Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

From Rivers to Landslides: Charting the Slopes of Sediment Transport

30.01.2014
In the Earth Surface Dynamics Lab at the California Institute of Technology (Caltech) the behavior of rivers is modeled through the use of artificial rivers—flumes—through which water can be pumped at varying rates over a variety of carefully graded sediments while drag force and acceleration are measured.

The largest flume is a 12-meter tilting version that can model many river conditions; another flume models the languid process of a nearly flat river bed forming a delta as it reaches a pool. Additional flumes are constructed in the lab on an as-needed basis, as in a recent study testing sediment transport in very steep channels.


Artificial river in the Earth Surface Dynamics Lab at Caltech built to model the behavior of steep channels.Credit: Jeff Prancevic

One such newly constructed flume demonstrates that the slope of streambeds has dramatic and unexpected effects on sediment transport. Logic would suggest that steeper streambeds should allow for easy sediment transport since, as the angle of the slope increases, gravity should assist with moving water and sediment downstream.

But experimental data from the flume lab show that gravity does not facilitate sediment transport in the expected manner. Furthermore, in very steep streambeds with a 22-degree or higher slope, sediment motion begins not with grains skipping and bouncing along the bottom of the streambed, but rather with a complete bed failure in which all the sediment is abruptly sent hurtling downstream as a debris flow.

"Most previous work was done on low-gradient channels with a gentle slope," says Michael P. Lamb, assistant professor of geology at Caltech. "These are the rivers, like the Mississippi, where people live and pilot boats, and where we worry about flooding. Low-gradient channels have been studied by civil engineers for hundreds of years." Much less attention has been paid to steeper mountain channels, in part because they are more difficult to study. "Counterintuitively, in steep channels sediment rarely moves, and when it does it is extremely dangerous to measure since it typically includes boulders and large cobbles," explains Lamb.

And so Lamb, along with Caltech graduate student Jeff Prancevic and staff scientist Brian Fuller, set out to model the behavior of steep channels on an artificial watercourse—a flume—that they created for just this purpose.

They intentionally removed key variables that occur in nature, such as unevenness in grain size and in the streambed itself (in steep channels there are often varying slopes with waterfalls and pools), so that they could concentrate solely on the effect of bed slope on sediment transport.

They created a uniform layer of gravel on the bed of the flume and then began running water down it in increasing quantities, measuring how much water was required to initiate sediment motion. Gradually they tilted the flume to steeper angles, continuing to observe when and how sediment moved as water was added to the system.

Based on studies of sediment motion in low-gradient channels, geologists have long assumed that there is a linear relation between a watercourse's slope and the stress placed by water and gravity on the streambed. That is, as the angle of the streambed increases, the quantity of water required to move sediment should decrease in a simple 1-to-1 ratio. Lamb and Prancevic's flume experiments did indeed show that steeper slopes require less water to move sediment than flatter streambeds. But contrary to earlier predictions, one cannot simply raise the slope by, say, 2 percent while decreasing the water depth by 2 percent and see the same pattern of sediment transport. Instead, as the flume tilted upward in these experiments, a proportionately greater amount of water was needed to initiate sediment motion. By the time the flume was tilted to a slope of 20 degrees, five times the depth of water as previously predicted was needed to move the gravel downstream.

At one level, this experimental data squares with field observations. "If you go out to the Mississippi," says Lamb, "sand is moving almost all the time along the bed of the river. But in mountain channels, the sediment that makes up the bed of the river very rarely moves except during extreme flood events. This sediment is inherently more stable, which is the opposite of what you might expect." The explanation for why this is the case seems to lie with the uneven terrain and shallow waters common to streams in steep mountain terrain.

