[A-HW28_30PM2] Water and material transport and cycle in watersheds: from headwater to coastal area

[AHW28-24] Study on Effect of Rainfall Distribution and Rainfall Intensity on Discharge at The Concentration Point of The Basin

*Kazuhiro YOSHIMI¹, Chao-wen WANG¹, Tadashi YAMADA² (1.Graduate School of Science and Engineering, Chuo University, 2.Department of Civil and Environmental Engineering, Chuo University)

Keywords:rainfall distribution, runoff analysis, peak discharge, average-rainfall over watershed

In resent years, the damage caused by flood comes obvious because of unprecedented record-breaking rainfall event or largest recorded rainfall. Because of this situation, river planning starts to be looked at again in Japan. For example, it has been reported by MLIT (Ministry of Land, Infrastructure, Transport and Tourism) that even if total rainfall is the same when the rainfall distribution differs in a basin, the discharge at a reference point may differ (refer to MLIT). In other words, it is necessary to innovate a new intellection considering the difference of rainfall distribution when creating river planning. This study aims to clarify the relation between rainfall distribution in a target basin and peak discharge at a reference point. The authors therefore verified how the peak discharge at a basing point responds to rainfall distribution at an intended basin. Moreover, the authors examine the impact on discharge at a reference point if rainfall intensity increases or decreases in a basin. A target basin of this study is Tone upper river basin. In addition, largest recorded flood of this basin is Kathleen typhoon which 3-days accumulated rainfall is about 320mm(refer to document of Japan Society of Civil Engineers) in the basin. First, the authors separated the target basin into 4 parts (refer to document of Science Council of Japan). Then, we did runoff analysis for a number of rainfall distributions using this typhoon event as basic rainfall pattern. The conditions of this calculation are, first, average rainfall of Tone upper river basin is the same in every rainfall distribution. Secondly, soil condition and geotechnical condition do not change in every case. After that, we compared every peak discharge at the reference point. Moreover, the authors also did runoff analysis using a number of average-rainfall over watershed 0.8, 0.9, 1.1, 1.2 times as much as basic rainfall event. And then, we compared the peak discharge in the same way. In consequence, the authors indicated that peak discharge at the point of reference was about from 20800m³/s to 23800m³/s in which case rainfall distribution differs. These range of values is ±7% in contrast with basic design flood of Tone river. In particular, the peak discharge of the reference point becomes 22000m³/s or more in which case heavy rainfall intensity occurred in a watershed nearby the point. Furthermore, it was found that the range of values of peak discharge grows wider as average-rainfall over watershed builds in intensity. Therefore, flood exceeding the designed level is necessary to be defined newly and exactly in Tone upper river basin.