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Author: Robert A. Dalrymple and David L. Kriebel
The design of civil engineering structures in tsunami-prone areas can be critical.
Living near the sea means living with the risk of a tsunami. In some areas, such as the Pacific Rim countries, the risk is high: the possibility of a significant tsunami occurs on a decadal scale in Japan. In other regions of the world—the Mediterranean Sea and the Atlantic Ocean—the risks are much lower. In the Indian Ocean, the risks are lower still, although tsunamis have occurred there in the past. The National Geographic Data Center (NGDC, 2005) lists 63 tsunami events in the Indian Ocean since 1750, eight of which were major events, including the wave caused by the eruption of Krakatoa in 1883 in the Sunda Strait (for a good read, see Winchester, 2003). The Boxing Day Tsunami of December 26, 2004, which killed more than 200,000 people in 11 countries around the Indian Ocean, showed the catastrophic effects of a major tsunami in a low-risk, and consequently unprepared, region.
Tsunamis are created in a variety of ways. Perhaps the best known generation mechanism is earthquake-induced displacement of the sea bottom, which causes a related sea-surface elevation that then propagates away from the generation area due to gravity. But submarine slumping of the offshore shelf or slope or the impact of a terrestrial landslide into the sea can also cause devastating tsunami waves. In fact, the highest known tsunami wave, caused by a large landslide in Lituya Bay, Alaska, is documented to have toppled trees growing 524 meters above the water level (Miller, 1960). Even higher tsunami waves are thought to have been caused by bolide impact—meteorites striking the ocean—during the millions of years of Earth’s history. The height of the wave would be related to the size of the bolide, which could be comparable to the depth of the ocean.
Despite the infrequency of tsunami events, their severity and extreme consequences make the design of civil engineering structures in tsunami-prone regions critical. In Japan, efforts are under way, at great cost and expense, to ensure that ports are tsunami-proof and that populations living near the sea are protected through appropriate construction and alerted through warning systems. For example, on Okushiri Island, a seawall nearly 4.5 meters high was built to protect the Aonae peninsula. Yet this wall was overtopped by a tsunami in 1993, and more than 185 people were killed. Since then, the wall has been rebuilt, and there is an ongoing debate about the wisdom of the wall, which is so high now it obstructs the view of the sea and was extremely expensive to build. Today, nations around the Indian Ocean are trying to decide whether to allow rebuilding on the coast, which structures to rebuild and which ones to relocate, and how to rebuild to minimize losses in future tsunamis.
As they have done after other natural disasters involving the loss of many lives, the American Society of Civil Engineers (ASCE) sent teams of engineers to India, Sri Lanka, and Thailand to examine the effects of the December 26, 2004, tsunami on civil infrastructure and lifelines, such as ports and airports, residential and commercial buildings, roads and bridges, and water supply and wastewater systems. Composed of experts in earthquake damage to lifeline infrastructure and coastal engineering, these teams were asked to determine how well the civil infrastructure fared in the tsunami. The goal was to learn something new about mitigating future tsunami damage in the United States and around the world in other tsunami-prone areas (for a good summary, see NTHMP, 2001).
As part of the ASCE investigation, the authors1 visited Thailand in early February 2005 to observe the tsunami’s effects on the civil infrastructure, as well as on beaches and coastal structures. Because the southwest coast of Thailand has become an international tourist destination that now hosts more than 1.5 million visitors during the winter high season, this area provided an opportunity for the team to assess tsunami impacts on modern infrastructure that was designed and built to high standards. Lessons learned there are relevant to many coastal resort areas in the United States that are subject to tsunami hazards—notably, parts of Hawaii and the highly developed coastal regions of California, Oregon, and Washington.
Nations around the
Indian Ocean are trying
to decide whether to allow
rebuilding on the coast.
The tsunami was generated less than 500 kilometers west of Thailand at 8 a.m. (local time) on December 26, 2004; it was caused by a magnitude 9.3 earthquake off the coast of Sumatra. Part of the tsunami propagated due east, and in 2 hours struck the populous southwest coast of the country at Phuket Island and nearby areas known for their beaches and tourist resorts. Unfortunately, because the wave arrived at high tide, the tsunami rode on top of the elevated tidal water level.
