When typhoon Mindulle flooded western Taiwan in 2004, it sent the equivalent of half of the Mississippi River’s annual sediment load into the Taiwan Strait via the Choshui River in just three days. Surprising new research shows that the incredible 70 million tons of mud then disappeared from the offshore seafloor in just a few weeks.
Oceanographers and geologists are only now beginning to understand the causes of Taiwan’s dark deluges and the secrets of its offshore mega mud-moving power
Among the lessons they are learning is that even short rivers can play major roles in moving earth and shaping the land and seafloors.
"The Mississippi has been nicknamed the ‘Big Muddy,’" said oceanographer John Milliman of the College of William and Mary in Virginia. But the average amount of sediment suspended in the Mississippi’s waters is about half a gram per liter, he said. Taiwanese rivers, on the other hand, can carry 500 grams of muck and sand per liter.
"That’s a thousand times the Mississippi River’s load," he said.
When river water carries more than 40 grams of sand and silt per liter, the flow is called "hyperpycnal," Milliman explained. But it’s more than just a fancy word for muddy water. When hyperpycnal waters reach the sea they do something other freshwater river waters do not: They sink.
This is exactly the opposite of what usually happens when freshwater flows into saltwater, which is usually denser.
Taiwan is the world’s leader in hyperpycnal flows, Milliman said. The reason is simple: Lots of landslides
and lots of torrential rain from being located in what meteorologists call Typhoon Alley.
"There are landslides everywhere," said Milliman. "Then they get heavy rains on this nicely exposed material."
To get a better handle on the effects of all this sliding soil and rain, Milliman’s Taiwanese colleagues went to work studying the effects of typhoon Mindulle. They sent out students to document the river flows and study the seafloor to watch the movements of the mud and sand
after the rains had ended.
"We knew the sediments didn’t stay for long," said Milliman.
Still, they were surprised to discover that the material had been moved away in just four or five days, he said. "This is much more dynamic." A report by Milliman and his colleagues appears in the September issue of the journal Geology.
Among the places which might benefit from a closer look at Taiwan’s hyperpycnal rivers is California, where it’s not uncommon for rivers to look more like streams for many years — until an unusually wet winter turns them into dark, muddy, destructive torrents that can drastically reshape the landscape in days.
"We’re finding that we’re just scratching the surface," said oceanographer Jonathan Warrick of the U.S. Geological Survey
, regarding the effects of hyperpycnal rivers.
They can, for instance, send large amounts of polluted sediments into the deep sea, he said. Or they can send sand beyond the reach of beaches, he said.
"One of the assumptions we make in California is that all the (river-discharged) sand remains along the shore," Warrick said. "But we actually don’t see all of it on the beach."
Instead, as is readily studied in Taiwan, the density of the hyperpycnal waters sends the sand down, out and away from shore, which is bad news for beachgoers
Via: Discovery Channel