Friday, July 4, 2014

Massive Kaua`i landslide--neither unique nor even rare

Volcanic eruptions are impressive, but massive erosion events are right up there on the breathtaking scale.

When a chunk of the side of Wai`ale`ale/Kawaikini sloughed off this week, it sent a catastrophic debris flow down a tributary of the Wailua River. 

A flash flood of water, rocks, dirt and logs scoured the river bed and its banks, destroying all vegetation and any wildlife in the area.

(Image: A screenshot of the Wailua streambed after the debris flow. The image is taken from a Kaua`i Fire Department video taken from a helicopter.)

The event turned the Wailua River a yellowish milk chocolate color, as the mud mixed with the flow from myriad streams feeding Wailua.

This isn’t new. It happens fairly frequently, though mostly in smaller events. You frequently see vertical brown streaks against the green steep cliffs of the older islands, indicating similar events.

The Wailua event appears considerably smaller than a massive slide at the back of Olokele Canyon around 1980, when an immense chunk of material plunged down a towering basalt wall, hit the bottom, and surged downstream. An image of that event is visible on the first page of this site from the Association of American State Geologists

In that case, the Makaweli River flowed milk chocolate in color for months. A most interesting view was where Makaweli and Waimea joined, and you could see swirling and mixing of the the tea-tinted Waimea water mix with the café-au-lait of the Makaweli River.

This landslide process is sometimes called mass wasting, the downward movement of soil and rock under the influence of gravity, often lubricated by water. It is an entirely natural process that has been going on for millennia.

Indeed, just as periodic volcanic eruptions are responsible for building the Islands, periodic landslide events are significant features in unbuilding them.

“Volcanoes are particularly prone to massive rock slope failure and can experience very large scale sector collapse or much smaller partial collapse,” says a 2006 paper, “Landslides from massive rock sloped failure and associated phenomena.” 

What is often far more dangerous—sometimes catastrophic—is the water, mud, rock and debris flow and lunges downstream as part of the collapse. It can rip out everything in its path, scouring the valley floor to bare mud and rock.

Such events also have significant impacts on nearshore waters, dumping tons of sediment into the coastal ocean, often temporarily blanketing reefs.

© Jan TenBruggencate 2014

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