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米兰理工大学Alessio Radice副教授:Bridge pier scour in live-bed:from a case-study prototype to an investigation of the process mechanisms

发布日期:2018-05-30访问次数: 字号:[ ]


国际水利与环境工程学会试验方法设备专业委员会主席、米兰理工大学Alessio Radice副教授做《Bridge pier scour in live-bed:from a case-study prototype to an investigation of the process mechanisms》的学术报告

yh86银河国际官网水工水力学研究所副所长王晓刚教授为Alessio Radice副教授颁证书

Abstract: Local scour at river bridges has been a challenging thrust on hydraulic research for several decades, mostly due to the significant implications of this process for structure and people vulnerability to high flows. The scour process has been studied taking a variety of approaches, the experimental approach being a leading one.

The manuscript documents a 15-year research activity on live-bed scour at river bridge piers. A laboratory-scale investigation for an existing river bridge is first described. The bridge under consideration is situated in Northern Italy across the Po, that is the largest river in Italy, 650 km long and with a hydrographic basin of 70,000 km2. The complex piers of the bridge are compound of two rows with three piles each. Large scour depths can be expected considering the possibility of debris accumulation against the pier. Therefore, experiments were performed to quantify the effect of trapped debris on the scour depth and to design a prototypal countermeasure for local scour in the presence of a significant amount of floating debris. In these experiments, achieving a full similarity for the sediment size was impossible, due to already fine sediment being present at the bridge section. Therefore, the laboratory model implied a partial similarity, with the ratio of the flow velocity to its threshold value for sediment transport being the key dimensionless ratio driving the choice of the hydrodynamic conditions for the experiments. The scour depth was measured using a non-touch, automatically moved optical proximity sensor. A countermeasure was conceived as a ‘roof’, or cover, supported by sacrificial piles, to be placed just upstream of the pier. The elevation of the cover was lower than maximum expected water levels, as the most dangerous situation was identified with lowering flow depth during the tail of the hydrographs. 

In the second part of the paper, attention is moved to laboratory experiments for an idealized pier (single, circular pile), describing a recent campaign that enabled different facets of the process mechanism to be investigated. Two aspect under consideration were a dune control of the scour fluctuations after attainment of a mean equilibrium scour depth, and the scour process under unsteady flow conditions, that has been studied in clear-water scour but never in live-bed scour. Measurements of the bed elevation were taken by laser distance sensors at different locations around the pier. The results suggested that a transformation of the incoming dunes took place as soon as they entered the scour hole, before they reached the bottom. Consequently, the slope of the scour hole continuously changed in time, differently from what is typically considered in analytical models of the process. The investigation of the live-bed scour process under unsteady flow revealed that the scour progression is affected by the multiple changes of the flow rate, and different plateaus seem to be reached during the time intervals with a certain discharge. These plateaus are not necessarily characterized by the same scour depth that is achieved in a steady experiment with the same flow rate, nor asymptotic values for a certain flow rate are equal in the rising and descending phases.

Based on the latest results obtained, some prospects are provided to orient future research. On the one hand, incorporation of the geotechnical behavior of the upstream hole slope will improve the capability to reliably reproduce the process. On the other hand, further analysis of pier scour in unsteady flow conditions shall clarify how the scour depth in a real situation (that is by definition unsteady) can be represented by an experimental run performed with a steady flow.

Keywords: River bridge; local scour; live-bed; experiments

 






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