Experiments with the tilting flume also allowed Lamb and Prancevic to simulate important transitions in sediment transport: from no motion at all, to normal fluvial conditions in which sediment rolls along the streambed, to bed failure, in which the entire sediment bed gives way in a debris flow, stripping the channel down to bedrock. The researchers found that with lower slopes, as the water discharge was increased, individual grains of sediment began to break free and tumble along the flume bed; this pattern is common to the sediment-movement processes of low-gradient riverbeds. As the slope increased, the sediment became more stable, requiring proportionately more water to begin sediment transport. Eventually, the slope reached a transition zone where regular river processes were completely absent. In these steeply sloped flumes, the first sediment motion that occurred represented a complete bed failure, in which all of the grains slid down the channel en masse. "This suggests that there's a certain slope, around 22 degrees in our experiments, where sediment is the most stable, but these channel slopes are also potentially the most dangerous because here the sediment bed can fail catastrophically in rare, large-magnitude flood events," Lamb explains.

Researchers previously believed that debris flows in mountain terrain primarily derived from rainfall-triggered landslides flowing into watercourses from surrounding hillsides. However, the flume-lab experiments suggest that a debris flow can occur in a steep river channel in the absence of such a landslide, simply as a result of increased water discharge over the streambed.

"Understanding when and how sediment first moves at different channel slopes can be used to predict the occurrence of debris flows which affect people and infrastructure," Lamb says. There are other, wide-ranging implications. For example, some fish, like salmon, build their nests only in gravel of a certain size, he notes, and so, "as rivers are increasingly being restored for fish habitat, it is important to know what slopes and flow depths will preserve a particular size of gravel on the riverbed." In addition, he adds, "a better understanding of sediment transport can be used to reconstruct environments of Earth's past or on other planets, such as Mars, through observations of previously moved sediment, now preserved in deposits."

The paper, "Incipient sediment motion across the river to debris-flow transition," appears in the journal Geology. Funding was provided by the National Science Foundation, the Terrestrial Hazard Observation and Reporting Center at Caltech, and the Keck Institute for Space Studies.

Written by Cynthia Eller

Contact:
Deborah Williams-Hedges
(626) 395-3227
debwms@caltech.edu

Cynthia Eller | EurekAlert!
Further information:
http://www.caltech.edu
http://www.caltech.edu/content/rivers-landslides-charting-slopes-sediment-transport

More articles from Earth Sciences:

nachricht GPM sees deadly tornadic storms moving through US Southeast
01.12.2016 | NASA/Goddard Space Flight Center

nachricht Cyclic change within magma reservoirs significantly affects the explosivity of volcanic eruptions
30.11.2016 | Johannes Gutenberg-Universität Mainz

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Novel silicon etching technique crafts 3-D gradient refractive index micro-optics

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...

Im Focus: Quantum Particles Form Droplets

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.

“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...

Im Focus: MADMAX: Max Planck Institute for Physics takes up axion research

The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.

The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...

Im Focus: Molecules change shape when wet

Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water

In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...

Im Focus: Fraunhofer ISE Develops Highly Compact, High Frequency DC/DC Converter for Aviation

The efficiency of power electronic systems is not solely dependent on electrical efficiency but also on weight, for example, in mobile systems. When the weight of relevant components and devices in airplanes, for instance, is reduced, fuel savings can be achieved and correspondingly greenhouse gas emissions decreased. New materials and components based on gallium nitride (GaN) can help to reduce weight and increase the efficiency. With these new materials, power electronic switches can be operated at higher switching frequency, resulting in higher power density and lower material costs.

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE together with partners have investigated how these materials can be used to make power...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ICTM Conference 2017: Production technology for turbomachine manufacturing of the future

16.11.2016 | Event News

Innovation Day Laser Technology – Laser Additive Manufacturing

01.11.2016 | Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

 
Latest News

UTSA study describes new minimally invasive device to treat cancer and other illnesses

02.12.2016 | Medical Engineering

Plasma-zapping process could yield trans fat-free soybean oil product

02.12.2016 | Agricultural and Forestry Science

What do Netflix, Google and planetary systems have in common?

02.12.2016 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>