Field investigation of tsunami impact on coral reefs and coastal sandy slopes

The objective of this study was to investigate the effects of tsunamis on coral reefs and sediment beds in the near shore region. A remotely operated vehicle (ROV) was used to survey the coral reef and collect sediment samples from the coastal regions surrounding American Samoa five weeks after the tsunami generated by a magnitude 8.3 earthquake on September 29th, 2009. The surveys recorded extensive damage to the coral reefs, as evident by the broken corals of almost every type in the region and by debris from onshore, along the coastal regions of Poloa (14° 19′ 11″ S, 170° 50′ 11″ W) and Leone (14° 20′ 30″ S, 170° 47′ 1″ W). The field observations suggest that sand was carried from the beach out to sea by the tsunami drawdown causing net beach erosion, which is consistent with findings from model-scale studies presented in (Young et al., 2010). The field observations also suggest that the high sheer stresses created by the rapidly retreating seaward flow, and the sudden release of the sediments and debris entrained in the water during the drawdown process, are responsible for majority of the damage to the coral reef.