Tuesday, February 14, 2006
I know that it is old news, but I figured it was worthy of mention here. The December tsunami of 2004 wreaked much havoc over land. However, it left much of the local reefs untouched. A multinational team of scientists did surveys, recording both fish sizes and invertebrates, including a method to determine new coral settlement, a measure of recovery. This survey concluded that relatively little damage was sustained by the coral reefs, and even in areas where there was coral damage, there was still considerable amounts of live coral. Damage included overturned corals and broken coral where land objects like trees where dragged along the corals with the receding water. The damage from the quake that caused by the tsunami, however, was much more severe. The damage here ranged from lifted reefs, shattered beds and overturned coral. Some of this was caused because the quake tilted some islands, where one end went up 2 meters and the other end sank 2 meters. One thing the survey did notice was high turbidity levels from runoff, something caused by stripped land from the tsunami and large rainfall events. More coral damage could come as a result of this turbidity and the land damage, but this remains to be seen. Low abundances of food fish also indicate overfishing. The combination of this sedimentation, turbidity and overfishing can be more devastating to the reef than the actual tsunami. Check out reefbase for more about this study.
This makes sense that the damage to reefs was minimal from the tsunami. Tsunamis are very fast but when they start they have a small wave height. So while there may have been a surge over the reef, it wasn't a devastating wave like the one that hits land. As a wave approaches land, the slope creates drag against the water and slows the fast wave, especially at thee bottom. The surface water is moving slightly faster and builds up on top of the slower moving water, creating the breaking wave. While this is how all breaking waves are created, this is greatly modified in tsunamis. The energy of thee wave remains constant, so as the high speed of a tsunami wave is slowed dramatically by the slope of the shore, the wave height must increase dramatically, creating large breaking waves that sweep over land when the amplitude over open ocean may only be one or two meters. So, it makes perfect sense that the damage from the actual tsunami was minimal. What you never think about it the tsunami's devastating impact on land can create greater problems for the local reefs. This is something that should be monitored, how local ecosystems react to natural disasters. Very interesting stuff...