Masaru Yamashiro

Analysis of directional wave spectra in shallow water areas using video image data

In this study, a technique for analyzing directional wave spectra in shallow water areas using video image sequences is presented. The video image was obtained and collected by a single digital video camera. The technique was based on time series of the pixel brightness on video images. The pixel can be treated as a fixed instruments through use of the rectification process. The extended maximum likelihood method and the Bayesian directional method were used to estimate directional wave spectra using two different configurations of arrays from pixel brightness on video images. The study examined video image data at HORS pier on Hasaki Beach, Japan. The results indicate that video image data could be used to estimate surface wave spectra in very shallow water areas.

EFFECTS OF WAVE-DISSIPATING BLOCKS ON REDUCTION OF SALINITY IN THE AIR GENERATED AT A VERTICAL BREAKWATER BASED ON FIELD OBSERVATIONS

At Waku fishing port in Yamaguchi Prefecture, Japan, which has a vertical breakwater at the mouth of the port, wave-dissipating blocks were installed in front of the breakwater to reduce the sea water spray generated at the breakwater. To clarify the effect of wave-dissipating blocks on the reduction of the salinity in the air, a large number of field observations were conducted at the port in the winter seasons from 2005 to 2011; the run-ups of the splash (heights and frequency), the wind (velocity and direction) and the salinity concentrations in the air were measured at about 30 observation points. The data obtained before and after the construction showed that the violent run-up of splash was efficiently blocked by fully covering the vertical breakwater with the wave-dissipating blocks. Consequently, the salinity in the air was also drastically reduced.

STUDY ON CHARACTERISTICS AND CAUSE OF SAND WAVES IN THE KANMON WATERWAY

IN THE KANMON WATERWAY Masaru Yamashiro, Kyushu University, yamashiro@civil.kyushu-u.ac.jp Kosaku Oka, Kyushu University, 2TE17283E@s.kyushu-u.ac.jp Noriaki Hashimoto, Kyushu University, hashimoto-n@civil.kyushu-u.ac.jp Yasuyuki Nakagawa, Kyushu University, y.nakagawa@civil.kyushu-u.ac.jp Masaki Yokota, Kyushu Sangyo University, m.yokota@ip.kyusan-u.ac.jp Yukifumi Ikki, Ministry of Land, Infrastructure, Transport and Tourism, ikki-y89ua@mlit.go.jp Masaru Takayama, Ministry of Land, Infrastructure, Transport and Tourism, takayama-m899v@mlit.go.jp INTRODUCTION The Kanmon waterway is one of the most important international routes in Japan. The waterway is in the Kanmon Strait, which is remarkably narrow and meandering considerably, and the tidal current is rapid and complicated in several places. In the waterway, sand waves, which may hamper the ship navigation, develop on the bottom at some places (Figure 1). The dredging is indispensable to the maintenance of required water depth in the waterway. In this study, for the purpose of accumulating useful knowledge for the efficient maintenance of the Kanmon waterway, the characteristics and cause of the sand waves in the waterway were investigated.

Numerical Experiments on Reduction Effects of a Submerged Breakwater with Vertical Plates on Mean Water Level Rise and Transmitted Wave Heights

本研究で新たに提案する連立壁潜堤の概略を図-1に示 す.これは,従来多用されている捨石とブロックによる 台形潜堤に替えて,低天端のマウンドと連立する複数の 鉛直壁を組み合わせた構造により,急激に波を砕波させ ることなく緩やかに減衰させることによって,潜堤背後 の水位上昇を抑制し,かつ通過波も大きく低減させるこ とを期待したものである. 信岡ら(1993)により提案された連立壁潜堤は,マウ ンドが無く,連立する壁が沖向きに傾斜したもので,傾 斜角により背後海域の平均水位や連立壁上の流れが変化 することが報告されている.しかし,連立壁の数や高さ 等に関する検討ケースは限られており,また,入射波も 規則波であった.本研究では,連立壁潜堤の効果に関す る基礎的な知見を得るため,連立壁は鉛直に設置するこ ととし,数や高さなどの諸元が異なる場合の効果の違い に焦点を絞って検討を行う. 連立壁潜堤の波浪制御効果と平均水位上昇抑制効果に関する数値実験 Numerical Experiments on Reduction Effects of a Submerged Breakwater with Vertical Plates on Mean Water Level Rise and Transmitted Wave Heights

