流下能力評価の基礎となる不等流計算は，既往洪水の再現計算を行いながら各種条件設定を定める．しかし神通川下流部では，既往洪水に対する再現性を十分確保できない事例が確認されており，その課題は出発水位，低水路粗度係数，洪水中の河床低下の3点にあると考えられている．これをふまえて，洪水時の縦断水位観測，河床の3次元測量，河床変動観測および分析を行うことで，洪水中の実態を反映した計算条件の設定に有効であることを確認した．

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本検討では，地理情報を活用して河川流可視化計測を手軽に実施する方法を例示する．すでに整備されているWEBで利用可能な地理情報について，可視化計測に利用しやすいものに絞り，整備状況や規格などを概説していく．続いて，既存の地理情報では使いやすい形に整理されていない河道内の標高分布データについて，本検討での整理例を示す．次にその地理情報，具体的には航空写真と標高分布をどう河川可視化計測に利用して，効率的な計測につなげるかを例示する． 最後に，本検討の内容を取りまとめるとともに，地理情報，特に河道内の標高分布データ整備の重要性や留意点などを論述する．

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River discharge is one of the most difficult hydrological quantities to measure both accurately and safely and several novel methods have been searched for in the past decades in addition to the conventional float method. The space-time image velocimetry (STIV), which measures streamwise surface flow velocity distributions from video images can be one of the candidates to meet the above requirements. However, as long as the measurement target is a surface flow, there raises a problem to be solved when converting the surface velocity distribution to discharge via the surface velocity coefficient α, because the standard value of 0.85 does not always provide an adequate discharge in practice. To improve the versatility of discharge measurement with STIV, the maximum entropy method (MEM) was utilized to derive a general equation for the surface velocity coefficient, taking into account the velocity dip phenomenon and wind effects. The applicability of the proposed method was demonstrated through application to two rivers, the Ohta River and the Nakamura River. The discharge estimated by the proposed method agreed well with the discharges measured by the acoustic Doppler current profiler (ADCP) which is considered to provide the most reliable data, and gave better results than those estimated by the conventional laws of vertical velocity distribution. A sensitivity check of the entropy parameter in flow estimation was also performed.

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