics of variation of significant heights and periods behind impermeable rectangular/trapezoidal submerged breakwaters. Moreover, numerical simulations are also conducted to reproduce such phenomena by using a Boussinesq-type wave model, which is often applied for harbor tranquility analysis. Here, a newly developed “step boundary technique” is installed to conduct a stable calculation of wave transformation on the submerged breakwaters which have large steps. In addition, the effects of uni-/multi-directional wave refraction on transmitted waves behind impermeable submerged breakwaters are investigated in both model experiments and numerical simulations in experimental and numerical basin. Finally, the harbor tranquility analysis is conducted to estimate cargo handling rates on a berth in cases of the submerged breakwater existing and the offshore breakwater missing.
In case that the top of breakwater has sunk by 2 meters below the sea level, the wave period of transmitted wave of which incident wave height is 1 meter is shortened by about 20% in a relative wide area behind the submerged breakwater due to wave fission. And the submerged breakwaters which can reduce multi-directional waves may allow vessels to be moored in a sheltered area, though a local wave height concentration is occurred by uni-directional wave refraction. In addition, the cargo handling rate in the case of the submerged caisson existing is at least 10% higher than one of the caissons missing because of reduction of wave heights due to breaking, and so on.

Key Words: submerged breakwater, variation of wave period, fission, convergence, step boundary, Boussinesq model, harbor tranquility