2006 Volume 12 Pages 223-228
An influence of Niu dam on sediment runoff and corresponding bed evolution of alluvial reach are investigated in Takatoki river basin based on numerical predictions, supposing an occurrence of a plan size rain-fall event with a large amount of sediment yield due to numerous landslides. Computations are conducted for three cases; present drainage condition without the dam (Case 1), flood control with the dam (Case 2) and no-flood control but with the dam (Case 3), using a specified six years-flood series and the plan size flood is included in the first year. In Case 2 and Case 3, it is supposed that all sediment delivered to the dam site is caught in its storage area. Predicted results suggest that (1) in Case 3, the amount of sediment volume transported from mountain areas into the alluvial reach reduces to about forty percent of the calculated amount in Case 1 and in addition, most of sediment delivered to the reach of flat area is carried from upstream as suspended loads whose grain sizes are smaller than 1mm, (2) bed evolution of the alluvial reach during the flood is influenced by pre-flood reach conditions such as the channel width and bed material as well as by deposition of suspended sediment, (3) Coarse sediment which behaves as bed load in mountain areas does not influence the bed evolution because it is still not propagated during the flood, and (4) the dam reservoir can suppress remarkably the bed aggradation even if the alluvial reach is located far downstream.