The tide-gauge at Ko Taphao Noi, an island about 7 kilometers southeast of Phuket City, is part of the Global Sea Level Observing System. Because the tide gauge was sheltered from the direct assault of the tsunami by Phuket Island to the west, the recorded wave magnitude was less than the magnitude that hit the western shorelines. The tide record shows that the tsunami arrived as a negative wave that dropped the sea level to a level corresponding to low tide in a matter of 20 minutes; this was followed by two major wave crests in quick succession (20 minutes apart). These were followed by continuing oscillations of the water surface for the rest of the day due to the excitation of the entire Indian Ocean by the original tsunami.
Due to the offshore bathymetry, the height of the tsunami that struck along the southwest coastline varied by nearly a factor of three, with 4-meter high waves striking Nai Yang Beach (near the Phuket International Airport), 6-meter high waves hitting the populous Patong Beach on Phuket Island, and 11-meter high waves inundating the popular Khao Lak resort area, 65 kilometers north of Phuket City. These run-up and inundation elevations were measured by Japanese-Thai survey teams just after the wave attack (RCDRS, 2005). The variations in wave height resulted in corresponding differences in the number of fatalities and injuries.
The official total death toll in Thailand was more than 5,000 people, and about 3,000 more are still listed as missing. Surprisingly, only about 250 people were lost at the densely populated Patong Beach on Phuket Island. In contrast, nearly all resorts in Khao Lak, a jumping-off point for diving in the Andaman Sea, were destroyed or severely damaged, and thousands died there.
At Phi Phi Don Island, 40 kilometers southeast of Phuket Island, each wave struck the crowded tourist area twice, because the geography of the island is comprised of two high rocky islands connected by a low sandy isthmus that has a maximum elevation of about 1.5 meters and a width of several hundred meters. Approximately 10,000 people were celebrating the holidays on this finger of sand when the first wave struck. The H-shaped island, with the isthmus running roughly east-west, was struck from the west, which forced the tsunami to refract and diffract into the north- and south-facing beaches (on Loh Dalam Bay and Ton Sai Bay, respectively). From the photographic evidence, the shallowness of Loh Dalam Bay and the deeply indented southern Ton Sai Bay caused the northern beach to be struck first and experience the largest wave heights (about 5 meters), which overtopped the isthmus. Immediately after, another part of the same wave struck from the south, washing the debris back over the island. Nearly 2,000 people died that day from drowning or being hit by debris.
Despite the high water levels, there was surprisingly little permanent damage to major civil infrastructure. At Patong Beach, despite the flooding, most of the structural damage was confined to beachfront structures, and within weeks, much of the town had reopened for business. The Phuket International Airport suffered no damage to the runways, despite its seaside location. Water supply and waste treatment plants were largely intact, except for seaside pumping stations and pipes in waterways that were damaged by scour. Most of the major bridges, roadways, and civil buildings survived the inundation from the wave well. However, low-lying coastal resorts, businesses, and private homes were severely damaged, as were fishing villages, such as Baan Nam Kem, which were visited by the ASCE team and, subsequently, by former Presidents Bush and Clinton.
After visiting a variety of beaches, ports, and fishing villages, the team was able to draw some very clear conclusions about structural design in a tsunami-prone region. Most of the well designed, reinforced concrete buildings with good foundations survived the wave attack. The survival rate was even higher for buildings that were elevated, allowing the water to flow under the structure. In addition, if the structure was constructed so water could flow through the first floor, structural damage was minimized, despite the loss of interior contents. This was actually part of the (accidental) design of many resorts in Thailand, which had reinforced concrete buildings that contained resort apartments with sliding glass doors facing the sea and the backs of the buildings. These buildings suffered little structural damage as the force of the tsunami broke through all of the doors and windows, thus reducing the force of the water on the building itself. By contrast, concrete buildings with solid masonry in-fill walls and no flow-through capability often experienced destruction of the walls and, in many cases, damage to the load-bearing structural frame.
At Kamala Beach, on Phuket Island north of Patong Beach, the Hotel Benjamin was one of the few buildings we saw that was constructed on concrete pilings that allowed water to flow under it. There was no obvious structural damage to the hotel, but the non-elevated buildings on either side of this shorefront hotel sustained considerable damage. Interestingly, the next row of non-elevated buildings landward showed a surprising pattern of damage because they took the brunt of the wave force from the water that flowed under and through the Hotel Benjamin. From this observation, the team concluded that if a seaside building is elevated to reduce damage from wave impact, the landward buildings behind it should also be elevated.
