Taro Arikawa

INITIAL REPORT OF JSCE-PICE JOINT SURVEY ON THE STORM SURGE DISASTER CAUSED BY TYPHOON HAIYAN

Japan Society of Civil Engineers (JSCE) and Philippine Institute of Civil Engineers (PICE) jointly collaborated to carry out the field survey on severe storm surge disasters caused by Typhoon Haiyan/Yolanda from 12 December to 16 December, 2013 in Leyte and Samar. Based on interviews with local residents, the joint survey team obtained measured inundation and run-up heights at almost 80 different locations along the coast and also at more than 70 locations inside the cities of Tacloban and Palo. Clear contrast of inundation characteristics was observed especially among east coast of Eastern Samar, inner and southern parts of San Pedro Bay. While inner part of San Pedro bay, as was expected, showed relatively high inundation heights, east coast of Leyte also had comparably high inundation heights even outside the San Pedro Bay. It should also be highlighted that surprisingly high inundation heights were observed along the east coast of Eastern Samar, which faces to the Pacific Ocean with deep Philippine Trench. This paper aims to report primary results obtained through the first joint field survey of JSCE and PICE.

Damage Caused by the 2004 Indian Ocean Tsunami on the Southwestern Coast of Sri Lanka

The Indian Ocean Tsunami on 26 December 2004 affected Sri Lanka, which is located 1,700 km from the epicenter. A field investigation was conducted along the southwest coast of Sri Lanka to measure tsunami trace heights, investigate the damage, and obtain correct information on the tsunami. The results of the field survey showed that the tsunami arrived on the southwest coasts two and a half hours or more after the earthquake. The average tsunami height was 5 m along the southwest coast and tsunami trace heights of 10 m were found locally. The tsunami destroyed a number of wooden and brick houses, damaged port and harbor facilities and coastal railways, washed away many vessels, and scoured the foundations of coastal structures. The port and harbor facilities such as the breakwaters, and rigid coastal houses continuing along the coast reduced the tsunami and lessened the damage that it caused. Hydraulic experiments were conducted to investigate the characteristics of tsunami flooding on flat or declining ground, since it was clarified by the field survey that the severer tsunami damage was caused at the place where the coastal land inclines inland. The experimental results show that the pressure acting on the structure on the sloping bottom was at least twice that of the flat bottom under the experimental conditions.

ISPH Simulation of Scour Behind Seawall Due to Continuous Tsunami Overflow

Tsunami overflow may destroy the coast-protection structures through continuous scour behind them. In this paper, the incompressible smoothed particle hydrodynamics (ISPH) method [Gotoh, H., Khayyer, A., Ikari, H., Arikawa, T. & Shimosako, K. [2014] “On enhancement of incompressible SPH method for simulation of violent sloshing flows,” Appl. Ocean Res. 46, 104–115.] is modified for simulation of erosion process behind the seawall under continuous tsunami overflow. Concepts of numerical turbid water particle (TWP) and clear water particle (CWP) are proposed to deal with the sediment-entrained flow. Reasonable value of 1250 kg/m3 is suggested for the initial density of the TWPs based on studies of bottom sediment movement. Sediment particle is judged to initiate its movement if the exerting shear stress exceeds the critical shear stress (CSS) of incipient motion of sediment calculated from a prescribed formula. The numerical results are compared with the data of flume experiment [Arikawa, T., Ikeda, T., Kubota, K. [2014] “Experimental study on scour behind seawall due to tsunami overflow,” J. Jpn. Soc. Civil Eng., Ser. B2 (Coastal Eng.) 70 (2), I_926-I_930 (in Japanese).]. The predicted results of the maximum depth as well as the shape of the scour pit reasonably agree with the experimental data.

Collapse Mechanisms of Seawall due to the March 2011 Japan Tsunami using the MPS method

On March 11 2011 a huge tsunami was caused by a magnitude 9.0 earthquake, which devastated the sea defenses in the Tohoku coastal region of Japan. We investigated one of the disaster areas, the Taro district, which had been very famous for having a 10m high seawall. This paper focuses on why the seawall in the Taro district collapsed during the tsunami. To this end the present work involves (1) the field disaster investigation, (2) the scale of the March 11 tsunami by performing its runup calculation, (3) the overflowing effects of the tsunami on the seawall by using the MPS (Moving Particle Semi-implicit) method, (4) the overflowing model test of a seawall and (5) the possible collapse mechanisms of the seawalls, in which they were mainly destroyed by the overflowing and the ensuing draw down wave of the tsunami.

Risk assessment and design of prevention structures for enhanced tsunami disaster resilience (RAPSODI) - Euro-Japan collaboration

The 2011 Tōhoku event showed the massive destruction potential of tsunamis. The Euro-Japan “Risk assessment and design of prevention structures for enhanced tsunami disaster resilience (RAPSODI)” project aimed at using data from the event to evaluate tsunami mitigation strategies and to validate a framework for a quantitative tsunami mortality risk analysis. Coastal structures and mitigation strategies against tsunamis in Europe and Japan are compared. Failure mechanisms of coastal protection structures exposed to tsunamis are discussed based on field data. Knowledge gaps on failure modes of different structures under different tsunami loading conditions axe identified. Results of the wave-flume laboratory experiments on rubble mound breakwaters are used to assess their resilience against tsunami impact. For the risk analysis, high-resolution digital elevation data axe applied for the inundation modeling. The hazard is represented by the maximum flow depth, the exposure is described by the location of the population, and the mortality is a function of flow depth and building vulnerability. A thorough search for appropriate data on the 2011 Tōhoku tsunami was performed. The results of the 2011 Tōhoku tsunami mortality hindcast for the city of Ishinomaki substantiate that the tsunami mortality risk model can help to identify high-mortality risk areas and the main risk drivers.

