Naoto Kihara

Three-dimensional sediment transport processes on tsunami-induced topography changes in a harbor

A three-dimensional hydrostatic numerical simulation on tsunami-induced topography changes near a harbor is carried out, and sediment transport processes on a significant local deposition near the center of the harbor caused by a tsunami, which was observed in an early experimental study, are investigated. This local deposition has not been well predicted by a vertically averaged hydrodynamic model. The results show that velocities, water levels and topography changes in the harbor predicted in this study agree with the experimental data. The local deposition has relations with a vortex generated in the harbor when the tsunami attacks the harbor. At areas near the vortex center, a secondary flow of the first kind develops, and it plays the role of transporting suspended sediment to the vortex center, located near the center of the harbor, and causes the local deposition there. In order to predict deposition areas with high accuracy, the secondary flow effects should be incorporated in prediction methods of tsunami-induced topography change. Key words: Tsunami, tsunami deposits, sediment transport, hydrodynamic model.

Improvement of Collision Force Formula for Woody Debris by Airborne and Hydraulic Experiments

The relationship between the collision direction angle and the collision force of woody debris and its behavior is investigated through large-scale bore flow and airborne experiments. In the case of the airborne experiments oblique collision angle being larger than 20°, the oblique collision force is reduced by up to half of the longitudinal collision force. In the case of the bore flow experiments initial arrangement along the transverse direction, the wood-log moved with a rotation along the channel floor. Collision forces in the bore flow experiment indicated the trend less than that in the airborne experiment. The effects of water cushioning between the wood-log and vertical wall, and the surrounding water caught by the rotation of debris were considered. Major collision force formulas for collisions in the long axis direction of woody debris are verified through comparison of this study and recent experiments. By considering the reduction in collision energy by the rotational motion of debris after oblique collisions, their collision force formulas are improved to estimate the oblique collision force. The improved formulas can reproduce the reduction of significantly oblique collision forces compared with longitudinal collision forces by the above experiments. The effect of the collision angles uncertainty on the estimation of the collision force was investigated as a case study on a probabilistic evaluation of the collision force.

A Model for Air–Sea Interaction Bulk Coefficient over a Warm Mature Sea under Strong Wind

A boundary layer model for evaluating sensible and latent heat fluxes over a mature sea accounting for sea spray effects at wind speeds of up to 28 m s 1 is presented. Heat exchange across the ocean surface controls the development of tropical cyclones, and Emanuel’s theory suggests that the ratio of the exchange coefficient of total enthalpy to the drag coefficient should be greater than 0.75 to maintain the intensity of tropical cyclones. However, traditional bulk algorithms predict a monotonic decrease in this ratio with increasing wind speed, giving a value of less than 0.5 under tropical cyclone conditions. The present model describes a decrease in the ratio with increasing wind speed under weak to moderate winds (20 m s 1 ), and a plateau at approximately 0.7 under strong winds (20 m s 1 ).

Computational fluid dynamics simulation and statistical procedure for estimating wide-area distributions of airborne sea salt considering local ground conditions

Abstract Steel corrosion under atmospheric conditions is a critical issue in the maintenance of structures such as electric transmission towers and bridges during their long-term operation, which are generally located at many places over a wide area. Since a major factor causing corrosion is airborne salt particles coming from the sea, wide-area distributions of the long-term cumulative amount of sea salt deposited on surfaces are needed. Moreover, since the amount of airborne sea salt varies locally with the topography, it is also important to consider the effects of topography. In this paper, a method combining a computational fluid dynamics model and a statistical procedure is proposed to efficiently estimate wide-area distributions of the cumulative amount of airborne sea salt by considering the local topography. The predicted amount of airborne sea salt decreases with increasing distance from the coast and varies with the topography and the offshore wind. A comparison between predicted and observed amounts revealed that: (1) this method appropriately estimates topographical effects on sea-salt transport and enables the estimation of deposited sea salt on structure surfaces, and (2) consideration of the trapping efficiency of sea-salt particles on structure surfaces improves the prediction accuracy.

Meteorological observation with Doppler and Raman lidars and comparison with numerical weather simulations

Meteorological observation data such as temperature, humidity, wind speed and wind direction are important for validating and improving numerical weather simulation models. Lidar is an effective method for acquiring such data with high range resolution and short time intervals. In this study, we carried out a field observation with coherent Doppler Lidar and Raman Lidar systems at the coastal area of Yokosuka, Japan, and compared the observed data with the results of numerical weather simulations. We obtained the vertical profiles of horizontal wind speeds and wind directions by Doppler Lidar with 65 m vertical range resolution, and the vertical profiles of the water vapor mixing ratio by Raman Lidar with 20 m vertical range resolution at the lower atmospheric boundary layer (200-600 m height from ground level). These data were acquired at time intervals of 10 minutes. We found an interesting phenomenon from observed data indicating that, under weak wind conditions, water vapor in the atmosphere significantly increased just after a definite change in wind direction from land breeze to sea breeze. A similar phenomenon was also predicted by the numerical weather simulation with the same meteorological and terrestrial conditions. We analyzed the numerical results and found that the change in water vapor mentioned above is mainly caused by the difference between the evaporation from land and sea surfaces, which were located upwind of the land and sea breezes, respectively.