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For instance, in the area around Hikkaduwa City in Sri Lanka, about 47% of the residents do not trust in the functionality of the present early-warning tower since it failed during the 2012 Sumatra tsunami (Samarasekara et al. 2011 Bernard and Titov 2015 Samarasekara et al. The danger of a malfunctioning of early-warning systems can aggravate the effects of insufficient additional countermeasures (Strunz et al. For instance, the 2004 IOT reached the town of Banda Aceh in northern Sumatra within 15 min after the earthquake. Although, in several potentially affected locations, authorities operate state-of-the-art tsunami early-warning systems, the available time for alerts or the evacuation of the threatened coastal population is often insufficient and the possibility of malfunctions cannot be ruled out (UNDRR 2019). 2017), which may be mitigated by hard or soft tsunami countermeasures. Apart from instantaneous destruction, tsunamis can cause medium-term impacts such as the destruction of power plants and long-term impacts such as salt-water intrusion in intensely cultivated delta plains (Villholth and Neupane 2011 Nakamura et al. The Indian Ocean Tsunami (IOT) on the Boxing Day of 2004 was the most destructive recent tsunami with about 230,000 fatalities (Telford et al. From 625 million people in 2015, the population growth in low-lying coastal areas is expected to raise by 68–122% resulting in about 1052 to 1388 million people by 2060 (Neumann et al. The threat due to tsunamis intensify since the already densely populated coastal areas are experiencing further population growth as predicted by Neumann et al. The highly destructive energy of tsunamis can cause large numbers of causalities, damages to infrastructure and affect the livelihood of coastal communities. Many coastal communities are exposed to the hazards of marine flooding induced by tsunamis or storm surges resulting in adverse impacts on the coastal ecosystem and built environment. An overview and comparison of such damages and dependencies are given and new approaches for mitigating tsunami impacts are presented. The recent large-scale tsunami events facilitate the assessment of performance characteristics of countermeasures and related damaging processes by in-situ observations. In this paper, the efficacy of hard countermeasures is evaluated through a comprehensive literature review. Whist hard countermeasures generally aim at reducing the inundation level and distance, soft countermeasures focus mainly on enhanced resilience and decreased vulnerability or nature-based wave impact mitigation. These countermeasures encompass hard measures to physically prevent tsunami impacts such as different types of seawalls or offshore breakwaters, as well as soft measures such as long-term tsunami hazard assessment, tsunami education, evacuation plans, early-warning systems or coastal afforestation.
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To counter their effects, several hard and soft mitigation measures are applied, the choice of which essentially depends on regional expectations, historical experiences and economic capabilities. Tsunamis pose a substantial threat to coastal communities around the globe.