The gyrokinetic PIC (particle-in-cell) code for MHD simulation, Gpic-MHD, was installed on SR16000 ("Plasma Simulator" in NIFS), which is a state-of-the-art scalar SMP (symmetric multiprocessing) cluster system consisting of 8,192 logical cores (128 nodes, each node includes 32 physical cores with SMP architecture, and one physical core is equivalent to two logical cores with multithreading technology). Gpic-MHD assumes a cylindrical coordinate system corresponding to the lowest order tokamak ordering. The hybrid parallel programming model of thread parallel (auto-parallelization) and process parallel (MPI) is used. The total simulation domain (cylinder) is decomposed in one (1d) or two (2d) directions. Replicas of field quantities are used to utilize logical cores larger than the number of decomposed domains (parallelization due to "particle decomposition"). Each process is responsible to one decomposed domain and includes the approximately same number of particles. Gpic-MHD with 1d domain decomposition in an axial direction, demonstrated a good scaling up to 8,192 logical cores. However this scaling will saturate for more than several tens of thousands of logical cores because the communication time between processes will increase as the number of replicas increases. To overcome this deterioration of the scaling, Gpic-MHD with 2d domain decomposition was made, in which the total domain is decomposed in axial and radial directions. The 2d domain decomposed version also showed a good scaling, but the computation time was a little bit longer than the 1d domain decomposed version for the relatively small number of meshes and cores studied in this work; the faster computation time was obtained for the 1d domain decomposed version. However, for the future simulation with much larger meshes and logical cores, it is expected that the 2d domain decomposed version with further optimization will exhibit better parallelization performance.

The similarity law of the wind velocity in wave-overtopping experiments is not known. Then, inthis study, the correspondence of the wind velocity in the wave-overtopping experiments to the one in the real coast was investigated experimentally based on the results of the field observation of wave-overtopping conducted by Fukuda, et al. (1974Fukuda, et al. ( ) in 1971Fukuda, et al. ( -1972 at Niigata east port. Wave-overtopping rate, incident waves, wind direction and wind velocity, etc. were obtained in that observation. The experiments were conducted by using two-dimensional wave tank using the 1/45 reduced scale model. Generating the wave and the wind at the same time, wave-overtopping rates were measured. And the rates in the experiments were compared with the ones of the field observation. It was found that the wind velocity in the experiment becomes about 1/3 of the wind velocity of the real coast.

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The effect of wind to wave-overtopping and water spray transportation behind the seawall was investigated with the Non Wave-Overtopping Type Seawall. It is confirmed that even for the strong wind , the seawall is effective for preventing wave-overtopping compared to the conventional upright seawall . But, as the wind velocity increases, the wave-overtopping rate increases and water spray was transported far and widely. In addition, a parapet was installed on the tip of the seawall as one of countermeasures of water spray, and its effect was investigated. It is found that the parapet is not so effective for reducing water spray, but it can prevent overflowing of water mass into just behind the seawall. It is also found that the parapet of about 1 m in height reduces the wave-overtopping rate from 30 percent to 40 percent even under strong wind.

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