Many building failures occurred as the tsunami floodwaters scoured building foundations, as shown in the picture of Yumei Wang (ASCE team member) and an exposed footer. Unlike hurricane-prone coastal areas of the United States, where coastal buildings are now elevated on deep foundation piles, buildings in Thailand are often elevated on shallow spread footings embedded less than 1 meter below ground level. Our team saw dozens of examples of sediment scour down to, and under, these shallow footings. Interestingly, the scouring appeared to occur mainly as the tsunami receded and floodwaters returned seaward. Thus, many damaged buildings may have survived the incident tsunami wave only to be damaged as the floodwaters ebbed.
Khao Lak, with its low-lying coastal plane and numerous tourist resorts, suffered tremendous damage and loss of life as the result of the 1-meter high turbulent bore that raced 1.5 kilometers landward. The height and bore-like nature of the wave were likely due to the much shallower shelf offshore of the beach compared to the shelf off Phuket Island. Tourist videos and pictures2 show that the waves in this region broke far offshore (in fact, well offshore of two coastal patrol boats) and came ashore as a nearly vertical wall of water moving at high speed (probably on the order of 40 to 60 kilometers per hour, based on measurements made after other tsunamis). This rapidly moving tsunami bore destroyed much of what was in its path. All timber structures, with few exceptions, were destroyed, creating hazardous floating debris.
Reinforced concrete structures fared somewhat better, and structures that were flow-through, such as the microwave tower and the Andaman Scuba shop did very well. The scuba shop fortuitously had a trapezoidal footprint and was oriented so that the short side faced the sea, streamlining the building. The first floor was a shop lined with display windows that were broken out by the wave, which further reduced the hydrodynamic load on the building. The building on the right shows the typical type of damage to concrete buildings with masonry in-fill walls. The garage-door size holes broken through by the water on both the seaward (to the left in the photograph) and landward sides of the building illustrate the need for flow-through design. (The building between these two was apparently under construction; the skeletal nature of the building is not likely due to tsunami damage.)
At least 5 meters of water flooded the Phang Nga (Thai) Navy Base near Khao Lak, causing scour, grounding a Navy frigate, and killing 80 land-based sailors. Buildings at the base of the major pier were destroyed as the wave attempted to plane off the pier structures. At Baan Nam Kem, the fishing village north of Khao Lak, the uplift pressure of the water displaced the concrete pier decks. Fishing boats broke their moorings and were found clustered in the inland swamps and among the residences in the town. By contrast, boats and ships at sea suffered no damage because the tsunami was small in amplitude offshore. (We also heard that scuba divers were unscathed by the wave, although they were scared by the sudden powerful surge as the wave passed.)
On Phi Phi Island, major resorts on the isthmus constructed of reinforced concrete were not structurally damaged by the wave; most of the wooden homes in the inundation zones were destroyed. The resorts’ foundations were designed well enough to prevent the scour from overpassing waves at the corners of the building from being deep enough (a meter or so) to create foundation problems. These multistory, reinforced concrete resorts also played a lifesaving role by providing a vertical evacuation route that enabled some people to find safety above the floodwaters.
In almost all cases, it was clear that buildings in the inundation zone, piers, and harbor support buildings should have scour-resistant foundations, small projected areas exposed to the horizontal flow of the wave, and good anchoring to their foundations. Flow-through structures are critical to reducing hydrodynamic forces on structures. Designing buildings with habitable areas on upper floors and using the ground floor for less vital space can also reduce fatalities.
However, structural engineers must be aware that buildings able to withstand wave attacks, as suggested above, might not survive the earthquake that triggers the tsunami. Earthquake resistance and flow-through lower floors are not obviously compatible. Therefore, in active earthquake-tsunami-prone regions, the design of structures is a multi-hazard exercise.
At a number of beaches, low seawalls were in place, presumably to protect against high perigean tides and storm waves. Most of these walls were not damaged by the tsunami, and, despite the fact that the walls were overtopped by the much higher tsunami waves, structures landward of the walls were somewhat protected. At Patong Beach, the most populous beach on Phuket Island, where a low seawall stretched across most of the beach front, the steep offshore bathymetry caused the leading crests of the tsunami to break close to shore as plunging breakers, much like large surfing waves in Hawaii, except that they were backed by a step increase in water level. When these plunging breakers hit the beach, they broke into tongues of water that jetted into the community. The low seawall deflected much of the momentum of the waves skyward, reducing the forces on landward structures.3 Although the wave flooded more than half a kilometer inland into the business district, completely flooding first floors in some areas, the loss of life was very low.
