Washington: A group of scientists working in the Kuril Islands off the east coast of Russia has documented for the first time the scope of tsunami-caused erosion and found that a wave can carry away far more sand and dirt than it deposits.
The fortuitous observations resulted because the Kuril Biocomplexity Project had made detailed surveys of some Kuril Island coastlines during the summer of 2006, and then returned for additional work in the summers of 2007 and 2008.
That provided a unique opportunity for before-and-after comparisons following a magnitude 8.3 earthquake and accompanying tsunami on November 15, 2006, and an 8.1 quake and resulting tsunami on January 13, 2007.
When the scientists revisited coastlines they had surveyed in 2006, they found that in some places, the amount of sand and soil removed by tsunami erosion was nearly 50 times greater than the amount deposited.
“It was so extreme. I was really surprised,” said Breanyn MacInnes, a University of Washington doctoral student in Earth and space sciences.
The team observed shorelines stripped of vegetation, small hills of soil and volcanic cinders washed away to expose boulders and, in one place, the unearthed rusty remnants of military equipment left behind at the end of World War II.
“We were there the year before and it had been completely covered with vegetation, and there were ridges closer to shore that had been completely removed when we returned,” MacInnes said.
The November 15, 2006, Kurils earthquake was large enough to raise alarms about the potential for a tsunami throughout the Pacific basin.
The Kurils were hit by tsunami waves more than 70 feet high in some places, and changes in topography were dramatic.
The amount of erosion from a tsunami depends somewhat on the topography of the land, but definitely is related to the force of the wave, the scientists found.
They noted that an area called South Bay on Matua Island lost about 50 cubic meters, or about 1,765 cubic feet, of sediment per meter of width, while an area called Ainu Bay lost an astounding 200 cubic meters, or about 7,060 cubic feet, of sediment per meter of width because of tsunami-induced erosion.
At a spot called Dushnaya Bay, where the tsunami was at a relatively low elevation at its greatest distance from shore, the biggest change occurred on the sandy beach, with about 5 cubic meters, or about 177 cubic feet, of sediment eroded per meter of width.
According to the scientists, some of the landscape scars will remain visible for decades, or even centuries.