STUDY ON CHARACTERISTICS OF DEPTH CHANGES IN KANMON WATERWAY BASED ON SOUNDING DATA

1正会員 九州大学助教 大学院工学研究院(〒819-0395 福岡県福岡市西区元岡744) E-mail:yamashiro@civil.kyushu-u.ac.jp 2正会員 九州大学助教 大学院工学研究院(〒819-0395 福岡県福岡市西区元岡744) E-mail:yokota.masaki.426@m.kyushu-u.ac.jp 3株式会社建設技術研究所 水工部(〒103-8430 東京都中央区日本橋浜町3-21-1) E-mail: y-ootani@ctie.co.jp 4沖縄県土木建築部 下水道建設事務所(〒900-0029 沖縄県那覇市旭町116-37) E-mail: maeshihi@pref.okinawa.lg.jp 5フェロー 九州大学教授 大学院工学研究院(〒819-0395 福岡県福岡市西区元岡744) E-mail: hashimoto-n@civil.kyushu-u.ac.jp 6フェロー 九州大学教授 大学院工学研究院(〒819-0395 福岡県福岡市西区元岡744) E-mail: kasugai@civil.kyushu-u.ac.jp 7関門航路事務所 航路保全課(〒802-0001福岡県北九州市小倉北区浅野3丁目7-38) E-mail: honda-k898d@pa.qsr.mlit.go.jp 8下関港湾空港技術調査事務所 環境課(〒750-0025 山口県下関市竹崎町4丁目6-1) E-mail: iseri-e89qt@pa.qsr.mlit.go.jp

Application based on the strictly combined method of BEM and CADMAS-SURF

The hybrid numerical model is developed by combining BEM that can calculate the wave motion rapidly under the potential theory and CADMAS-SURF that solves Navier-Stokes equations for the free surface variation near the structure, In the hybrid model the calculation of wave motion in a wide field of wave reflection for deep water area is conducted by BEM but for shallow water area by CADMAS-SURF. Especially the hybrid model can calculate random wave motions for long term period more rapidly with almost similar accuracy than the calculation of wave motion which was carried out by CADMAS-SURF only. In this study the coupling model was applied to the calculation of the strong nonlinear wave motion such as wave runup and overtopping at the coastal structure on the mild-slope bottom and the results of numerical model were compared with the Toyosima`s experiments of regular wave runup and Goda`s design diagram of ramdom wave overtopping, respectively,

Development of A Simplified Method for Judging Storm Surge Risk around Ariake Sea

For the purpose of developing a simplified method for judging storm surge risk around Ariake Sea, many numerical computations for storm surge were carried out. A lot of various typhoon data, which are the input data for the storm surge computations, were created by a stochastic typhoon model with the Monte Carlo simulation. The relationships between the storm surges and typhoon parameters (the central atmospheric pressure, the radius of the maximum wind speed and the course) were investigated on the basis of the computation results. Then, the conditions of typhoons that may cause severe storm surges were clarified at several coastal areas around Ariake Sea.

BASIC VERIFICATION ON BOUNDARY PROCEDURES IN BOUSSINESQ CALCULATION FOR LONG-PERIOD WAVES

When waves around harbors are computed with Boussinesq models, so-called “sponge layer” is placed along the computational region to absorb reflected waves going out to open sea. The wave absorbing characteristics of the sponge layer for long period waves have not yet been fully clarified. Using two-dimensional numerical wave-tank with a sponge layer just behind the wave generating boundary, regular waves of 20, 60, 100, 200 and 300 sec were generated, and its wave absorbing characteristics were investigated through the wave height of the standing waves generated in the wave tank. It was found that the reflected waves entering from the region to the sponge layer are almost completely absorbed by properly setting the sponge stiffness and the sponge thickness. Further investigation on water surface oscillations near the boundary of the harbor was carried out. It was found that by providing thin sponge layer along the impermeable boundary, abnormal water surface oscillations, which occasionally occur around the boundary corner, can be effectively suppressed.

EFFECT OF WIND ON LANDWARD TRANSPORT OF SPLASH GENERATED BY WAVE-OVERTOPPING AT VERTICAL BREAKWATER

To clarify the effects of wind and wave run-up on the transportation of the splash caused by wave overtopping, field observations were conducted at Waku fishing port in Yamaguchi prefecture in the winter season over two years from December 2005 to February 2007. The violent wave run-up at a vertical breakwater and the transportation of splash by wind were recorded with 3 digital video cameras. Besides, incoming waves were measured with a wave gauge and wind was also measured with a wind anemometer. On the basis of observation data, several findings were obtained: 1) there are high correlations among the wave heights, wave run-up heights and the distance of splash transportation, 2) contrary to expectation, relatively low correlations are found between the distance of splash transportation and wind velocity, 3) the distance of splash transportation can be estimated from the wave run-up height and wind velocity with a simple expression obtained by multiple regression analysis.

APPLICABILITY OF COMBINED BEM-VOF METHOD TO RANDOM WAVE COMPUTATIONS

We have developed a strictly combined BEM-VOF method for solving non-linear wave-structure interactions, and have verified the accuracy and stability of the method so far. This method uses VOF method in the area around the structure where violent wave motions such as wave breakings occur and BEM in the other area where wave motion can be assumed to be a potential motion. The method is particularly useful for random wave calculations for which long and stable computation is required to obtain statistically meaningful results. In this study, to demonstrate the efficiency and utility of this method, it is applied to random wave run-up on a vertical wall, and the experimental results and the numerically estimated results for frequency distributions on run-up height were compared.