国内外で大規模な洪水氾濫による被害が発生している中，気候変動の影響により洪水の更なる高頻度化，激甚化が進行すると推定されている．洪水の被害を最小限にする対策として，平常時から河川整備を進めた上で，発災時には洪水予測に基づき洪水予警報などを実施することが一般的である．ここで，河川整備を進めるためには整備目標を定める必要があり，そのためには洪水時のピーク流量を計測することが重要である．洪水時の流量は流速を計測した上で，洪水前後に計測した河床の横断形状を用いて算出される．一方，洪水時は，安全面の観点から作業員が現場に近づくことが難しい場合もあり，データが安定的に取得されない場合もある．また，洪水時は河床の横断形状が時々刻々と変化する場合もあり，洪水前後の横断形状を用いることによる流量値への影響は明らかでない．また，洪水予警報のための洪水予測においては，近年，降雨流出現象のみならず，氾濫現象も一体的に表現するモデルが開発されている．大規模氾濫を伴う洪水の激甚化が懸念される中で，このようなモデルを活用することは効果的である．ここで，降雨流出と氾濫を一体的に表現するモデルを扱う上では，河道の流下能力のモデル化が氾濫水の計算に影響すると考えられる．一方，これまでの研究事例では，河道の流下能力，すなわち河道形状と粗度係数の設定については，単純な仮定を用いて設定することが多く，それが氾濫水の計算にどれほど影響するかは明らかでない．また，特に大陸河川の大流域などでは，氾濫水の詳細な調査が困難であるため，計算結果を検証することは難しい．本研究では上記に述べた課題，すなわち流量観測における安全面と精度の課題と，洪水予測における河道の流下能力のモデル化と結果の検証の課題を解決する技術を提案する．具体的には，洪水中の河床変動の把握や流下能力の把握のためには，洪水中の流水抵抗の変化を理解し予測する必要があるため，最初に実河川の流水抵抗の分析を実施する．その上で，作業員が洪水中に現場に赴かなくても，河床変動を考慮しながら流量を観測できる手法を提案し，従来の流量の算定手法と比較してその効果を確認する．また，河道の流下能力をモデル化した上でシミュレーションを実施し，従来手法によるシミュレーション結果と比較してその効果を確認する．さらに，衛星情報を活用した氾濫水の推定方法を提案し，上述のシミュレーション技術と併せてパッケージの技術として提案する．
This study aims to propose methods to estimate discharge within river channel and inundation volume over flood plain. Some devastating floods have occurred in the recent years and such floods are expected to become more serious and frequent due to climate change. Countermeasures are generically taken in both ordinary and emergency situations. In ordinary situation, river development is promoted based on the planned discharge. Hence discharge observation during flooding is highly important to design the planned discharge. In general, discharge is calculated by using observed velocity during flooding and measured cross sectional shapes before and after the flooding. However, there are some difficulties observing discharge. For example, laborer cannot reach the observation site due to harsh flow condition, which causes missing data. In addition, cross sectional shape sometimes varies by the minute during flooding, which could affect discharge value. In emergency situation, flood forecasting is conducted to issue a flood warning. Some models that can simulate rainfall - runoff process have been developed to conduct flood forecasting. Furthermore, a model which can simulate inundation process in addition to rainfall – runoff process has been proposed. It is expected to be used especially in the emergency case that causes large scale inundation. In case that utilize the rainfall – runoff – inundation model, modelling of river channel capacity probably affect the inundation volume. However, river channel capacity, i.e. cross-sectional shape and roughness coefficient, is set with very simple assumption in the previous studies. The influence of the river channel capacity on inundation process has not been understood completely. Besides, it is difficult to validate the calculation results because conducting surveys at many locations in a widespread inundation area is substantively impossible because of labor, time, and cost constraints, particularly in developing countries. This study proposes methods to approach the challenges mentioned above, i.e. safety and accuracy of discharge observation during flooding, and modelling river channel capacity and validation of simulation result. To understand the varying of cross-sectional shape and river channel capacity, flow resistance during flooding has to be understood. Hence the analyzing of flow resistance was conducted in this study at first. Then, the method to observe discharge considering river bed evolution was developed. Furthermore, inundation simulation with precise river channel condition was conducted and its result was compared with computation results with simple assumption of river channel condition. Besides, a method to detect inundation water with satellite information was proposed. The methods to simulate inundation process and to detect inundation water with satellite data are expected to be used as a packaged technique to grasp information of inundation in the emergency situation.
洪水中の流水抵抗変化を踏まえた流量・氾濫量の推定に関する研究
A413
(11.33MB)
