Assessment and mitigation proposal in case of major tsunami impact

Abstract

Iquique is an important city in the northern of Chile. For the coast of Iquique a seismic gap with a return period of 111 ± 33 years is present. Since the last big event in 1877 only 20 % of the accumulated seismic slip is released. The configuration of Iquique is such that a large future earthquake and subsequent tsunami would generate a serious impact in terms of loss of life, economical andmaterial damage and post-event disruption of the society. In this research an answer is given to the question how to reduce the impact of a major tsunami. First the size of the impact is assessed with a high resolution numerical model, NEOWAVE. Various possible scenarios are analysed. Governing parameters are flow depth, flow velocity and the arrival time of the wave in city. The worst case tsunami scenario predicts significant inundation on three locations, the port, the commercial centre Zofri and the popular Cavancha region. The port is outside of the scope of this research. Detailed analysis of the Zofri and Cavancha areas give relevant information for future disaster mitigation. In Zofri, the natural barrier at the shore is overtopped, after which the hinterland, including the Zofri mall, is inundated. The topography of this area shows a hollow profile therefore a ’bathtub’ effect is present. After 60 minutes 50 % of the area is inundated with a maximum of flow depth 2,5 meters and a runup of 4.7 meters. The total inundated area is 760.000m2 In the Cavancha region the water enters 17 minutes after the earthquake event, mainly from the north side,inundating the hinterland. After 5 additional minutes, the Cavancha peninsula is cut off, creating an island. The numerical results give a maximum flow depth of 5mand flow velocities between the 3 and 5 m/s and the runup reaches 6.8meters. The total inundated area is 700.000m2. As the size of the impact is determined, the mitigation of the impact is assessed. A method of developing and assessing mitigation measures is put forward. Essential to this method is the framework, in which elements of existing calamity frameworks are captured. The used framework addresses the phase to which the disaster has evolved (strategies), techniques that can be applied with respect the the event (methods) and the concept of safety ensured on multiple levels. The use of the framework ensures the development of a multilateral complex solution. Sub-research questions corresponding to the stages of analysis, design and validation are introduced to guide the process of obtaining the answer to the main research question. In the analysis stage a short list of mitigation measures, specifically suited for the Iquique areas, is derived. In the subsequent design stage, the methods, derived in the framework, are used to compose three possible systems of mitigation. The first system focusses on retaining water at the coastline, preventing the hinterland from being inundated by the use of a Dyneema tsunami barrier. In order to secure safety, mitigation measures related to escaping the water are implemented. Since the system also has to reduce impact related to possible post-disaster disruption,also the method preparing for the aftermath is employed. The second system evolves around the concepts of delaying and diverting the incoming water. In order to do this, an submerged breakwater and flow channels integrated in existing infrastructure are introduced. Similar to the first system, this system makes use of measures related to escaping the water and preparing for the aftermath. The third system focusses completely on the concepts of escaping the water and preparing for the aftermath. A new evacuation protocol and the design of a public vertical evacuation building are proposed. The systems are validated on their effectiveness and their side effects are evaluated. At this point, the three complex solutions are evaluated on the impact criteria. All the systems can be implemented in Iquique and will reduce the impact of a future tsunami. When mitigation measures for Iquique are considered the systems proposed in this report are a guidance for the possibilities and scale of impact reduction