Evaluation of flood control under changing conditions

This paper constitutes an analysis of the historic flood of Sept. 15-16, 1947, on the Tone River, Japan, to make possible technological assessment of the flood control works applied to Japanese river basins since the early twentieth century. The Tone River basin has the largest river basin in Japan, and the Tokyo metropolitan area lies within it. The flood control works on the Tone River, which were constructed since 1900, and current flood projects on the Tone River are described. The evaluation of the flood control works leads to the following conclusions: 1) evaluation should consider the passage of time in relation to human activities; 2) the expression of magnitude of floods in terms of return periods has limited significance; 3) it is difficult to evaluate the net effect of flood control measures because they are often executed for other purposes as well; and 4) the influence of flood control works upon regional environment and development should be considered in evaluation of flood control projects.