Shunya Matsuba

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.

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...

Destruction of coastal structures after the 2011 Great East Japan earthquake and tsunami

Abstract Coastal dikes, breakwaters, and seawalls are generally used to control floods, debris flows, erosion, and even massive catastrophic events such as tsunamis. However, these structures will only perform well if they are firmly constructed in terms of strength and stability of slopes and toes and can resist the dynamic forces imposed by the incident wave energy. During a Level 2 tsunami such as the 2011 Tohoku Earthquake Tsunami many concrete coastal defenses in the NE of the country could not withstand the forces imposed by the tsunami overflowing water and as a result failed either partially or completely. The authors carried out detailed field surveys in Miyagi and Fukushima prefectures during the period from 2011 to 2013 in order to explore the failure modes and mechanisms of coastal structures due to the 2011 Tohoku Earthquake Tsunami. Eight locations which were severely affected by this event were considered in the analysis. Subsequently, six major failure modes could be identified from the collected field data, namely: (i) leeward toe scour failure, (ii) crown armor failure, (iii) leeward slope armor failure, (iv) seaward toe scour and armor failure, (v) overturning failure, and (vi) parapet wall failure. At most surveyed locations the critical failure mode was identified as the scour initiated at the leeward toe. The present chapter discusses all six failure mechanisms, highlighting how the lessons learnt from them can help to improve the design of coastal structures under tsunami attack.

Fluid Motion around Coastal Dyke due to Overflowing Tsunami

Fluid motion around a coastal dyke due to overflowing tsunamis was investigated by conducting hydraulic experiments and analyzing videos recorded during the 2011 Tohoku Earthquake and Tsunami. The results of the hydraulic experiments showed that overflowing tsunamis could be classified into three types, and velocity field around coastal dykes and pressure acting on coastal dykes were different in these three types. This classification can be helpful to consider the mitigation of damage on coastal dykes. The videos showed that the type of overflowing tsunamis around coastal dykes varied with the location, and this type was considered to have an influence on how overflowing tsunamis affected coastal dykes in the 2011 tsunami.