Takahito Mikami

Classification of Tsunami and Evacuation Areas

On March 11, 2011, a large earthquake that occurred offshore the north-east coast of Japan generated a large tsunami which devastated extensive areas of the Tohoku coastline. Despite Japan being considered a country well prepared for these types of disasters, large casualties were recorded, with numerous discussions amongst the Japanese coastal engineering community ensuing. As a result, two different levels of tsunamis have been proposed and now recognized in Japan, depending on the frequency of such extreme events. The idea that hard measures can protect the lives of inhabitants of coastal areas has been abandoned, and these measures are only considered to be effective in protecting properties against the more frequent but lower magnitude events. Soft measures should always be used to protect against the loss of lives, and to this respect, the authors of the paper propose the introduction of a Classification of Evacuation Areas, to show which of these should be prioritized by residents as they seek to evacuate.

Track analysis, simulation, and field survey of the 2013 Typhoon Haiyan storm surge

The authors have examined the characteristics of Typhoon Haiyan (Yolanda), which made landfall on the coast of the Philippines on November 2013, generating a substantial storm surge. In order to better understand the storm surge phenomenon, its nature and severity were analysed by means of a numerical simulation and a field survey. Unlike most other tropical cyclones that weaken before they hit land, Haiyan struck Leyte Island at near peak strength, with maximum sustained wind speeds of 160 knots, the strongest in the recorded history of the Western North Pacific. Haiyan approached very quickly with a forward speed of 41 km/h towards Leyte, which was also the fastest among typhoons with similar intensities. As a result of these extreme gusts and the exceptionally low central pressure of the typhoon (895 hPa), Haiyan caused the largest storm surge in the recorded history of the Philippines. Numerical simulations show that the maximum storm surges occurred in Leyte Island and Samar Island. A storm surge field survey conducted by the authors corroborated this, with maximum inundation height of 7 m recorded in Tacloban, located at the northern end of Leyte Gulf. The simulation results also corroborate the fact that water levels at some locations first lowered and then rapidly began to increase after an hour.

Stability of Breakwater Armor Units against Tsunami Attacks

The design of breakwater armour units against tsunami attacks has received little attention in the past because of the comparative low frequency of these events and the rarity of structures designed specifically to withstand them. However, field surveys of recent events, such as the 2011 Great Eastern Japan Earthquake Tsunami and the 2004 Indian Ocean Tsunami, have shown flaws in the design of protection structures. During these extreme events, many breakwaters suffered partial or catastrophic damage. Although it is to be expected that most normal structures fail due to such high order events, practicing engineers need to possess tools to design certain important breakwaters that should not fail even during level 2 events. Research into the design of critical structures that only partially fail (i.e., “resilient” or “tenacious” structures) during a very extreme level 2 tsunami event should be prioritized in the future, and in this sense the present paper proposes a formula that allows the estimation of armour unit damage depending on the tsunami wave height.

Storm Surge Heights and Damage Caused by the 2013 Typhoon Haiyan Along the Leyte Gulf Coast

Typhoon Haiyan (Yolanda) struck the islands of Leyte and Samar, the central part of the Philippines, on 8 November 2013, bringing large-scale devastation to coastal areas due to the high winds and large storm surge and waves associated with it. In order to obtain the distribution of storm surge heights and damage covering the wide stretch of affected coastline, the authors carried out a series of field surveys in the aftermath of the typhoon (in December 2013, May 2014, and October 2014). In the present paper, the authors detailed the results of these field surveys and summarized the characteristics of the storm surge and main causes of the damage, especially focusing on the Leyte Gulf coast, which is surrounded by the eastern coast of Leyte and the southern coast of Samar. Finally, the wide range of types of storm surge disasters were also discussed by comparing Typhoon Haiyan with other recent major events.

Storm Surge Awareness in the Philippines Prior to Typhoon Haiyan: A Comparative Analysis with Tsunami Awareness in Recent Times

Category 5 Typhoon Haiyan was one of the strongest typhoons to hit the Philippines in modern times, and introduced the term “storm surge” to the vocabulary of many local residents, who had not heard about such phenomena prior to this event. The storm surge manifested itself on the 8th November 2013 as Haiyan made landfall, devastating large areas in the islands of Leyte and Samar. To attempt to gain an understanding of the level of awareness that local residents had about storm surges the authors conducted structured questionnaires (n = 172) and focus group interviews with local residents, and discussed the results obtained with key informants (government officers and disaster risk managers). One of the key problems identified during the interviews was how people were not able to clearly understand the concept of a storm surge, with many respondents emphasizing how it would have been better for authorities to describe it as a “tsunami”. A discussion is also made on the recent evolution in world-wide coastal disaster awareness, comparing it to recent developments in storm surge awareness in the Philippines and other countries in the area, such as Vietnam and Japan. The authors conclude that, in terms of its influence on worldwide disaster awareness, typhoon Haiyan constitutes an event of similar importance to major recent high-impact tsunami events such as the 2004 Indian Ocean and 2011 Tohoku events. As a result, the authors will outline a number of lessons learnt from Haiyan, such as the necessity for creating multi-layer safety strategies and improving how information about storm surges is transmitted to local residents.