All along the business district of Patong Beach, the seawall had regularly spaced openings for pedestrian access to the beach. Damage to inland shops appeared to correlate to these openings.4 Scour of the beach berm also appeared to be related to these openings, probably because of the draining of the receding tsunami floodwaters seaward through the openings. If access had been provided by cross-over access paths over the wall, instead of openings in the wall, the constant wall elevation along the beach would presumably have reduced damage significantly.
At the north end of Patong Beach, the design of a masonry seawall created a problem. Rather than having a vertical seaward face, the wall sloped inland, creating a ramp for the tsunami run-up jet that essentially launched the water into the attic of the building the wall was supposed to protect. This surprising phenomenon reinforces the idea that seawalls should be vertical or concave seaward.
Located at the north end of Phuket Island, the Phuket International Airport has one runway, running east and west; the western end of the runway is located at the shoreline. However, because the runway was protected by a vertical concrete wall set high up on the beach face, the airport was shut down for only two hours from flooding of 100 meters of the seaward end of the runway. The wall suffered no damage and probably prevented significant damage and scour at the airport.
At Kamala Beach, south of the Hotel Benjamin, a vertical seawall fronted the playground of a school. Although the vertical wall was damaged, the playground and the school buildings were not seriously damaged. Fortunately the school was closed on the Sunday of the tsunami.
On Phi Phi Island, a seawall on the south beach built with a core of sandbags and a hard outer covering was supposed to protect shops. The wall failed in numerous spots because of scour, hydrostatic force of the waves, and, in one spot, because a house dropped on it. It is not clear how much protection this poorly constructed wall afforded.
The tsunami removed an immense amount of sand from the beaches in Thailand. Presumably, most of the sand was carried landward by the wave and deposited as a thin lens across the wave-inundation zone. (Geologists use lenses of sand in the geological record to determine the occurrences of paleo-tsunamis.) Figures 9 and 10 (see PDF version), taken from the IKONOS satellite, show the beaches of the Khao Lak region of Phang Nga province one year before the tsunami and just after the event. The wide beaches (in the low tide image, Figure 9) prior to the tsunami disappeared along with most of the vegetation between the sea and the limit of wave incursion (up to 1.5 kilometers inland). The images document the flushing of sediments from tidal creeks and streams. The incoming wave run-up and subsequent seaward draining of the floodwaters produced abnormally high stream-flow rates and sediment transport in these streams. This sand was apparently carried into the nearshore area.
The loss of the beaches, if prolonged, would have been devastating to tourism, because the beaches are a major attraction to the millions of tourists who visit southwestern Thailand. However, one month after the tsunami, we found that the beaches had recovered remarkably. In most places, the new beaches were more than 30 meters wide and showed no obvious permanent damage from the tsunami, suggesting the existence of significant offshore and nearshore sources of beach sand to replenish the destroyed beaches. Some of this sand was probably the very material that was flushed from creeks and rivers. (This is not to say that there was no permanent damage—the northwest tip of Cape Pakarang at the top of the pictures was still missing.)
Although there are few sand dunes along the Thai coast, in locations with wide, elevated, vegetated sand dunes, damage was reduced. In the Kata-Karon area of Phuket, residential and commercial development was set back behind a sand dune that had been preserved by local authorities. Although the dune was overtopped by the advancing tsunami, interviews with local inhabitants and observations of residual damage suggest that the dune dramatically reduced flow velocities. Damage in this area was limited to direct flooding and did not convey the kind of impact-related structural damage that was seen elsewhere.
As the waves came ashore and flowed across streets and through structures, they generated an immense amount of debris. Demolished wood buildings, cars, furniture, clothing, and objects of day-to-day life were picked up by the waves and carried inland. As the waves receded, the debris receded with them; subsequent waves then returned the debris to shore once again. Numerous photographs after the tsunami of downtown Patong Beach reveal automobiles in shops, piled on top of one another, and on rooftops. This flotsam was a major source of injury and death as it struck people struggling in the water or trying to hold on to fixed objects.
