| Summary: | 碩士 === 逢甲大學 === 水利工程與資源保育研究所 === 97 === The purpose of this study is to use the hydraulic model to increase our understanding of the mechanism of the local scour around non-uniform bridge piers. The tests were conducted under the following conditions: steady flow ( V/Vc = 0.6,0.7,0.8,0.9,0.99), non-uniform bridge pier-diameter ratioD D* = 0.4, two sediment standard deviations of 1.22 (uniform sand) and 2.88 (non-uniform sediment), and five soil heights (Y=0、±1、±2).
The results show that local scour depth in front of the pier is very small when the initial bed level is same as the non-uniform plane (Y=0 cm) under all five relative flow intensities. It was probably because the downflow was
retarded by the foundation top. Local scour depths for the conditions where the bridge foundation is unexposed(Y=+1、+2cm) were less than those where the bridge foundation is exposed(Y=-1、-2cm). In addition, the scour depth increases with an increase of the exposure depth.
Under the uniform sediment condition, when the initial bed level relative the foundation top elevation is same, both the scour depth and the time to equilibrium increase with an increase of relative flow intensity. On the other hand, under the non-uniform sediment condition, with the same
initial bed level relative to the foundation top elevation, the time to equilibrium only increases slightly with an increase of relative flow intensity which was probably due to the hiding and armoring effects. The scour depth usually reaches the equilibrium condition at the initial stage, and is less than that under the uniform sediment bed.
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