Dams are supposed to prevent floods. Some may make them worse | Science

Dams are often built to control floods, but on certain kinds of rivers they may make big deluges worse, a new study finds. The finding suggests river managers might need to rethink their flood control strategies on silty and sandy lowland rivers.

“It’s a counterintuitive finding,” says Gordon Grant, a hydrologist and geomorphologist with the U.S. Forest Service who was not involved with the work. “What this provocative paper is showing us is that we don’t fully understand” how dams influence flooding, he says.

Dams provide multiple benefits. They can generate relatively clean electricity; they store water and release it in the dry season, which helps farmers and other users, and they can hold back floods. Dams also have downsides, such as displacing people when they are built and preventing fish migration and causing other ecological harm. But no one had suggested dams might make flooding worse than before a barrier was built, says Ellen Wohl, a river scientist at Colorado State University, Fort Collins.

In addition to storing water, engineers expect dams to reduce flood risks by modifying the river downstream. Because dams trap sediment, they release relatively clear water that cuts deeper into the river’s bed. This incision creates a roomier channel that can carry more water and prevent floodwaters from spilling over riverbanks.

Hongbo Ma, a geomorphologist at the University of California (UC), Irvine, was interested in how dams alter riverbeds’ typical sediments—and this led to the surprising result of his study. The phenomenon involves erosive water released from a dam, which removes more of the finer particles and leaves behind the larger grains. This coarsening of the riverbed then creates underwater dunes.

Ma and his colleagues were studying the Yellow River, which flows from mountains on the Tibetan Plateau to the East China Sea. While using sonar to scan the riverbed of the lowermost reaches, they were struck by the absence of dunes. This is likely due to the high silt content of the riverbed; the Yellow River—the muddiest in the world—is named for the heavy loads of silt it carries. The fine particles hinder the formation of dunes. But closer to Xiaolangdi Dam, the riverbed was coarser and had large dunes. “We have a really astonishing example of how the bed roughness can change so much,” Ma says.

Ma wondered whether the rougher, dune-filled riverbed would impede the flow of floodwaters, causing them to back up, overflow riverbanks, and spill onto the floodplain. To test the idea, he and colleagues first ran calculations based on the shape of the river channel and other factors. The results suggested large floods are now about twice as deep compared with before the dam was built in 1999, despite a channel that is up to 3.4 meters deeper, the team reports this month in Nature Communications.

By checking flood records from 1980 to 2015, the scientists discovered that the magnitude of moderate and large floods had in fact increased. Over the same period, however, the magnitude of smaller floods decreased—likely because the river’s deeper channel is better able to contain them.

Luckily, large floods have seldom struck the Lower Yellow River since the dam was built. That’s because the climate has become drier and the reservoir still has enough capacity to hold back the greater flows that come from extreme rainstorms. But climate models suggest rainfall in the Yellow River Basin will increase by up to 30% this century. And as the river continues to dump sediment into the reservoir—it’s already 75% full—the dam will have less room to contain floodwaters.

“We need to start thinking how we should re-evaluate the flood risk, not only for the Yellow River, but also for other rivers,” Ma says. The team estimates that new dams would exacerbate large floods on more than 80% of lowland rivers.

The Yellow River could be an unusual case, notes Michael Singer, a hydrologist at Cardiff University, because most large dams are built on rivers carrying coarser sediment that doesn’t readily form dunes. Still, it would be worth seeing whether a similar phenomenon is occurring on other rivers that carry fine sediment—known as alluvial rivers—and have large dams, says Nicholas Pinter, a geologist at UC Davis. In the United States, such rivers include the Missouri and the Tennessee. If so, engineers might need to recalculate local flood risks.

More broadly, engineers should also pay more attention to the complex behavior of rivers when designing new dams, Pinter says. “It’s amazing how much we’ve gotten wrong by thinking that these big alluvial rivers are just pipes,” he says. “We continue to underestimate the importance of bedforms and roughness.”

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