Takehisa Saitoh

WATER WAVE INTERACTION OF TWIN LARGE SCALE CAISSONS WITH A SMALL GAP BETWEEN

A reduced two-dimensional source distribution method is used for systematic computations on wave interaction of twin vertical cylinders with rectangular sections aligned with small gap between in order to get a clear fundamental view of small gap influence to the force response on potential very large coastal and marine structures. Strong hydrodynamic interaction between caissons with small gap is observed and sharp peak force responses are proved in the paper by both numerical evidence and theoretical verification due to newly discovered narrow open channel resonant phenomena. This strong interaction feature has its own important practical significance for design of links of modules for the large structure and attentions on terms of work for linking the modules. And moreover, the importance is also closely related to hydro-elasticity analyses for the very large structures, in which local loads may be as important as integrated loads.

'Effects of Zero-Overtopping Data in Artificial Neural Network Predictions'

This study examined the applicability of artificial neural networks (ANNs) to the estimation of wave overtopping over sloping seawalls, especially with regard to the best structure for an ANN. Correlation coefficients between measurements and predictions were best when 6 input units and 12 hidden layer units were employed. Bayesian Regularization, recommended in this study, does not require a validation data set. It was found that the ANNs could not recognize when wave overtopping failed to occur if data on zero overtopping were omitted.

Wave Transformation and Cross-Shore Sediment Transport on Sloping Beach in Front of Vertical Wall

Abstract SAITOH, T. and KOBAYASHI, N., 2012. Wave transformation and cross-shore sediment transport on sloping beach in front of vertical wall. An experiment was conducted in a wave flume to examine cross-shore sand transport on a sloping beach in front of a vertical wall located above the still water level (SWL). The irregular wave transformation and beach profile change were measured using eight wave gauges, four velocimeters, a laser line scanner, and three ultrasonic transducers. An initial semi-equilibrium beach accreted slightly above SWL where approximately 5% of incident waves reached the vertical wall. The cross-shore numerical model CSHORE is calibrated and used to predict the cross-shore variations of the maximum free surface elevation and horizontal velocity that are needed for coastal flood hazard mapping. The cross-shore variations of the suspended sand and bedload transport rates predicted by CSHORE are examined to explain the accretion in front of the vertical wall. The experiment in this study was very limited but indicated that accretion could occur in front of the vertical wall exposed to infrequent wave action.

Kinematics and Transformation of New Type Wave Front Breaker over Submerged Breakwater

This study presents the discovery of a new type of wave breaking over a submerged breakwater. Our findings were made through laboratory experiments. In this new type, wave breaking first occurs at a waves front part, not at the top of the wave crest. Next, the wave crest passes over the breakwaters crown without breaking. In this study, we call this wave breaker a wave front breaker , and the process of wave transformation and the conditions for the wave front breaker to occur are clarified. The wave kinematics of the wave front breaker is also investigated by PIV and LDV measurements. Consequently, it is shown that the occurrence of the wave front breaker strongly depends on the incident wave length, and the wave transmission coefficients of the wave front breaker become larger. It is also found that an inverse layer of the mass transport velocity exists near the seabed against the main region of the mass transport velocity. Furthermore, the direction of the mass transport velocity near the toe in the inverse layer is onshore, while that of the maximum acceleration is offshore and the value of the acceleration increases as the measuring points get closer to the seabed and away from the toe.

Theoretical analysis of wave and structure interaction around a composite-type coastal structure - a case study of a seawall and detached breakwaters

This study presents a theoretical method for analyzing flow fields around a composite-type coastal structure. A dividing region method, extended from previous work [1],[2],[3] is adopted, and the case composed of a seawall and detached breakwaters is the focus for a fundamental stage of this topic. Laboratory experiments are also conducted to examine the validity of this method. Theoretical results of water surface elevation around the composite-type structure agree well with the experimental results and the validity of this method is confirmed.

Laboratory Experiments for Mass Transport Velocities and Turbulence Flows due to New Type Wave Front Breaker over Artificial Reef

In this study, laboratory experiments were conducted in order to investigate mass transport velocities and turbulence flows around an artificial reef due to a new type of wave front breaker over it. In addition to this, the paper examined a classification of an occurrence condition for the wave front breaker. Consequently, it was found that the occurrence condition of the wave front breaker was classified using the Ursell Parameter and relative crown depth, and an increase of mean water level behind the artificial reef did not affect the occurrence of the wave front breaker. Furthermore, reverse layers of mass transport velocities were found around the toe area of the artificial reef. The thickness of the reverse layers varied with the incident wave height and had its peak. Strong turbulence occurred during the wave run-down to the run-up phase at the breaking point. In addition, the time-averaged turbulence intensity maintained a large value not only beneath the jet time-averaged turbulence intensity maintained a large value not only beneath the jet mixing due to the wave breaking, but also near the bottom at the breaking point.