EVALUATION OF DISCHARGE HYDROGRAPHS AND FLOOD STORAGE IN THE MAIN RIVER AND TRIBUTARY CONFLUENCE

Rivers possesses the function of storage and peak discharge attenuation of flood water. Author's recent studies have deepened basic understanding of these important characteristics of the flood flow. At the river confluence, downstream boundary conditions of a tributary change with temporary change in water level of the main river. Waters flowing down the tributary and transmitted back into the tributary from the main river are stored in the tributary section. Furthermore, at the river confluence, a water level changes with time and adverse water surface slope occurs under the temporal change in the downstream boundary conditions. To estimate accurate discharge hydrographs under such complicated hydraulic conditions is important for the river planning of the main river and tributary. The authors, focusing on the confluence between the Tone and Watarase Rivers—for which temporally detailed flood level hydrographs were available— decided to use two-dimensional unsteady-flow analysis to re-create the temporal changes in water surface profile with the goal of creating highly precise estimated discharge hydrographs and storage rate for the main river–tributary confluence. Key Words: water surface profile, flood discharge hydrograph, flood storage volum