The Vaiont Dam was built in 1961 in northeastern Italy. Two years later, a massive rock slide produced a 200-ft wall of water that spilled over the top of the dam and drowned over 2,000 people. The Vaiont Dam Disaster was not due to a poor foundation or other structural flaw. The problem was that geologists and engineers underestimated the probability of a slope failure.
Two years after the Vaiont Dam was built, its reservoir was nearing capacity and raised the surrounding water table. This change, when coupled with heavy rainfall, contributed to increased pressure between consolidated sediments. Tragically, a slope failure occurred, sending millions of cubic meters of land mass into the Vaiont Dam's reservoir. The resulting wave spilled over the top of the Vaiont Dam. Although the 860-ft high structure survived, the surrounding villages were swept away.
The ecological impact of placing a dam in an ocean-bound river is significant, of course, but there are other geologic considerations that are just as important when building such a structure. The geology of the Hoover Dam near Las Vegas, Nevada was studied extensively before construction began. The drilling of bedrock and an investigation of the stability of neighboring sediments was of key interest. The builders needed a solid foundation that would stand the test of time before the river evolved and eroded a spillway underneath the dam, which may still happen some several hundred years from now.
Tragically, the builders of the Vaiont Dam believed that a catastrophic failure of the valley side was unlikely. Understanding the importance of building any structure on a solid foundation and understanding its impact on the local environment has resulted in many guidelines and regulations. Today, these guidelines seem over taxing to some people, yet we still watch houses on the U.S. West Coast fall into the Pacific Ocean as million-dollar mansions suffer the same fate as the mountainside upstream from the Vaiont Dam.
Reference:
http://www.geocities.com/geogsoc2000/Vaiont1.htm
http://www.answers.com/topic/vajont-dam?cat=travel
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