James Bond had his Q. The Jackson School has Joel Johnson. No, he can’t supply you with a helicopter in a suitcase or rocket launching cigarettes. He does however have one of the coolest gadgets in the field of sediment transport: smart rocks.
These are cyborg-like rocks that can sense accelerations in all three axes and record them on a memory chip. These accelerations can be used to work out when the rock was moving, how and where it moved, how often it hit the riverbed, and with how much force. Johnson builds them out of real rocks ranging in size from tennis balls to cantaloupes. He slices them open, removes a section from the middle, and glues in a metal compartment. Inside, he mounts a little off-the-shelf black box of electronics that includes a three axis accelerometer and then he screws the whole thing shut.
When non-experts hear the term sediment, they may think of particles the size of sand grains. But to a geologist, these gravel to cobble-sized smart rocks are somewhere in the middle of a broad size scale that ranges from fine clay particles (measured in microns) to large boulders (measured in meters).
The idea of smart rocks has been talked about in the geologic community for at least 20 years, but only recently have the electronic components become compact, affordable, and reliable enough to be practical for field and experimental work. A few years ago, other researchers placed an expensive instrumented boulder prototype in a ravine in the Alps to record the conditions inside a landslide. When a slide finally came, it swept up the rock never to be seen again. It, and the data it collected, lie buried to this day, tantalizingly out of reach.
Johnson hopes to avoid that fate with most of his rocks. He worked with a company to build customized sensors that can collect data for months rather than minutes like his earlier models. He and a student plan to place the upgraded smart rocks in a mountain river in Idaho in early 2011 and wait for a flood. Radio antennas positioned at regular intervals along the shore will help keep track of the rocks via embedded radio frequency identification (RFID) tags. If that system fails, the fallback plan is to wade into the water with a metal detector and locate them based on their metal content. Once they fish the rocks out, they’ll open them up and download the accelerometer data.
These data will tell them how high the water flow rate had to be to start moving the rocks and how flow rate relates to the forces rocks feel during transport. By putting out rocks of different sizes, they’ll also study how sediment size affects transport. Their ultimate goal is to improve predictions of sediment transport rates (how much mass moves past a point in a given time) and how those rates depend on local water flow rate and river morphology.
“I’m excited about the potential to measure sediment transport from the view of rocks themselves,” he says. “It’s basically a killer app for people who study mountain river geomorphology.”
Oversimplified
Scientists like Johnson who study why landscapes look the way they do, called geomorphologists, often try to distill the processes that create natural landforms?mountain ranges, meandering rivers, deep canyons on Earth and Mars?down into simple but predictive models. Sometimes, says Johnson, they take it just a little too far.
“We all take these complex systems and we say let’s cut out this part and let’s cut out that part, let’s reduce the complexity to something we can understand,” he says. “But you have to be careful not to take out the parts that actually make it interesting, and that subtly control system behavior.”
Johnson’s approach to complex systems was strongly influenced by David Mohrig, a member of his dissertation committee at MIT.
“You want to embrace the complexity in these systems,” says Johnson, “and really try to understand it rather than thinking about it as something that just makes the problem too hard, that you can assume away.”
When Johnson was searching for an academic home while completing a postdoctoral fellowship at the U.S. Geological Survey, Mohrig, who had since moved to The University of Texas at Austin, again influenced his thinking. Johnson, who has now completed his first year as assistant professor in the Jackson School, liked the idea of coming back to Texas (he went to junior high school in Lubbock).
He also liked the resources available to him, the kinds only a handful of institutions can provide. For example, he’s remodeling an existing 40 meter flume — a long narrow tank that recreates river flow under controlled conditions — for his own research. And now he’s part of a small community of Jackson School researchers developing unique experimental flumes to study surface processes.
“If a place doesn’t already have a flume lab, nobody wants to take the space and do the things necessary to build a lab, not for a new junior faculty member,” he says. “So I was and still am excited and very appreciative that I was able to show up here and have a hydraulics lab ready and waiting in a sense.”
Sweet Spot
Johnson has also started a National Science Foundation funded research project on the big island of Hawaii, to better understand how climate controls landscape evolution.
The Kohala Peninsula, which juts out from the northwest corner of the island, is in some ways the perfect real world experiment. One side is dry, receiving just 25 centimeters (10 inches) of rain each year. The other is wet, getting as much as 4 meters (13 feet) a year. Meanwhile, the dark basaltic bedrock is the same on both sides. Johnson and a colleague are looking to see how the differences in rainfall affect the depths that river channels have carved down into the rock.
At first, you might assume that the greater the local rainfall rate, the greater the river flow rate, and the greater the river downcutting rate. Flow rate and erosion should go in lockstep, right? But Johnson’s preliminary results suggest more complexity. In addition to river discharge, rainfall influences how deeply weathered and physically weakened the bedrock becomes, and also how much sediment washes into river channels. That’s because sediment is created as a result of rock weathering and hill slope erosion.
A key goal of the project is to determine which is the most dominant controller of erosion: the sedimentation rate, the bedrock weathering rate, or the actual river flow rate.
If there is too little sediment in a river, the water isn’t very effective at scouring away the rocky bottom of the channel. At the other end of the spectrum, if there’s too much sediment, the bottom is in a sense insulated from erosion.
“There might be a sweet spot where you get the right amount of sediment coming into your channels and causing more erosion,” says Johnson. “In what sounds like a simple system, indeed in all of these geomorphic systems, once you look deeper, there are a lot of feedbacks that cause a lot of complexity.”
By Marc Airhart
For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.
