A big flood disaster struck the Hue lagoon area of the Middle Vietnam in the November 1999. More than 700 people were killed by very high flood both in the riverside and littoral lowlands, and killed by many debris flows in the mountain area. In the lagoon area the water level reached to 4 m high from the sea level, which caused washout of the sandbar or the barrier beach between the lagoons and the sea. Consequently some former inlets of the lagoon opened again and rapid coastal erosion has occurred in each collapsed places.
The purpose of this study is to show the factors of the washout of the sandbar or the barrier beach, and to know the mechanism of the rapid coastal erosion after the flood. The authors collected and compared three kinds of topographic map issued in 1950, 1965-68 and 1990s, and three kinds of remote sensing images made by Landsat 7 ETM data taken before and after the flood.
The trigger of the flood was the heavy rainfall more than 2000 mm in a week, but the background of the washout is the bad drainage through the narrow inlets at the flooding time, and the continuous coastal erosion from 1960s in the whole area of this coastal zone. And the main cause of the rapid coastal erosion after the flood is the occurrence of some depression of 5 m or more in depth, 400-1000 m in length, 100-250 m in width at the offing of the washouts. The beach sand moved into the depression from the northwest to the southeast by the winter monsoon from the north or northeast. So northwestern beach were eroded so rapidly in the last 4 years just after the flood.