ON-SITE EXPERIMENTS AND HYDRAULIC EVALUATION ON RUNOFF CONTROL BY RAINWATER STORAGE IN PADDY FIELDS

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In recent years, flood disasters have become to occur frequently by change of runoff characteristics due to urbanization of a river basin. In order to deal with serious disasters by heavy rainfall, in addition to the river improvement works, construction of the rainwater storage facilities has been planed in many urbanized basins of Japan. However, the preparing plan has not been in progress because construction of such facilities costs high and the suitable building sites are limited. Thereupon, a utilization of paddy fields remained in the suburbs of a city gains attention as an alternative measure for the rainwater storage within a basin. But during a summer as a rainy season, paddy fields are filled with water for the rice fanning. In order to control a flood runoff by utilizing paddy fields, I suggested new methods to improve the rainwater storage function of the paddy fields in this retherch.Generally in a paddy field, there is an inlet aperture from an irrigation channel and an … More outlet aperture to a drainage channel. New suggested methods to substitute paddy fields for rainwater storage facility consist following two procedures.1. To set up the automatic sluice gate with wath-level sensor at the inlet aperture. This equipment can shut inflow water from an irrigation channel by detecting rise of wathrlevel during a rainfall2. To replace a wide weir of the outlet aperture by a narrow weir in order to reduce outflow water to a drainage channel. This method is better than the orifice because it can protect the crops from submergence during an excess heavy rainfall.I have carried out the on-site experiments for some actual paddy fields in the Shinkawa basin, Aichi prefecture, Japan, since 2004. From comparison of wate-levels in paddy field with conventional weir and paddy field with a narrow weir it is clear that the rainwater storage function is improved by the suggested method. Next, for a drainage district that consists of 30 paddy fields, a flood control effect by the improvement in each paddy field is evaluated through the runoff simulation analyses. The runoff model is based on the kinematic wave theory, and the model parameters are identified by observed data in the on-site experiment district. According to the simulation results on the runoff processes from the paddy fields district for a probabilistic design rainfall, it is possible to reduce the peak discharge of outflow largely and to delay the peak time for several hours by contracting the weir width at the outlet aperture. Therefore, I can recognize that the paddy fields become useful as alternative of the flood control facilities by the improvement methods suggested in this research. Less