| Summary: | 碩士 === 逢甲大學 === 水利工程所 === 95 === This research focuses on hydraulic models to explore the time variations of the local scour depths for two consecutive non-uniform piers with steady flows. The upper and lower pier diameters are 20mm and 50mm, respectively, i.e. the pier diameter ratio D/D* is a constant of 0.4. In addition, there are five different longitudinal intervals between the two consecutive piers (Ld =1.25, 2, 3, 4 and 5D*), and 15 different initial bed levels in the experimental design.
The time to equilibrium for the local scour of the downstream pier increases with the increase of the distance between the consecutive piers. The local scour of the downstream pier could extend upstream, which causes the secondary scour of the upstream pier. The phenomenon of alternate scour and deposition between the consecutive piers terminates as the downstream pier reaches the equilibrium condition. This is the major difference for the scour processes between the consecutive two piers and the single pier.
Probing into the change of riverbed’s scouring and deposition in different intervals of consecutive two piers, the downstream pier is protected by the upstream pier because the flow intensity is reduced due to the energy dissipation caused by the upstream pier. When Ld =1.25D*, the sheltering effect of the upstream pier is negligible and the case could be regard as single pier scouring phenomenon.
While comparing three kinds of initial bed level, the volume decrease ratio of the scour hole reduces as the interval of the pier increases. In words, the volume decrease ratio of scour hole is the best when the interval of piers is 5D* under the condition that the bridge foundation is un-exposed, and it’s a ideal allocation interval of piers. As the downstream non-uniform pier is almost not influenced by the upstream pier scour, the interval of piers 4D* is a critical interval for non-uniform pier scour. When Ld =1.25D*, it has a better volume decrease ratio for the scour hole because of the sheltering effect under the condition that the bridge foundation is exposed.
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