GEOMETRIC CHARACTERISTICS OF BRAIDED STREAMS UNDER UNSTEADY WATER SUPPLY CONDITIONS

摘要

Braided streams under unsteady water supply conditions were reproduced using numerical analysis, and the effects of the unsteady characteristics of water supply on the geometric characteristics of braided streams are discussed. Furthermore, the mechanism of decreasing number of streams under unsteady flow conditions is discussed. The hydraulic conditions tested were restricted to those with short period hydrographs, to avoid complete transitions of bed configurations in a flood. The results show that bars with shorter wavelengths are more stable under small non-dimensional shear stress conditions. Hence, wavelength of submerged bars does not primarily cause to lengthen streams under unsteady flow conditions, but sediment deposition in smaller channels during floods and erosion at riffles during low flows does. These phenomena force water to flow into larger streams. As a result, the number of streams decreases under unsteady flow conditions. Furthermore, the presence of vegetation enhances sediment deposition at higher locations and contributes to a decrease in the number of streams under the condition that vegetation on islands is not washed away completely by floods. These results also indicate the possibility that an unsteady condition of water supply is absolutely essential for formation of single thread meandering channels.