Ravindra Jayaratne

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.

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.

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.

A SIMPLE NUMERICAL SIMULATION MODEL FOR BEACH PROFILE EVOLUTION

Abstract: The prediction of suspended sediment concentration and cross-shore beach deformation has been researched broadly over the last few decades. However, theoretical and numerical simulation models still produce results to a considerable uncertainty and developing such models to satisfactory levels is a challenging task. Hence, a complete set of explicit theoretical models has been derived to predict field-scale suspended sediment concentration on rippled bed, from sheet flow, and under wave breaking agitation by Jayaratne and Shibayama (2007, 2010). The present study leads on to develop a simple numerical simulation model for predicting beach profile evolution with onshore sand bar formation and erosion under monochromatic wave action based on the sediment concentration formulae of Jayaratne and Shibayama (2007, 2010). A good agreement was found between measured and computed field-scale beach profiles.

Stability of breakwaters against Tsunami attack

Abstract Despite the regularity with which tsunami waves attack a variety of countries around the world, comparatively little attention has been paid in the past to the design of tsunami-resilient breakwaters. This chapter will highlight some of the problems related to the failure mechanisms of caisson and armored caisson breakwaters against tsunami attack, focusing mainly on two modes: bearing capacity failure at the heel of the caisson and the stability of primary armor units. A methodology for the design of such armor will be provided for the case of overtopping waves, based on real life analysis of the failure of such armor. This method takes into account not only the tsunami level and height, but also the function of the structure (in terms of the importance of what it has been designed to protect), highlighting the need for an approach that can minimize the cost of such countermeasures.

Hydrodynamic perspectives of soil scour in fluvial environment

The major concern for many hydraulic structures is the effect of scour at the toe, when the racing floodwater scours away the bed just downstream of the piers. Therefore, understanding the soil-hydrodynamic interaction needs to be investigated. In this study, a series of 2D laboratory tests have been carried out to study the likelihood of soil scour due to the soil-hydrodynamic interaction and influence of sediment properties. Characteristics such as sediment deposition patterns, longitudinal/lateral spreading length/area, and bed scour profiles for three sediment diameters (i.e. 0.26, 0.30 and 2.40 mm) under dry and wet soil conditions are studied intensively. Experimental results revealed that soil of identical diameters under wet and dry conditions caused significant changes in soil scour rate and deposition patterns. Transport rates in dry condition were much slower than wet condition. It was observed that, for the same flow condition, different soils gave different long term equilibrium deposition patterns due to the grain size distribution and particle shape. Eddies were generated behind the soil samples which resulted in forming a series of ‘crescent’ zones. Findings of this study could offer a qualitative outline of the effects of various parameters to demonstrate a better representation of estimating scour rate in fluvial condition.