Flow structure and bed evolution after weir removal

Hideyuki Sumida, Yasunori Muto, Takao Tamura

Monday 29 june 2015

16:30 - 16:45h at Oceania (level 0)

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

Parallel session: 3B. Sediment - River

River crossing structures have contributed to flood control and irrigation. However, they adversely affect the river’s biota and sediment transport. Recently methodology of full or partial removal of a weir in order to eliminate discontinuity of the river is under consideration. Nevertheless, impact on bed morphology and channel development due to the removal has not been fully explored. In this paper flume experiments were conducted to study effects of partial removal of a weir on bed morphology in its upstream. Detailed velocity measurements were conducted in order to clarify a mechanism of bed evolution process after the weir removal. The depth and width of the removed part were systematically changed. It is found that a downward flow is induced on the upstream wall of the weir and an eddy with the transvers axis is formed near the bed. Then the eddy extends toward the opening of the weir, i.e., the removed part, as if detouring the wall of remaining part, showing like a hose-shoe vortex motion. The detouring flow is accelerated on the eddy, then causes local scour in the vicinity of the remaining wall. On the other hand, a channel formed from the removed part toward the upstream is closely related to flow concentration after the removal. Velocity distributions in several cross-sections clearly shows flow concentration toward the opening, however the concentration becomes not clear in a certain distance from the weir, where a simple sand bar is formed.