Dam Break Floods - Unnatural Disasters?

Steve's picture

 

    You might not classify flooding resulting from a dam break to be a natural disaster. Still, if you live or own property downstream from one of the 1000s of reservoirs in the United States, exposure to dam break flooding  -- natural or unnatural  -- ought to be on your radar screen. While dam safety and security are remote from my field of interest or expertise, computer simulations of actual or potential dam break floods do fall within my sphere of hazard forecasting. 

 


   To set the stage for that next event, consider two historical examples:  The 1889 Johnstown Flood and the 1928 Saint Francis Dam Failure.

     In 1889 Johnstown Pennsylvania was an industrial workingman’s town, sited on a flat triangle of ground at the confluence of the Little Conemaugh and Stoney Creek Rivers.  Years of industrialization narrowed and channelized the valley’s three rivers to a fraction of their natural widths. As a consequence, nearly every spring Johnstown flooded. On May 30 and 31, 1889 the upper reaches of the river basins experienced the strongest rains in anyone’s memory. The Conemaugh River at Johnstown rose from 1 foot to 23 feet in just over 24 hours. By mid-day on the 31st, water crept over most of the lower town. Like floods of the past, the 1889 one would have faded from history if not for South Fork Lake, 24 km up the Little Conemaugh River and 140 meters higher than Johnstown. Also swollen by the strong rains, South Fork Lake rose steadily throughout May 31 and water began running out its spillways at mid-day. Had it been rebuilt to its original 1853 specifications, South Fork Dam may have withstood even this torrent. However, blunders in the dam’s 1886 reconstruction and later management led to its overtopping and failure. Collapse of the 22 m high dam released 16 million cubic meters of water into the Little Conemaugh River targeting Johnstown. When the surge arrived eighty minutes later, it found no match for the wood frame construction in the valley. Hundreds of families waiting out high water in upper floors were carried away -- house and all. Until the terrorist attacks on September 11, 2001, the 2200 casualties in the 1889 Johnstown Flood amounted to the largest single-day disaster in United States history.

 

 

   At the turn of the twentieth century, Los Angeles was a growing and thirsty city. To satisfy people’s needs, 1908 saw an ambitious plan set into motion to bring water to southern California from distant snow melt sources east of the Sierra Nevada mountains. The colossal Los Angeles Aqueduct project spearheaded by William Mulholland, a self-styled engineer, was by and large hugely successful in its mission. Still, of the many components of its remarkable story, the short-lived Saint Francis Reservoir forms one of the darker chapters. Built in 1926 in the upper reaches of the San Francisquito canyon about 30 miles north of the Los Angeles, the Saint Francis Dam and Reservoir intended to be both a storage facility and a power generating facility. After completion of the initial design blueprints, Mr. Mulholland unwisely directed that the dam be increased in height twice, by 10 feet each time, with no corresponding expansion of the dam’s footprint. This turned out to be a fatal mistake for sure. Just days after its first filing in 1928 a large portion of the structure toppled over, releasing all 45 million cubic meters of water in less than an hour. On its 52 mile course to the Pacific the flood killed 600 people. Mr. Mulholland endured years of subsequent law suits and ended his life in disgrace. Recognizing that one man, no matter how good of reputation should have full authority over a building project with such potential for disaster, the State of California soon afterwards required that all Civil Engineers be licensed.

Why run dam break flood models like these? Successful simulations of historical disasters give scientists faith in their ability to forecast consequences from future “what if” hazards  --- such as might happen at that dam just up the river from you.

  Please view video portraits of these disasters here 

 Johnstown Flood

http://www.youtube.com/watch?v=tMc9kP9q-d8

St. Francis Dam

http://www.youtube.com/watch?v=3MdB_s6KhwA

 

Steven N. Ward,   Santa Cruz

 

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