Present Situation and Cause Analysis of Diversion Dam Disasters

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Diversion dams are constructed on riverbeds over which sand and gravel move seriously during floods. Dam structures and attaching and bank protection works are sometimes destroyed due to riverbed scouring below dams. In 2001, many examples of diversion dam disasters were analyzed. In addition to these analyses, failures of several diversion dams on the Tama River that were destroyed again by the flood in September 2001 were investigated, comparing them with past disasters.These investigations clarified that it was very important to predict natural and artificial change of riverbed form and watercourse precisely and accurately. Channel changes according to downstream migration of alternate bars and man-made changes by rierbed mining could cause troubles on diversion dams. In 2002, the following two methods were applied for studying these problems.1. Investigating method for present situation and yearly changes of riverbed form and watercourse It is very difficult to study the mechanisms … More and causes of river disasters, including diversion dam disasters, because of lack of direct observations and measurements during floods. For this reason, the first step of these studies is to grasp the changes in riverbed form and watercourse before and after a flood. It is also important to investigate long-term changes, so fieldwork approaches, such as comparison investiagtions of aerial photographs and analyses of survey results on riverbed changes, are very useful and fruitful.2. Prediction method for flood-flow route and its future change on a small-scale model experiment An experimental method to simulate riverbed form and water route on a small-scale model flume was studied. The most important point for obtaining similarity between the river and the flume is to represent the shape and connecting pattern of alternate bars. A small-scale model experiment for the Uetsu-Arakawa River succeeded according to Kinoshita's method. This success revealed the possibility of small-scale model experiment in order to investigate the causes of disasters and preventive measures. Less