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建围堰是水坝安全拆除的第1步。小型低水头坝拆除所需时间通常为几个月。为减少围堰修建费用,通常需要在低水时期进行水坝拆除,因此围堰的设计应根据水坝拆除期间相应的河流流量系列而不是全年流量系列进行。对近期拆除的俄勒冈州桑迪(Sandy)河上马莫特(M armot)坝的时均和日均流量记录,按4种拆坝期限进行分析,期限长达1~4个月(7~10月)。为代表现场的最大流量系列,将L-矩比率图和概率图相关关系系数法用来选择皮尔逊Ⅲ型,二参数和三参数的对数正态概率分布函数。从水文工程角度看,建议拆坝的最优时间期限为3个月(7~9月)。根据年最大流量系列估算的流量设计值较高,会导致围堰设计不经济。
Cofferdam construction is the first step in the safe demolition of a dam. The time it takes to disassemble a small low head dam is usually a few months. In order to reduce the construction costs of cofferdam, it is usually necessary to carry out dam removal in low water period. Therefore, the design of cofferdam should be based on the corresponding series of river flow during the dam demolition instead of the annual flow series. The records of average and daily average flow rates of M armot dams on the Sandy River in the recent removal of oregon in Oregon are analyzed according to four kinds of dam removal deadlines of 1 to 4 months (7 to 10 month). To represent the maximum flow series on site, the correlation coefficient method between L-moment ratio and probability was used to select the log-normal probability distribution function of Pearson type III, two parameters and three parameters. From hydrological engineering point of view, it is suggested that the optimal time for removing dam should be 3 months (July-September). The higher flow design values estimated from the annual maximum flow series result in uneconomical cofferdam designs.