Issue: November 2009
Playing in the Sandbox Yields Clues about River Restoration
by Evan Galloway
Civilization began on the banks of rivers. Ever since, humans and cities have gravitated towards rivers — with good reason. Rivers provide freshwater, fertile plains, transportation routes, and even power via dams or waterwheels.
|
|
Photos.com
|
|
Civilization began on the banks of meandering rivers. Ever since, humans and cities have gravitated towards rivers. ABOVE: Houseboats cluster on a riverbank in China.
|
While humans have benefited enormously from rivers, the relationship has not always been mutual — rivers and the ecosystems
they support have suffered and, in some cases, disappeared because of human overuse. Realizing this, many conservationists are now trying to restore rivers to more natural conditions. For instance, rivers naturally meander back and forth, so people have tried digging new snake-like paths for them. However, such efforts often fail, with the rivers becoming fractured or straight again.
To find better methods for restoring the natural course of rivers, researchers at the University of California, Berkeley, have built a small river model and investigated the elements necessary for meandering. The research, led by Christian A. Braudrick and William E. Dietrich and published in the Proceedings of the National Academy of Sciences, indicates that stronger banks, such as those bolstered by plant life, are essential to the formation of meandering rivers.
Rivers are never completely straight; even ones straightened by humans have some curvature. This small curvature is what eventually becomes a dramatic bend in natural river systems. The reason is that the water on the outside of the curve moves faster than the water on the inside of the curve.
|
|
Photos.com
|
|
Rivers are never completely straight; even ones straightened by humans have some curvature. This small curvature is what eventually becomes a dramatic bend in natural river systems. ABOVE: An aerial view of the natural bends of a river in Siberia, Russia.
|
This should make sense if you have ever tried negotiating a curve on your bicycle. Taking the corner wide allows you to maintain a higher speed than if you try to stay on the inside corner. The same is true for water.
This difference in speed does two things. One is that the riverbank on the outside of the curve is eroded more quickly. The other is that the water on the inside of the curve is more likely to deposit sediment on the riverbank. This is because slower-moving water has less energy to carry the sediment around the corner. The net effect is that the riverbank on the inside of the curve creeps into the river's path, while the riverbank on the outside of the curve slowly disappears, producing a more pronounced curve.
This seems pretty simple and automatic, making it even more mystifying why many river restoration projects fail. Braudrick, Dietrich, and colleagues decided an ambitious project was needed to answer this question. They decided to build a physical model of a river system running at a pace that would simulate seven years of activity in only a few days.
The model consisted of sand (made up of grains of varying sizes) on a rectangular platform 6 meters wide by 17 meters long (roughly 20 feet by 55 feet). An initial river was carved in the sand, which was fed by a controlled flow of water; the water flow could be a trickle or a flood. The researchers also planted a bunch of alfalfa sprouts to simulate plant growth along the banks. Finally, they used a camera to document the progress of the river at five-minute intervals for the course of the experiment.
As hoped, the river system meandered impressively (a bit like slacking studiously). When they examined the development of the snaking river, they noticed a simple fact: the erosion of the outer bank and the deposition of sediment on the inner bank had to be in balance. Otherwise, meandering breaks down and the river branches or straightens instead.
Two things in their model were important for maintaining this balance. One was the presence of vegetation on the riverbanks. This strengthened the banks and prevented the fast-moving water on the outside of the curve from eroding the outside bank too quickly.
The second important element is an abundance of fine sediment or sand. Lighter sediment is more likely to be carried and deposited all along the river, helping the inner banks keep pace with erosion on the outer banks.
