Jun Mitsui

Armor Block Stability of Submerged Breakwaters Predicted by Numerical Wave Flume

ABSTRACT Matsumoto, A., Mano, A., Mitsui, J., and Hanzawa, M., 2013. Armor block stability of submerged breakwaters predicted by numerical wave flume Submerged breakwaters are one of the most fundamental structures to protect coastal areas from wave action. To ensure the stability of a whole structure, it is primarily important to determine the required mass of armor units accurately. This study aims to evaluate the armor block stability based on a flow field calculated by a numerical wave flume. The flow field was calculated by using a 2D VOF-type numerical wave flume. The wave force acting on an armor block was calculated using the Morison formula. Drag, inertia and lift coefficients for the Morison formula were determined by using an exact 3D VOF-type numerical model. The resultant stability of the armor block was judged from a comparison between the wave force and the stabilizing force originating from the mass of the armor block. An evaluation method for the critical condition of armor block stability of submerged breakwaters is proposed. Parameters for the evaluation were calibrated through comparisons between experimental and numerical results. An impact wave force with short duration acted when the water depth above the breakwater was shallow. In such a situation, even if the peak value was large, the impulse was not so large. To estimate the displacement of the block, the equation of the motion of the block was solved. The agreement between predicted and experimental stability was improved by using the fluid force coefficient corresponding to an individual water depth condition. That is to say, the applicability of the model was shown to have improved.

Estimation Method of Armor Stability Against Tsunami Overtopping Caisson Breakwater Based on Overflow Depth

This paper presents a practical design method for armor units to cover a rubble mound at the rear side of a caisson breakwater against tsunami overflow. In this method, the overflow depth of tsunami is used to represent the external force. This enables the estimation of the required mass of the armor units to be done more robustly and easily than in the conventional method based on the flow velocity. Hydraulic model experiments were conducted to investigate the armor stability. We found two important factors for armor stability. These were the impingement position of the overflow jet and the harbor-side water level. These effects were taken into account in the method. Numerical analysis on the fluid forces acting on the armor blocks was also conducted to explore the fundamental expression of the new design formula for armor stability. Empirical formulae for the stability estimation were then derived based on the findings from experiments and numerical analysis. The overflow depths of the stability limit corresponding to two failure modes, overturning and sliding, were obtained by two formulae. The stability numbers for each armor unit were determined through the experiments. The estimated results by this method agreed well with the experimental ones.

New Fish Reef Block Ensuring Accurate Placement by Free-Fall Method

AbstractA new fish reef block ensuring accurate placement by a free fall was developed to enable a more economical construction of nursery reefs for snow crabs. The block is based on a cube-shaped frame structure with extra shelf areas attached to the upper and lower frames to act as stabilizers. The falling behavior of the block in water was investigated by hydraulic model experiments and three-dimensional (3D) numerical computations. A stochastic model based on the experimental data was used to confirm a narrow distribution of the placement position as a result of the swing motion of the block during its fall. The numerical computation results indicate that the upper stabilizer produces a restoring moment for a tilted block and the lower stabilizer reduces instability in the wake of the falling block. In addition, a quick estimation method for the placement position was developed to determine the release point of the block during actual construction in the presence of ambient currents.