Numerical Simulation of Hydraulic Overflow Pressure Acting on Structures behind the Seawall

The 2D numerical wave flume based on VOF method, called CADMAS-SURF, was applied to hydraulic pressures acting on the structures because of the flooded water. The validity of this model is verified due to the comparison with experimental data. Then the numerical simulation of the hydraulic pressure was carried out. The numerical results suggested the importance of detached breakwaters to decrease the amount of flooded water and hydraulic pressures.

A Consideration Aimed at Improving the Resiliency of Protective Structures Against Tsunami

In this paper the effectiveness of breakwaters to provide protection against huge tsunami was considered using numerical simulations. Kamaishi Bay, where large breakwaters had been installed, was selected as the target area, because about half the breakwaters were washed away by the tsunami produced by the Tohoku Earthquake in 2011. Therefore, the effect of breakwater protection against the 2011 tsunami is verified by comparing the different states of damage to breakwaters with numerical simulations. The results show that the protective effect provided by breakwaters against tsunami depends on the rate of opening gap, and suggest that this is an efficient way to improve the resiliency of deeper-region breakwater structures.

Tsunami Afeti Sonrası Yapısal ve Sosyal Planlama, Yapılanma Aşamaları ve Farkındalık: Japonya Örneği

11 Mart 2011 tarihinde gerceklesen 9.1 siddetindeki depremin yarattigi tsunami, Japonya’nin dogu kiyisi boyunca cok sayida can kaybina ve ciddi hasara yol acmistir. 2011 Tsunamisi’nin ardindan Japonya’da yapilan planlama ve yeniden yapim calismalari alinmasi gereken onlemlere onemli bir ornek teskil etmektedir. Bu calismada afetin bes yil ardindan bolgeye yapilan iki saha arastirma gezisinde gozlemlenen yapisal ve sosyal planlama, yapilanma, afet yonetimi ve afet farkindaligi yaratma calismalari ayrintilariyla sunulmus ve Turkiye’deki mevcut calismalar tartisilmistir.

DEVELOPMENT OF HIGH PRECISION TSUNAMI RUNUP CALCULATION METHOD COUPLED WITH STRUCTURE ANALYSIS

The 2011 Great East Japan Earthquake (GEJE) has shown that tsunami disasters are not limited to inundation damage in a specified region, but may destroy a wide area, causing a major disaster. Evaluating standing land structures and damage to them requires highly precise evaluation of three-dimensional fluid motion – an expensive process. Our research goals were thus to develop a coupling STOC-CADMAS (Arikawa and Tomita, 2016) coupling with the structure analysis (Arikawa et. al., 2009) to efficiently calculate all stages from tsunami source to runup including the deformation of structures and to verify their applicability. We also investigated the stability of breakwaters at Kamaishi Bay. Fig. 1 shows the whole of this calculation system. The STOC-ML simulator approximates pressure by hydrostatic pressure and calculates the wave profiles based on an equation of continuity, thereby lowering calculation cost, primarily calculating from a e epi center to the shallow region. As a simulator, STOC-IC solves pressure based on a Poisson equation to account for a shallower, more complex topography, but reduces computation cost slightly to calculate the area near a port by setting the water surface based on an equation of continuity. CS3D also solves a Navier-Stokes equation and sets the water surface by VOF to deal with the runup area, with its complex surfaces of overflows and bores. STR solves the structure analysis including the geo analysis based on the Biot’s formula. By coupling these, it efficiently calculates the tsunami profile from the propagation to the inundation. The numerical results compared with the physical experiments done by Arikawa et. al.,2012. It was good agreement with the experimental ones. Finally, the system applied to the local situation at Kamaishi bay. The almost breakwaters were washed away, whose situation was similar to the damage at Kamaishi bay.

EFFECT ON SCOUR DEPTH OF FALLING HEIGHT OF OVERFLOW

2011年東北地方太平洋沖地震における津波により,防 波堤など港湾構造物が甚大な被害を受けた.根木1)によ ると,防波堤の被災パターンは,津波波力型,越流洗掘 型,堤頭部洗掘型,引き波水位差型の主に4つに分類さ れる.その中で,有川ら2)は,ケーソン背後が洗掘され ると,マウンド支持力が低下しケーソンが倒壊すること を水理模型実験から明らかにした.また,東山ら3)によ ると,越流洗掘型の港内側マウンドの被災に対して港内 水位が重要であること,最も危険な水位が津波水位ピー ク時であるとは限らないこと,そのため,港内側マウン ドの越流洗掘対策を検討する際には,最も危険な水位お よび港内水位の変化の状況を見極めることが重要である ことを指摘した.つまり,落下高と洗掘深の関係を明確 にすることで,最も危険な水位を検討する基礎情報とな ると考えられる. 越流による洗堀に関する既往研究として,野口ら4)は 孤立波の護岸における戻り流れによる前面洗堀に関する 実験を行い,越流落下高さと洗堀深の関係および洗堀深 が渦の大きさに関係することを明らかにしている.また, 有川ら5)は,直立型堤防背後における越流による洗掘量 に関する実験を行い,越流量が同じであっても,ある落 下高さ以上ではそれ以上に洗堀深が大きくならないこと を示した.しかし,渦と洗堀深の大きさに,それぞれの 実験結果の違いがあることや,落下高さと洗堀深の関係 が不明瞭であることが課題としてあげられる.そこで本 研究では,落下高と洗掘深の関係について検討すること を目的として,水理模型実験を行い,その結果を整理し た.