Preparations have begun to relocate about 20,000 residents from the vicinity of China's massive Three Gorges Dam because their homes are at risk from "constant landslides," authorities in Central China's Hubei province said.

The residents account for one-fifth of the population in Dongba county in Hubei, said Zhao Wenxing from the county's relocation headquarters, state-run news agency Xinhua reports.

That follows an announcement this week that 100,000 people may have to be relocated over the next three to five years due to the threat of natural disasters near the dam on the Yangtze River.

Scientists have said that fluctuating water levels of the 410-mile-long reservoir behind the 591-foot-tall dam have destabilized hundreds of miles of slopes, triggering massive landslides.

Landslides and other incidents have increased 70 percent since the reservoir reached its high-water mark in 2010.

Liu Yuan with the Ministry of Land Resources told China National Radio said rock falls and landslides at 335 sites would be addressed but there are more than 5,000 potential danger sites.

"Due to the complexity and uncertainty of the problems, the pattern of geological disaster cannot be accurately predicted," Liu said. "It's difficult to know what's going on."

During the engineered flooding required for the building of the $22.5 billion dam, started in 1994, 13 cities, 140 towns and 1,350 villages were submerged and 1.8 million people displaced. The project, completed in 2006, started generating power in 2008 ahead of the Beijing Olympics.

The power station has an installed capacity of 21,000 megawatts.

Noting that 20 years ago this month the Chinese government approved construction of the colossal project, Patricia Adams, editor of online news portal Three Gorges Probe, in an editorial published in Canada's Financial Post, wrote, "The critics said the dam would be an environmental and economic nightmare that would flood millions of people off their land, induce landslides and earthquakes, cripple navigation and produce unaffordable electricity.

"Twenty years later, the critics have been proven right on all counts."

Last May, the Chinese government acknowledged that Three Gorges "caused some urgent problems in terms of environmental protection, the prevention of geological hazards and the welfare of the relocated communities" but maintained the project "is now greatly benefiting the society in the aspects of flood prevention, power generation, river transportation and water resource utilization."

The government also said the dam had affected downstream shipping, irrigation and water supplies. The same day, it announced that measures would be carried out to improve the living conditions of the displaced people, protect the Yangtze's ecosystem and prevent geological disasters.