|
The Black Hills Flood of 1972, also known as the Rapid City Flood, was one of the most detrimental floods in the history of South Dakota.〔 It took place on June 9–10, 1972〔 in the Black Hills of Western South Dakota. of rainfall over six hours sent Rapid Creek and other waterways overflowing, flooding many residential and commercial properties in Rapid City. It also caused flooding of Battle, Spring, Bear Butte, and Boxelder Creeks.〔 During the night of June 9, Canyon Lake Dam became clogged with debris and failed, resulting in 238 deaths and 3,057 injuries. Several bodies were never found.〔(【引用サイトリンク】 publisher = United States Geological Survey )〕 Over 1,335 homes and 5,000 automobiles were destroyed. The value of the damaged property was over US $160 million in 1972 dollars. ==Causes== A few days before the Rapid City Flood “earlier rains had left the soil saturated, increasing the amount of runoff of the flood to come.”〔 On the afternoon of the 9th, substantial rains fell on the area, caused by “an almost stationary group of thunderstorms.”〔Carter, Williamson, and Teller〕 Cloud seeding experiments being conducted by the Institute of Atmospheric Sciences on clouds west of Rapid City were speculated to have contributed to the unusual amount of rain. However, there is no evidence that the two phenomena were related.〔Arnett Dennis〕 Additionally, "a strong low-level easterly flow which forced the moist unstable air up-slope on the hills. This sustained orographic effect helped the air to rise, cool, and release its moisture in repeating thunderstorms. Another contributing factor was the unusually light winds at a higher atmospheric levels which did not disperse the moist air nor move the thunderstorms along to prevent an extreme concentration of rainfall."〔 According to Herbert Thompson the air pattern causing this storm was noticed in big measure over the Great Plains, with a minor scale to the east of the Rockies. There were only what appeared to be light winds, so nothing indicated that there was a huge storm underway. Thompson further indicates that a cold high pressure region was pushed from Canada into the Great Lakes region leading into the western part of South Dakota. A “mesoscale cloud mass” from Colorado also moved towards Rapid City.〔Thompson 164〕 One of the cloud masses amplified the “mid-level moisture over Rapid City,” while the other mass caused the pressure to stay, as the high pressure pushed downward, thus creating the right conditions to produce rainfall.〔Thompson 164-165〕 The rainfall from the upper “cloud mass” above the Black Hills formed into new smaller masses downwind which reprocessed the rain allowing for the constant rainfall.〔Nair, Hjelmfelt, and Pielke 1756〕 The storm was described by Nair, Hjelmfelt, and Pielke as “convective cells of high precipitation efficiency a characteristic of tropical precipitation systems.〔 The immense precipitation was based over the “Rapid, Boxelder, Spring, and Battle Creeks,” creating run-off, along these creeks which lead to flooding throughout the surrounding areas.〔Nair, Hjelmfelt, and Pielke 1753〕 The intense rain began on the afternoon of June 9 and continued until after midnight on June 10. The immense amount of rain the Black Hills received during the thunderstorms ranged from 4 to 15 inches, causing Rapid Creek and surrounding creeks to overflow and creating massive amounts of runoff that resulted in flood waters. The run-off carried rubble to the Canyon Lake Dam creating a barrier in front of its spillway.〔 This resulted in the “increase in depth of water behind the dam of 11 to 12 feet,” which caused the release of more water adding to the already moving floodwaters.〔Thompson 167〕 The flash flood hit Rapid City the hardest around midnight on June 9; the flood waters also hit the small town Keystone nearby.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1972 Black Hills flood」の詳細全文を読む スポンサード リンク
|