Evaluating Tsunami Risk and Vulnerability Along the Vietnamese Coast

The Vietnamese coast faces the East Sea, which has been recognized as having the potential to generate devastating tsunamis, thus requiring a careful consideration of tsunami risk. In this chapter, tsunami risk and vulnerability are analyzed based on the numerical simulation of tsunami propagation and the social and geographical background of Vietnam. The numerical simulation of a potentially hazardous earthquake scenario in the Manila Trench shows the distributions of the maximum tsunami amplitude and arrival time along the Vietnamese coast. When discussing the vulnerability of the coastline, particular attention is given to three main elements affecting this type of potential disaster in Vietnam: the extensive area of low-lying ground, namely large deltas; a lack of past experience regarding tsunami events; and underdeveloped tsunami mitigation measures. Tsunami mitigation measures implemented recently in coastal communities in Vietnam, such as tsunami warning stations and tsunami drills, are also mentioned.

Entrainment and Transport Dynamics of Shipping Containers in Extreme Hydrodynamic Conditions

An experimental study was performed to examine multiple debris entrainment and transport in tsunami-like flow conditions. The study was performed in the Tsunami Wave Basin at Waseda University. The wave basin used a falling-head driven elongated solitary wave that broke at the edge of a horizontal apron to generate a tsunami-like surge. The debris, modeled as scaled-down (1:40 length scale) 20-foot standard shipping containers, was placed on the horizontal apron. The debris was tracked using a camera-based object tracking algorithm. The study examined the effect of the debris configuration, the number of debris, and orientation of debris on the entrainment and transport within the inundating surge. In examining the transport of multiple debris, the debris tended to be transported within the surge as an agglomeration. Using a previously derived model of debris transport, the study determined experimentally derived coefficients to explain the difference in debris transport between various configurations.

EFFECT OF SEA LEVEL RISE AND INCREASE IN TYPHOON INTENSITY ON COASTAL STRUCTURES IN TOKYO BAY

Sea level rise and an increase in typhoon intensity are two of the results expected from future climate change. In the present work a methodology to change the intensity of tropical cyclones in Japan is developed based on the work of Knutson and Tuleya (2004). An example of how this would affect one of the worst typhoons to hit the Tokyo Bay area in the 20 century was thus developed, highlighting the considerable dangers associated with this event, and how current sea defences could be under danger of failing by the end of the 21 century.

Experimental Investigations of Debris Dynamics over a Horizontal Plane

Abstract This study presents the results of an experimental research program dealing with spatial debris motion on a horizontal apron depicting a typical harbor wharf. Accordingly, scaled-down 6.1-m (20-ft) shipping containers were equipped with a novel yet nonintrusive real-time tracking system and motion sensors. The instrumentation allowed for the spatiotemporal tracking of debris specimens moving across the apron while entrained by an incoming tsunami-like broken bore. The system proved its capabilities and accuracy; this was particularly challenging since this was the first time the system was used in water. The experiments involved using various numbers of shipping containers that were either arranged in one layer or stacked in two layers. In addition, the effect of different numbers of container rows was also investigated to study the influence of the overall container count and placement with respect to their longitudinal displacement and dispersion (spreading) across the apron. Linear relationshi...

Estimation of the current risk to human damage life posed by future tsunamis in Japan

Abstract On the 11th of March 2011 an earthquake offshore the northeast coast of Japan generated a huge tsunami which overwhelmed coastal defences and inundated extensive areas of the Tohoku coastline. Japan was considered to be well prepared against such disasters but nevertheless casualties were high, though death rates were not homogeneous within the affected regions. In the present paper the authors investigate the relationship between human damage, maximum tsunami wave height and arrival time during the 2011 Tohoku Earthquake Tsunami. Generally speaking, the results indicate a positive correlation between the casualty rates inside inundated coastal settlements and tsunami height and a negative one regarding the arrival time (i.e. longer arrival times allowed residents more time to successfully evacuate). Based on these results a new tsunami category is introduced to clarify the potential threat that various earthquake scenarios pose to coastal communities in the Kanto area in Japan. These results contain useful insights for managing risks and prioritizing the investment in tsunami countermeasures to reduce future human suffering due to these disasters.