Even without sophisticated ocean monitors and computer-generated tsunami forecasts to help warn of approaching waves, some Indian Ocean communities might have experienced less devastation during the December 2004 tsunami had they heeded the tragic lessons of the past.
Mounting evidence suggests that humans make themselves particularly vulnerable to monster surges by destroying protective natural barriers-coral reefs, seaside forests and sand dunes-and by building homes in low-lying areas near the coast. Although most such evidence is still anecdotal, many scientists are encouraging officials in tsunami-prone areas to consider such factors when designing new building codes and evacuation routes.
The protective nature of coral reefs came to light during a major tsunami that hit Nicaragua in 1992. Scientific surveys following the event found indications that the waves had funneled through a shipping channel excavated in the coral reef near the town of El Transito. As a result, the tsunami inundated the coastline landward of the channel most severely and flattened the town. In contrast, even beach umbrellas survived the milder surge along an adjacent beach where the reef was still intact.
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In 2004 a similar disaster struck Sri Lanka. An estimated 1,500 people died when the Indian Ocean tsunami derailed and overturned a passenger train along the island's southwest coast near Sinigame. Dozens of local residents later told an international tsunami survey team that the reef offshore of the accident had suffered major losses due to commercial coral mining; detailed scientific surveys confirmed that flooding was much less severe in other areas where the reef was undamaged.
Another research team investigating the 2004 disaster underscored the importance of coastal forests-particularly mangroves-in southeast India. Along the 21-kilometer stretch of coastline they examined, villages sheltered behind stands of trees incurred much less damage than those exposed directly to the ocean. Similarly, satellite photos of western Sri Lanka reveal that the tsunami penetrated farther inland where a break or clearing in the forest occurred. Previous studies indicate that both reefs and forests may attenuate wave energy by as much as 90 percent.
Coastal sand dunes also played a critical role during the 2004 tsunami. Near Yala National Park in Sri Lanka a hotelier told scientists that a dune seaward of the hotel had been removed to give guests a better view of the ocean. Not only did the tsunami level the building, but the resort grounds also experienced substantially higher flooding than neighboring areas located behind unaltered dunes. Nearly 12 years earlier, in 1993, investigators had noted the protection sand dunes offer when a tsunami pounded the south side of Okushiri, Japan. There, a combination of man-made dunes and concrete barriers sheltered landward regions while channeling much of the wave energy elsewhere.
A related problem is communities that build homes in low-lying coastal areas without developing a reliable tsunami evacuation plan. Headlands are particularly prone to severe flooding because the tsunami can attack from two sides, as observed during relatively small tsunamis that struck East Java in 1994 and both Biak (Indonesia) and Peru in 1996. Computer models of tsunami inundation predict the same for some stretches of the California coast of the U.S. Yet many the residents of the headland city of Banda Aceh, Indonesia, did not have this information and were totally unprepared for the tsunami of 2004.
More detailed studies and computer modeling of potential flooding along tsunami-prone coastlines will help quantify the degree to which natural coastal features will reduce or enhance the fury of a tsunami. Still, experts say that policy makers who take current findings into consideration when developing coastal areas and when preparing for disasters will clearly help save lives and property. - The Editors
