Dams are supposed to prevent flooding. Some may make it worse Science

Flood dams are often built, but some types of rivers can make large floods worse, according to a new study. The findings suggest that river managers should rethink their flood control strategies on muddy, sandy lowland rivers.

“It’s an intuitive discovery,” says Gordon Grant, a U.S. Forest Service hydrologist and geomorphologist who didn’t participate in the work. “What this provocative verdict shows us is that we don’t fully understand” how dams affect floods, he says.

Dams provide many benefits. They can generate relatively clean electricity; they store water and release it on dry land, which helps farmers and other users, and can withstand flooding. Dams also have disadvantages, such as the displacement of people when they are built and the prevention of fish migration and other ecological damage. But no one has suggested that dams could be worse than floods before they built a barrier, says Ellen Wohl, a river scientist at Colorado State University in Fort Collins.

In addition to storing water, engineers hope to reduce the risk of flooding by shifting the river downstream. As the dams trap the sediments, they release relatively clean water, which sinks to the bottom of the river. This cut creates a wider channel that can carry more water and prevent flooding from flowing into the riverbanks.

Hongbo Ma, a geomorphologist at the University of California, Irvina, studied how dams change typical river sediments, which led to a surprising result in his research. The phenomenon involves the erosion of water released from a dam, which removes more fine particles and leaves larger grains behind. This thickening of the riverbed then creates underwater dunes.

Ma and her colleagues were studying the Yellow River, which flows from the mountains of the Tibetan plateau to the East China Sea. While they were using sonar to scan the course of the river below, they were surprised that there were no dunes. This is probably due to the high silt content of the riverbed; The Yellow River, the mudiest in the world, is named after the heavy mudslides it carries. Fine particles interfere with the formation of dunes. But closer to the Xiaolangdi Dam, the riverbed was thicker and had large dunes. “We have an amazing example of how bed roughness can change so much,” says Mak.

Mak wondered if the rougher, dune-filled waterways would interfere with the flow of floods, backtracking, flooding the riverbanks, and flooding the floodplain. To test the idea, he and his colleagues made the first calculations in the form of the river canal and other factors. The results suggest that major floods are now twice as deep as before the dam was built in 1999, despite being a 3.4-meter-deep canal, the group reported this month. Nature Communications.

Checking records of floods from 1980 to 2015, scientists found that the magnitude of moderate and large floods had increased. At the same time, however, the magnitude of the smaller floods has decreased, probably due to the fact that the deeper channels of the river are better able to hold.

Fortunately, there have been few major floods since the Lower Yellow River dam was built. As a result, the climate has dried up and the reservoir still has enough capacity to withstand higher flows from extreme storms. But climate patterns suggest that rainfall in the Yellow River Basin will increase by up to 30% this century. And as the river continues to pour sediment into the reservoir, which is already 75% full, the dam will have less space to contain the floods.

“We need to start thinking about how to assess the risk of flooding, not only for the Yellow River, but also for other rivers,” says Mak. The group estimates that the new dams would increase major flooding in more than 80% of the earth’s rivers.

The Yellow River could be an unusual case, according to Michael Singer, a cardiologist at Cardiff University. However, it remains to be seen whether similar phenomena occur in other rivers with fine sediment — known as alluvial rivers — and in other rivers with large dams, says UC Davis geologist Nicholas Pinter. In the United States, there are Missouri and Tennessee rivers, among others. If so, engineers may need to reconsider local flood risks.

In general, engineers should also pay more attention to the complex behavior of rivers when designing new dams, Pinter says. “It’s amazing how badly we’ve done that thinking these big alluvial rivers are just pipes,” he says. “We continue to underestimate the importance of bed shapes and roughness.”

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