Anatomy of a disaster
On May 2, 2008, Extremely Severe Cyclonic Storm Nargis made landfall in Myanmar. It was a Category 4-equivalent storm with sustained winds reaching 217 km/h (135 mph). The cyclone's path was unusual; most storms in the Bay of Bengal strike further north along the coasts of Bangladesh or western Myanmar. Nargis, however, tracked eastward and struck the low-lying Ayeyarwady Delta, a region unprepared for a storm of this magnitude.
The primary destructive force was not the wind, but the immense storm surge it created. A wall of water, estimated in some areas to be between 3.5 and 7 meters high, was pushed ashore. This surge penetrated as far as 50 kilometers inland, inundating the densely populated delta. The official death toll was 84,537, with 53,836 people listed as missing, though the final number of fatalities is likely much higher. The storm affected approximately 2.4 million people and caused an estimated US$10 billion in damage, making it the costliest cyclone in the North Indian Ocean at the time.
Modeling the megaphone
In the aftermath, scientists worked to understand how the surge became so destructive. The answer lay in the unique geography of the Ayeyarwady Delta. The delta itself is extremely flat, with vast areas sitting just 3 meters above sea level. Offshore, the continental shelf is very wide and shallow; the water depth is less than 5.5 meters for up to 28 kilometers from the coast.
Post-disaster computer modeling, using hydrodynamic systems like Delft3D and ADCIRC, revealed a catastrophic combination of factors. These models solve shallow-water equations to simulate the interaction of tides, winds, and complex coastlines. The simulations for Nargis showed that the wide, shallow shelf allowed the cyclone's powerful winds to pile up a massive volume of water. As this surge approached the coast, the numerous wide, bell-shaped river mouths of the delta were a series of funnels. This geometry channeled the water, amplifying its height and force as it was pushed deep inland through the river system. The geometry concentrated a megaphone, concentrating the energy of the wave over a vast inland area. The slow movement of the cyclone further exacerbated the flooding, allowing more time for the surge to propagate. These models, combined with post-storm field surveys measuring high-water marks, confirmed the deadly efficiency of the delta's shape in magnifying the disaster.