In flood-prone areas, it makes good sense to locate potential debris-producing objects and structures as far landward as possible. For example, parking lots should not be seaward of hotels, because floating cars are very destructive. At Patong Beach, local officials are even limiting the number of beach umbrellas allowed on the beach; nearly 7,000 umbrellas were transformed into projectiles that washed inland during the tsunami.
Rebuilding in a Tsunami Zone
Because beach tourism is the prime economic driver for southwest Thailand, a major tool of hazard reduction—prohibiting people from living in now known inundation areas—is not really a viable option. Thus tsunami-proof structures with flow-through designs, stronger buildings, and deeper scour-resistant foundations are mandatory. In addition, evacuation strategies must be part of the re-design. Although horizontal evacuation routes provide clear escape routes leading inland from inundation areas, vertical evacuation routes that ensure easy access to the upper floors of tsunami-resistant structures, may be better, given the lack of warning time. In all cases, public education about tsunamis and warning systems are critical.
Cost is one of the most difficult impediments to tsunami-proofing structures. Although the tsunami showed that Thailand is not immune to these disasters, historically, they have occurred infrequently. In addition, because this most recent earthquake relieved much of the stress in the fault, it may be more than 100 years before the next major tsunami occurs. With such a low level of risk, calculating acceptable investments for elevating structures, refitting buildings to reduce damage, and zoning people out of inundation regions may be difficult.
The wisest choices may be to revive the tourism industry by tsunami-proofing tourist resorts to reassure patrons they will be safe (or safer) and to dedicate remaining public funds to public education about tsunamis. Public knowledge about tsunamis, in this case about the association of tsunamis and earthquakes and the import of a large initial drop in water level, can play a major role in saving lives. In Thailand, astute coastal residents, visitors, and some local lifeguards recognized the hazard and evacuated the beaches just minutes before the tsunami hit.
Some of the tsunami protection solutions are available at reasonable cost. For example, the presence of a coastal sand dune at Karon Beach, just south of Patong Beach, reduced the force and velocity of upland flooding. However, today coastal dunes are nearly nonexistent along the southwestern Thai shoreline. If they existed at all, they have been removed either for construction or to provide a better view of the sea. Dunes are a simple, low-tech construction that can be implemented fairly easily. In addition, replanting mangrove swamps at appropriate locations can reduce the intensity of the waves.
Although it is impossible to guarantee safety in the event of another disaster of the same magnitude, the prudent implementation of the knowledge gained from this tsunami could dramatically reduce the loss of life the next time, whenever it occurs.
Miller, D.J. 1960. Giant Waves in Lituya Bay, Alaska. U.S. Geological Survey Professional Paper, 354-C. Washington, D.C.: U.S. Government Printing Office.
NGDC (National Geographic Data Center). 2005. Tsunami Event Database. Available online at: http://www.ngdc.noaa.gov/seg/hazard/tsevsrch_idb.shtml.
NTHMP (National Tsunami Hazard Mitigation Program). 2001. Designing for Tsunamis—Seven Principles for Planning and Designing for Tsunami Hazards.
RCDRS (Research Center for Disaster Reduction Systems). 2005. The December 26, 2004, Sumatra Earthquake Tsunami Field Survey around Phuket, Thailand. Available online at: http://www.drs.dpri.kyoto-u.ac.jp/sumatra/thailand/phuket_ survey_e.html.
Thailand Navy. 2005. Available online at: http://www.navy.mi.th/hydro/tsunami.htm.
Winchester, S. 2003. Krakatoa: The Day the World Exploded, August 27, 1883. New York: HarperCollins.
1. Other team members were Robert Lo, Yumei Wang, Curt Edwards (team leader), Robert Barnoff, Martin Johnson, and Anat Ruangrasseme.
2. The video Tsunami2004-Tsunami-Hits-Khao-Kak-by-
German-Tourist-Uncut.wmv (available online at http://www.waveofdestruction.org) shows the waves and the two patrol boats being struck by the wave. One of the ships may have been the police patrol boat, which was surfed ashore and deposited upright 1.5 kilometers inland, along with other flotsam.
3. The tourist video taken from the Novotel Coralia Phuket of the beach at Patong and the inundation of the shorefront Franco Roma Restaurant shows the waves striking the seawall (evidenced by the splash-up) and the inundation of the business district. Available online at: http://www.waveofdestruction.org/download.php?f=www. waveofde struction.org-patong_beach.wmv.
4. Team member Robert Lo first noticed this correlation.