| Summary: | 碩士 === 國立交通大學 === 土木工程學系 === 99 === Due to rivers with steep gradients and excessively concentrated rainfalls in Taiwan, floods generate severe local scours around bridge piers that reduce the bearing strength of pier foundations; more seriously they will result in the collapse of bridges. Present engineering methods of constructing gabions, groundsill works, ripraps, and armor units for pier protection are taking lots of time and uncertainly reaching its best effectiveness; therefore, it is necessary to develop the new modes of engineering methods for bridge pier protection.
This research is to study on the local scour under the condition of clear water scouring with a pair of symmetrical submerged vanes upstream of bridge pier. Local scour at bridge piers occurs mainly because of the down flow and horseshoe vortex, the two major mechanisms. This experiment is conducted to study different layouts of submerged vanes and vortex fields between the vanes and the bridge piers that both affect the scour depth on the river bed, and estimate the protective effectiveness of submerged vanes against the local scour around bridge pier; moreover, it studies on the field observation of local scours at bridge piers with the method of non-intrusive color particle image velocimetry to understand the variation of 2-D flow field of local scours at bridge piers. In addition, the field images captured by charge-coupled devices will be analyzed to obtain the velocity vector of the field and the layouts of vortex field. When the constant flow is 0.007cms (Froude number, Fr = 0.35), same bed material ( mm), the diameter of the pier is 2cm, and the length of submerged vane is 5cm, the result of this experiment testing on local scours at bridge piers and vortex fields by estimating the river bed and conducting field experiments shows when the angle of attack of the submerged vane is round 15°, the distance between the submerged vane and pier is three times length of the pier diameter, the front opening of submerged vane is 0.5 times length of the pier diameter, and the submerged water is around 0.48 times than the average depth of water; reduce the scouring depth to 54.6%, further, under the condition of greater scouring submerged vanes with the most effective layouts still function well.
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