The most obvious conclusion of this study is that vegetation is an important part of any river restoration project. By planting trees, bushes, and other plants along the banks of rivers, the erosion can be calibrated to produce the desired bends in the river. Introducing sufficient amounts of fine sediment to a system is probably more difficult, but could also be considered in some instances.
|
|
Zina Deretsky/National Science Foundation
|
|
|
Less obvious is the meaning of this study for more unusual rivers or channels — extraterrestrial ones, for instance. The authors note that studies such as this one may help "to understand the conditions necessary to support meandering channels observed on Mars and Titan." [See Mars: Wet and Wild, December 2006; Mars Lives! Water Sculpts Planet's Surface, August 2000; Triumph on Titan, January 2005; Saturn's Moon Gives Sneak Preview, November 2004]. For instance, the understanding that strong banks are necessary will encourage scientists to examine the area around the channels for evidence of some stabilizing influence. In this way, research that will help restore our planet will also help us appreciate and understand the wonders of other planets. Research often meanders like that.
Christian Braudrick currently is a doctoral candidate in geomorphology in the Department of Earth and Planetary Science at the University of California, Berkeley. He earned his undergraduate degree from the University of California, Santa Cruz in 1993 and his M.S. in geology from Oregon State University in 1997. From 1999 to 2004, Braudrick worked at Stillwater Sciences as a river restoration project manager. In 2002, he was a summer instructor at UC Berkeley and, since 2005, he has been working at Berkeley with William E. Dietrich on fluvial geomorphology.
Braudrick's research focuses on "the controls on channel form and the effects of sediment supply on bar morphology." His goal is to fill in "the gaps in scientific knowledge that can be used to address applied problems in stream restoration and the interaction between physical and ecological processes using physical modeling and field studies."
Below are Braudrick's November 9, 2009 responses to questions posed to him by Today's Science.
|
|
Courtesy of Maya Hayden
|
|
"A key part of being a good scientist is a curiosity about how the world works, and the initiative to try and understand it."
|
Q. When did you realize you wanted to become a scientist?
A. I didn't realize that I wanted to become a scientist until I took my first geology class on a lark in college. I had always thought I would study history before that. I was a poor student in high school, and didn't realize I had the aptitude for the math, physics, and chemistry required to be a good scientist. After working for a year after high school, I went to a junior college, and realized that if I actually worked I would have an aptitude for the things necessary to be a good scientist.
Q. How did you choose your field?
A. I chose geology by accident, but it suited me because I loved the outdoors and loved problem solving. I spent a lot of time hiking, camping, and backpacking when I was a kid, and learning about nature was part of what we did.
I think geology is relatively unique in that relatively early on in geology you are asked to come up with hypotheses about how particular places work and their history. Looking back, this freedom and encouragement for thinking rather than rote learning appealed to me. As I went on in geology, I was drawn to geomorphology (the study of the Earth's surface) because it was tangible; I liked that it was a field you could see.
Q. When you tell people you study rivers, what is their reaction? What do you think is the biggest misconception about your profession?
A. People always get excited when you say you study rivers. I think this is because it's something everyone can relate to. All of us have been on a river at some point, and many of the processes that are important in determining how rivers function are easily observable. This is particularly true in California, where water issues are so important, and most everyone has thought at least a little about water resource issues.
The biggest misconception about geologists is that geologists only study things like oil and mineral deposits. While there is a strong history of geology in these fields, the earth sciences are changing their focus towards environmental issues, particularly those associated with climate change and land use. In my career as a geologist I've worked on issues ranging from assessing how to improve fish habitat in rivers to the effects of dam removal on how rivers function. I really have enjoyed working on problems that will make a difference and hopefully slow the effects man is having on the world around us.
Q. Is it possible to make riverbanks too resistant to erosion by overplanting or using especially vigorous species?
A. Yes, too much vegetation can prevent the channel from migrating. In this case, the channel freezes in place, and migration ceases. In our experiments, had we started with too much vegetation, the channel would have remained straight. In nature, vegetation encroachment can be a problem for channels that have had their floods reduced by dams. High flows often erode banks and remove vegetation. When high floods are absent, vegetation can grow very large along the channel margins, constricting the channel and preventing channel migration. Maintaining migration is important because as the channel migrates, fresh sediment is supplied to the channel, exposing new surfaces for diverse types of vegetation.
