Numerical modelling of braided streams in unsteady flow

摘要

Braided streams with vegetation growth under unsteady water supply conditions were reproduced using horizontal two-dimensional bed deformation numerical analysis. The effects of both vegetation and the unsteady characteristics of the water supply on the dynamic characteristics of braided streams are investigated. The mechanisms of stream lengthening and the decrease in the number of streams under unsteady flow conditions are discussed. To avoid complete transitions of bed configurations during a flood, tested hydraulic conditions were restricted to those with short-period hydrographs. The results show that wavelengths of submerged bars under unsteady water supply conditions are shorter than those under steady conditions. The results further suggest that both sediment deposition at higher locations and the formation of islands primarily cause stream lengthening under unsteady flow conditions. The wavelength of submerged bars was not a primary cause. Sediment is deposited in smaller channels during floods; the bed at riffles is eroded during low flows. These phenomena force water to flow into larger streams. Consequently, the number of streams decreases under unsteady flow conditions. Furthermore, the presence of vegetation enhances sediment deposition at higher locations and this contributes to a decrease in the number of streams under the condition that vegetation on islands is not completely washed away by floods.