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4 COASTAL DAMAGE The coastal damage produced by the tsunami can be grouped into three main classifications: (l) damage from flooding; (2) damage to and caused by floating structures and damage caused by floating debris; and (3) damage to protective structures. DAMAGE FROM FLOODING As the tsunami, which was generated in relatively deep water, approached the coast and entered shallow water, the wave amplitude increased and was then transformed, mostly into bores. These bores rushed up the beach and entered harbors and rivers. Even though many areas were flooded, damage to houses was minor because few structures were located in low coastal areas. During the tsunami, l00 individuals were drowned, most of them at Noshiro, Hachimori, and the Oga Peninsula. These individuals were caught on the shoreline or on offshore construction sites. Tsunami warnings were issued approximately l4 minutes after the earthquake occurred; however, the leading wave reached the shoreline at these locations at approximately the same time. In Korea, three deaths were attributed to the tsunami. Since the tsunami reached the Korean shoreline approximately l-l/2 hours after the earthquake occurred, these deaths could have been prevented. A tsunami warning was issued by the Central Meteorological Office in Korea, 20
2l providing ample time for proper evacuation. Part of the Korean people's response to the tsunami, however, is indicative of the general lack of understanding concerning the dangers of tsunamis. Figure l4 shows Korean spectators following and watching the receding tsunami as it spills from a dock. Minutes before the photograph in Figure l4 was taken, this area was flooded to the top level of the tetrapods resting on the dock. This action by the spectators reveals a lack of understanding of tsunami dangers, in general, and of the possibility of subsequent waves, in particular. FLOATING STRUCTURES AND DEBRIS DAMAGE As the tsunami approached the shoreline and entered shallow waters, it created high water levels and strong currents. These caused mooring lines to break and ships to capsize. Most damage to floating structures resulted when they were carried by the high water levels and strong currents toward land, where they collided with other fixed structures. During the tsunami, significant numbers of boats were damaged when they collided with piles. Sometimes, these piles were designed to keep boats out and let water flow through. This type of damage is illustrated in Figure l5. In Korea, over 70 ships were damaged by the tsunami. The two photographs in Figure l6 graphically illustrate the destructive damage to boats and floating structures that can occur from the waves and currents produced by a tsunami. Figure l6A shows the tides rushing over a breakwater and inundating low-lying portions of a harbor. Figure l6B shows large ships being thrust onto and into the city. These ships received and caused significant damage during their movement. Experience from these localities suggests that damage from tsunamis could be significantly reduced if ships could be moved in time to deeper water, where both the waves and currents would be smaller.
22 FIGURE l4 Sightseers observing tsunami action in Imweon, Korea. Source: Korea Maritime and Port Administration. DAMAGE TO PROTECTIVE STRUCTURES Many coastal flood protective structures were severely damaged during the tsunami. For example, concrete armor units weighing 4 metric tons were moved hundreds of feet inland near the area of maximum run-up (see Figure l7). In addition, many levees and seawalls were overtopped and damaged. These protective structures were designed for storm waves and were unable to accommodate the waves produced by the tsunami. However, the nature of the damage observed could be useful in improving the future design of protective structures for tsunamis.
23 FIGURE l5 Damage in Noshiro, Japan. Note the large ship lodged on the pile. Source: Akitakai Newspaper Publishing Company, l983.
24 FIGURE l6A Coastal inundation in Imweon, Korea. Source: Korea Maritime and Port Administration. FIGURE l6B Damage to floating structures in Imweon, Korea. Source: Korea Maritime and Port Administration.
25 FIGURE l7 Protective concrete units weighing 4 metric tons were scattered by the tsunami at Mizusawa Beach near Minehama Village in Akita Prefecture. Source: Akitakai Newspaper Publishing Com- pany, l983.