ON THE EFFECT OF DISTRIBUTED RETARDING BASINS USING AREAS CLOSED BY EMBANKMENT IN A VALLEY BOTTOM PLAIN

Abstract

 Continuous embankment in the upstream valley plain generating many "closed floodplains" makes the natural retarding basin function of the floodplain decline. In this study, we investigated the characteristics of the closed floodplain in the Kitakami River, which has a long valley bottom plain of 85 km, and proposed a method of using them as a series of retarding basins that functions only for excessive floods. Numerical flow simulations were conducted to estimate the performance of retarding basins under the condition that the design flood of 150 years in return period attacks the river reach which is constructed to the level of 90 years in return period. The result showed the possibility that the flood peak discharge can be reduced by nearly 2-6 % by several a closed floodplain with a section length of about 5 km.