New mechanism of erosion revealed
Local surface uplift can block rivers, particularly in mountainous regions. The impounded water, however, always finds its way downstream, often cutting a narrow gorge into the rocks.
Geländearbeit in der Schlucht des Da’an Chi in Taiwan. Die ein Kilometer lange und bis zu 20 Meter tiefe Schlucht hat sich in weniger als 10 Jahren nach dem Jiji-Erdbeben im Jahr 1999 gebildet. Der Felssporn in der Bildmitte wurde während eines Hochwassers 2012 im Zeitraum von einer Stunde weggespült. (Foto: Kristen Cook, GFZ)
Subsequent erosion of the rocks can lead to a complete eradication of this initial incision, until not a trace is left of the original breakthrough.
In extreme cases the whole gorge disappears, leaving behind a broad valley with a flat floodplain. Previously, the assumption was that this transition from a narrow gorge to a wide valley was driven by gorge widening and the erosion of the walls of the gorges.
A team of scientists from the GFZ German Research Centre for Geosciences in Potsdam has now revealed a new mechanism that drives this process of fluvial erosion (Nature Geoscience, 17.08.2014). The geoscientists analyzed the development of a gorge on the Da'an Chi river in Taiwan over a period of almost ten years.
There, uplift that was caused by the Jiji earthquake of 1999 (magnitude 7.6), and that runs transverse to the river, had formed a blockage. Earthquakes of that size occur there every 300 to 500 years. "Before the quake there was no sign of a gorge at all in this riverbed, which is one and a half kilometers wide", explains Kristen Cook of the GFZ.
"We have here the world's first real-time observation of the evolution of gorge width by fluvial erosion over the course of several years." Currently the gorge is roughly a kilometer long, 25 meters wide and up to 17 meters deep. Initially, the gorge walls were eroded at a rate of five meters per year, and today are still retreating one and a half meters per year.
The scientists identified a hitherto unknown mechanism by which the gorge is destroyed. "Downstream sweep erosion" they termed this process. "A wide braided channel upstream of the gorge is necessary," explains co-author Jens Turowski (GFZ).
"The course of this channel changes regularly and it has to flow in sharp bends to run into the gorge. In these bends, the bed-load material that is transported by the river hits the upper edge of the gorge causing rapid erosion."
This mechanism gradually washes away all of the bedrock surrounding the gorge and, therefore, is the cause for the planation of the riverbed over the complete width of the valley. Assuming the current erosion rate of 17 meters per year, it will take here at the Da'an Chi River only 50 to 100 years until again a flat beveled channel again fills the valley.
In contrast, lateral erosion in the gorge would be too slow to eradicate the gorge in the time of one earthquake cycle. The newly discovered downstream sweep erosion is far more effective.
Kristen L. Cook, Jens M. Turowski and Niels Hovius: "River gorge eradication by downstream sweep erosion", Nature Geoscience, Advance Online Publication, 17.08.2014, DOI: 10.1038/ngeo2224
F. Ossing | Eurek Alert!
Explosions of Jupiter’s aurora linked to extraordinary planet-moon interaction
25.03.2015 | American Geophysical Union
Rethinking wetland restoration: Smaller wetlands more valuable than previously thought
25.03.2015 | University of Waterloo
In an experiment at the Department of Energy's SLAC National Accelerator Laboratory, scientists precisely measured the temperature and structure of aluminum as...
The IPH presents a solution at HANNOVER MESSE 2015 to make ship traffic more reliable while decreasing the maintenance costs at the same time. In cooperation with project partners, the research institute from Hannover, Germany, has developed a sensor system which continuously monitors the condition of the marine gearbox, thus preventing breakdowns. Special feature: the monitoring system works wirelessly and energy-autonomously. The required electrical power is generated where it is needed – directly at the sensor.
As well as cars need to be certified regularly (in Germany by the TÜV – Technical Inspection Association), ships need to be inspected – if the powertrain stops...
When an earthquake hits, the faster first responders can get to an impacted area, the more likely infrastructure--and lives--can be saved.
The Atlantic overturning is one of Earth’s most important heat transport systems, pumping warm water northwards and cold water southwards. Also known as the Gulf Stream system, it is responsible for the mild climate in northwestern Europe.
Scientists now found evidence for a slowdown of the overturning – multiple lines of observation suggest that in recent decades, the current system has been...
Because they are regularly subjected to heavy vehicle traffic, emissions, moisture and salt, above- and underground parking garages, as well as bridges, frequently experience large areas of corrosion. Most inspection systems to date have only been capable of inspecting smaller surface areas.
From April 13 to April 17 at the Hannover Messe (hall 2, exhibit booth C16), engineers from the Fraunhofer Institute for Nondestructive Testing IZFP will be...
25.03.2015 | Event News
19.03.2015 | Event News
17.03.2015 | Event News
27.03.2015 | Agricultural and Forestry Science
27.03.2015 | Materials Sciences
27.03.2015 | Ecology, The Environment and Conservation