Scientists debate the role of sediment in helping or hindering flooding
The Yellow River, China's second largest river, has always been known for its large amount of sediment, but it has become gradually clearer over the last decade and this has experts worrying about the possible greater frequency of flooding and the need for more flood control projects.
The river saw 70 percent less sediment from the loess plateaus upstream in the 2000 - 2015 period, somewhat of a rarity in historical records, Overlook Weekly reported Saturday.
During the dry season, 80 percent of the Yellow River has been looking clear and it was carrying less than 0.8 kilograms of sand per cubic meter in May, according to recent data from the Tongguan Hydrological Station. When we compare this with the 911 kilograms per cubic meter that it carried at one point, the highest record since 1919, the change is quite shocking, the report said.
"The Yellow river has been clear for more than 10 years and, sometimes it even looks green," one villager in Yonghe County, North China's Shanxi Province who has lived along the river since childhood, told the Overlook Weekly.
The decrease in the amount of sediment in the river is caused by construction, engineering projects and economic development, experts have said. The Yellow River Conservancy Commission also says that attempts to halt soil erosion, such as converting cropland to forests, has cut 435 million tons of sediment annually over the last two decades.
In response to this, Mu Jianxin, of the China Institute of Water Resources and Hydropower Research's Irrigation and Drainage Department, told the Global Times Monday, "It's great news hearing that the Yellow River has less sediment, since it means there are fewer raised bed river places above the surrounding areas on either side, hence a reduction in the amount of flooding along the lower reaches."
However, Overlook Weekly quoted Zhang Yuliang, the head of the Yellow River Engineering Consulting Company, as saying that, when the rainfall is heavy enough so that the plants and soil can't absorb it, the water will carry away large amount of sediment. And the water and soil erosion would worsen the more intense the rainfall gets.
If banks along the lower reaches burst, the flooding can affect more than 100 million people in 5 provinces, Overlook Weekly said.
Mu agreed that when the river had less sediment, the water would flow much faster and apply greater force to the watercourse, which would increase the possibility of flooding.
Still, Cheng Xiaotao of the Water Resource Ministry's Flood and Drought Disaster Reduction Research Center disagreed with Zhang.
"The Sanmenxia dam and other flood-prone areas can all reduce risks, so it's no different with the Yellow River flood controls," Chen said in a talk with the Global Times.