Study on Bar Formation with Bed Material of Low Density

Shunsuke Tanaka, Yoshihisa Kawahara, Ryota Tsubaki

Friday 3 july 2015

13:45 - 14:00h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 16B. Sediment - Transport

Channel bed patterns significantly change during floods. Many laboratory experiments have been carried out to deepen the understanding of bed formation process and sediment transport phenomena. Experiments with mobile bed have frequently adopted sand as a bed material, which sometimes makes the bed forms different from real rivers. One of the limitations of the similarity law comes from the bed material density. Hence this study aims to clarify the applicability of an artificial material of low density, chamotte. This study focuses on the influence of the low density material on bar formation under different hydraulic conditions through experiment and numerical simulation. Laboratory experiment was performed in a tilting flume 12.6 m long and 0.7 m wide using chamotte and sand as bed materials. The density of chamotte is 1.4 g/cm3 and its mean diameter is 1.3 mm, whereas the density of sand is 2.7g/cm3 and its mean diameter is 0.85 mm. Flow rate, bed slope and bed materials were changed in nine cases. Chamotte was used in six cases and sand was adopted in the other three cases. Bar formation was recorded by video recorder, and the height and length of bars are compared to those in the previous studies with sand. 2-d numerical simulations based on shallow water equations were also conducted to discuss the effects of chamotte on the bed configuration. We have obtained three findings. First, experiment with chamotte has provided similar bed patterns to those with sand. Second, chamotte with 1.3 mm diameter has successfully suppressed the formation of ripple and dunes since chamotte allows the flow at lower Froude number. Third, 2-d numerical simulation has reasonably reproduced the bars with chamotte. These claim that chamotte is useful to analyze the bar formation process in real rivers.