Q. What do you find most rewarding about your job? What do you find most challenging about your job?
A. I really like working on problems that fill gaps in our understanding of how rivers work, but that have application to improving ecosystems. This particular paper has application to stream restoration, which focuses on improving streams to try to mitigate for human impacts to streams, such as dams and other land use, which can affect the amount of water and sediment in streams. Because of the changes to rivers, habitats for fish, plants, bugs, and amphibians have been severely degraded in most rivers in the U.S. One of the changes is that rivers have widened and are now shallow relative to their former morphology, with fewer pools and shaded areas important to many aquatic species. One solution to this problem is to rebuild rivers to try and improve their value for biology. Unfortunately, we don't have a guidebook that tells us the necessary conditions to create healthy rivers, and restoration often is guided by previous experience and hunches. Our goal was to provide some guidance for the conditions required to have streams that migrate but maintain a single channel path.
Q. What has been the most exciting development in your field in the last 20 years? What do you think will be the most exciting development in your field in the next 20 years?
A. In the last 20 years the most exciting discovery in geomorphology is that rainfall and erosion by rivers can control tectonics and uplift rates. Previous to this discovery, it was thought that tectonics built mountains and rainfall and erosion responded to the mountains built by plate tectonics. In the last 20 years, modeling and field data have found that rapid erosion rates can actually increase the uplift rate of mountains. This occurs because the Earth's crust is less dense than the Earth's mantle (which underlies the crust). Erosion reduces the mass of the crust, and the response is similar to when you try to push down a single ice cube in a glass of water. The ice cube will want to bounce back. [See Paradox of the Himalaya Mountains: Tearing Them Down Builds Them Up, September 1992]
In the next 20 years, I think the most important discoveries will revolve around what sets the width of rivers. Although river width increases in a predictable way, we have yet to develop a model that can predict channel width. This is vital for improving stream restoration projects and also to help understand how channels evolve.
Q. How does the research in your field affect our daily lives?
A. Rivers are vitally important for both society and the natural world. Rivers often provide unique habitats, not just for fish, but for plants, birds, and wildlife. But people, by building dams, levees and the like, change how rivers function. In California, where I live, there is a constant tension about how to divide our relatively scarce water resources among irrigation for farms, and industrial and drinking water for cities, while still providing enough water to maintain healthy ecosystems.
Q. For young people interested in pursuing a career in science, what are some helpful things to do in school? What are some helpful things to do outside of school?
A. I think the most important thing to do in school is to get a strong background in math, physics, and chemistry. They are the building blocks of science, and are key to being successful in any branch of science. Also important is taking as many science classes as you can. Although you hear many people saying that scientists are becoming more specialized, I think it's crucially important to understand as many fields of science as possible, even if they don't seem directly applicable to what you might be interested in.
Outside of school, I think it is important to get outside and look at the world around you. When you go somewhere, ask yourself why does it look like this, and how might it have changed. A key part of being a good scientist is a curiosity about how the world works, and the initiative to try and understand it.
|
What might have strengthened the banks of extraterrestrial meandering channels?
(Researchers' own descriptions of their work, summary or full-text, on scientific journal websites.)
"Experimental Evidence for the Conditions Necessary to Sustain Meandering in Coarse-Bedded Rivers." www.pnas.org/ content/ 106/ 40/ 16936.abstract.
Berardelli, Phil. "Riverbed Model Works Like the Real Thing." ScienceNow (October 7, 2009) [accessed October 27, 2009]: sciencenow.sciencemag.org/ cgi/ content/ full/ 2009/ 1007/2.
Braudrick, Christian A., et al. "Experimental Evidence for the Conditions Necessary to Sustain Meandering in Coarse-Bedded Rivers." Proceedings of the National Academy of Sciences, October 6, 2009, pages 16936-16941.
riverbed restoration, sandbox model, Christian A. Braudrick, William E. Dietrich
PRINT
EMAIL
