Before Shihmen Reservoir was completed, engineers estimated that silt would flow into it at a rate of 790,000 cubic meters per year, giving it a useful life of at least 71 years. The reservoir began filling with water in May 1963, but when Typhoon Gloria struck northwestern Taiwan in September of the same year, it washed more than 19 million cubic meters of silt into the new reservoir, equivalent to one-third of its silt capacity. This knocked 23 years off its life expectancy at a stroke.Morakot did the same thing.....
In August 2009, Typhoon Morakot struck Taiwan with record-breaking rainstorms and devastating floods. As a result, the Cengwun and Nanhua Dam were ravaged, the massive flood water caused Cengwun Dam to silt up with 90 million cubic meters of mud, while Nanhua also suffered silting up of 17 million cubic meters of mud. Consequently, the water retaining capacities of the dams were significantly reduced, seriously affecting the water supply in the Greater Tainan region.A DPP legislator commented in the Taipei Times in 2010:
Following Typhoon Morakot last August, the bed of the Laonong River (荖濃溪) in southern Taiwan rose in some places by more than 23m, and the cross-watershed transfer project built to supply water to the Zengwen Reservoir (曾文水庫) in Taipei County was almost completely ruined. That was followed on March 4 by the Jiasian earthquake, after which environmentalist groups found that the transfer conduit tunnel was crossed by four geological faults, presenting the danger of collapse in case of a powerful earthquake.The conduit is discussed in detail in this 2009 post on The View on the destruction of Hsiaolin Village. The DPP legislator's piece stated that Tsengwen Reservoir's current capacity is less than 40%; it is so shallow that in the rainy season it often overflows and has to be lowered. UPDATE: Mike Fagan, who has traveled all over the south looking at its dams, reminds me that the 2010 program to combat the silting problem of the Tsengwen and Nanhua Reservoirs was implemented and $16 billion was spent on it. A 2011 piece from the China Post notes an important connection between the reservoirs and the construction-industrial state:
The Ministry of Economic Affairs (MOEA) had earlier planned to remove 52 million cubic meters of sand and mud from December 2010 to November 2011 but is now shortening the plan's completion time to the end of June.Heh. Fagan told me of the tributary widening: "you can see the difference - the newer weirs are much wider than the older stuff, and use different designs." Here are his posts on his trips to Nanhua and Tsengwen.
According to Wu, the government's dredging efforts have achieved good results, with sand prices having fallen by almost 25 percent from July to December last year, lowering the costs of public construction projects.
A UDN editorial sketched the future of Taiwan's dams:
To exacerbate things even further, extreme weather patterns have become ever more prevalent because of global warming. Both the Research Center for Environmental Changes at Academia Sinica and the United Nations-affiliated Intergovernmental Panel on Climate Change have already confirmed that every time the world’s average temperature rises by one degree Celsius, the amount of moisture in the atmosphere increases by 7 percent. This means that convection has intensified and there are now more severe heavy downpours and fewer light showers.The bashing by severe rains washes more silt down, meaning that in our warming world dams will have shorter life spans and serve up less water.
What is more, after analyzing Taiwan’s rainfall data from the last 45 years, Academia Sinica discovered that over this period the number of “severe” and “extremely severe” downpours in Taiwan has increased by 100 percent. The island has thus been affected by extreme weather conditions to a far greater degree than the global average.
If four typhoons strike Taiwan every year, and each typhoon affects the nation for two days, then 40 percent of annual precipitation would fall in eight short days. The number of days when it rains more than 1,000 millimeters per day has also become more frequent. But most of this rainfall flows rapidly directly to the ocean, and very little remains. All this means that the situation whereby Taiwan “rains a lot, but has very little water to use” is getting worse.
On the other hand, the number of days when there has been a light drizzle or sprinkle has declined sharply. Statistics show that during the last few decades, moderate showers fell an average of 70 days per year. In the last few years, this number has gone down to fewer than 30 days per year. Light rains moisten the earth, gradually percolate underground, then slowly migrate to rivers, and finally fill dams with water. This regulating system has today been weakened and is coming to a halt, so that the number of days when rivers run dry has increased, and droughts are becoming more common. The land is undergoing desertification, the rivers are filled with mud and dams are becoming silted.
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