<治水>降雨の極値統計と流出・氾濫現象の現地調査及びモデリング

Abstract

本稿は2017年九州北部豪雨を対象に, 降雨の極値統計と降雨流出・洪水氾濫の解析結果を報告する. 朝倉地点, 日田地点の時刻降雨量を用いて統計解析した結果, 比較的短い継続時間の降雨量がいずれも既往最大を記録しており, 特に朝倉地点では継続時間が4-20 時間の降雨が既往第二位の2 倍を超過していた. また災害前後の地形データを用いて, 筑後川右岸に流入する中小河川の降雨流出と洪水氾濫を解析した. その結果, 今回の洪水現象を説明する上で地形変化の影響が重要であり, 多数の表層崩壊地から流出した多量の土砂が, 河床および底部段丘面に堆積し, 洪水氾濫の拡大に結びついていることが示された.

This article reports statistical analysis of extreme rainfall and rainfall-runoff-inundation simulation for the 2017 Northern Kyushu flooding. Statistical analysis based on hourly rainfall at Asakura and Hita indicated that rainfall depths with relatively short durations record broke the historical maximum records at each location. In particular, rainfall depths recorded at Asakura during this event exceeded more than two times of historically recorded values in case of rainfall durations between 4 to 20 hours. In terms of flood simulations, this study used both original and post disaster topographic data to simulate flooding in tributaries of the Chikugo river. The simulation suggested that the topographic change associated to large volume of sediment inflow from upstream landslides is important to simulate this flooding. The deposit sediment in the rivers and floodplains caused the expansion of flood inundation areas.