VARIATION AND RUNOFF AND FLOOD DAMAGES CAUSED BY URBANIZATION IN THE SHAKUJII RIVER BASIN, TOKYO

Abstract

Of late the western upland suburbs have experinced frequent flood damages, especially in dissecting valleys. These social problems are largely attributed to rapid urbanization in the area. In this paper, the writer set the principal study object in the research for the relation of runoff with urbanization occurring in the Shakujii River basin (47.98km2). As the first procedure, typification of typhoons was conducted so as to get two sample typhoons of the same type. As a result, Typhoon No. 22 (1958) and Typhoon No. 4 (1966) were obtained for this purpose. A hydrological analysis was then conducted in the study area for this object by using impermeable area as urbanization index and runoff ratio, peak flow and others as runoff indices. Of these questions, S. MIZUKOSHI and T. KINOSHITA have already made analyses. Impermeable area here employed was obtained by airphoto interpretation and statistical data. Comparison was further made with the Meguro River basin in Tokyo in order to scrutinize whether the values obtained in the Shakujii River basin are applicable. 1) Relation of runoff ratio with urbanization ratio (impermeable area ratio). Urbanization ratio and runoff ratio in the Shakujii River basin……(1) Typhoon Urbanization ratio Runoff ratio No. 22 (1958) 0.26 0.50 No. 4 (1966) 0.42 0.52 Relation of runoff ratio with basin area ………………………………(2)_??_ where f is runoff ratio, a the basin area, and affixes imp and p are impermeable area and permeable area respectively. Substituting these values of (1) in equation (2), 0.26fimp+0.74fp=0.50……(1958)0.42fimp+0.52fp=0.52……(1966) Answering these as simultaneous equation, fimp=0.59_??_fp=0.47……………………………………………………… Substituting these values of (3) in equation (2), and considering the following equation : _??_Hence, _??_…………………………………………………(4) Therefore, the relation of runoff ration with basin area is a simple-power function. Investigation into the Meguro River basin which shows a very similar geographic condition to the Shakujii River basin presents the following data : urbanization ratio is 0.82 and runoff ratio for Typhoon No.4 is 0.55, both for 1966. Subsituting these values in equation (4), ft=(0.12×0.82)+0.47 _??_0.56This value is nearly equal to the calculated runoff ratio 0.55. Hence, it seems possible to apply equation (3) obtained by the Shakujii River basin to the Meguro River basin. Important is the possible estimation of future runoff in the Shakujii River basin which will experience further urbanization. The following estimation could be made from this relation regarding stages of urbanization : Urbanization ratio Runoff ratio 60% 0.56 100% 0.59 2) Relation of peak flow with urbanization. Flood routing method was employed to make clear this relation on the basis of storage function and rainfall of Typhoon No.4. As a result, simple-power function correlation was found as in the case of 1). Peak flow was estimated by the above method for different stages of urbanization as follows : Urbanization ratio Peak flow 60% 126.5m3/s 100% 179.6m3/s