Dynamics of flood flows and bed variations in river sections repaired to ship-bottom shaped channels from compound channels

Takuma Sasaki, Shoji Fukuoka

Monday 29 june 2015

14:50 - 15:05h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 2B. Sediment - River

Compound river channels in Japan have caused some problems in recent years, such as riverbed degradation, local scours, excessive growth of vegetation, and so on. As a result, changes in longitudinal and cross-sectional channel forms have become worse for flood controls and river environments. In order to deal with these problems, the authors focused on the cross-sectional form of natural rivers and aimed at forming dynamic stable channels from viewpoints of sediment transports and had a concept that the channel cross-sections like the ship-bottom boundary were suitable for both flood controls and river environments. We call such a cross-section ship-bottom shaped channel. The Onga River in Japan challenged to change a part of channel sections to ship-bottom shaped channels from compound channels in the view of river environments_There have been no ideas whether the ship-bottom shaped channels are good or not for the discharge capacity and formation of stable channel in large floods. We have experienced a large flood in 2010 after completion of the ship-bottom shaped channel in the Onga River. In this study, we clarified dynamics of flood flows and bed variations in the ship-bottom shaped channel by quasi-three dimensional analysis of flood flow and two-dimensional bed-variation using observed temporal changes in the water surface profiles for large flood in 2010. In addition, we compared analysis of the compound channel before river improvement works with that of ship-bottom shaped channel and evaluated river improvement works by using the relationship between dimensionless width, dimensionless depth, and dimensionless channel-forming discharge. These results were also compared to observed results in 2010 flood. From the calculation results, the following conclusions were drawn. The ship-bottom shaped channel brought improvement of velocity distributions and low hydraulic resistance due to less momentum exchanges associated with mixing between the main channel flow and the flood channel flow. Furthermore_longitudinal changes in bed load become small and bed variations decrease markedly in the ship-bottom shaped channel_These numerical computation results were coincided fairly well by the measured results of the 2010 flood.