Effect of complex shape of pier foundation exposure on time development of scour

Local pier scour has been recognized as one of the most significant causes of bridge failures. Despite numerous studies on the subject of local pier scour have been conducted, there remain problems in accurate scour prediction because almost of the past studies have focused only on scour depth around simple shape of a bridge pier with or without considering their time development, even though prototype bridges usually have complex pier configurations. Thus, the objective of this study is investigation of the scour process around complex shape of pier including various shape of foundations and columns other than cylindrical shape over time for the purpose of understanding the relationships between hydraulic variables and resulting time development of scour. In this study, two field sites in Georgia, USA were selected, and corresponding scale-down bridge models and river geometries were fabricated in a hydraulic laboratory to address objectives of this study. Three different sizes of sediment and several geometric scales of bridge pier models were used to investigate the effect of relative sediment size, which is defined as the ratio of the pier width to the median sediment size. Through the visualization experiment, it is observed that exposure of complex shape of pier footing has important role for the temporal variations of scour depth over time. In addition to the quantitative visualization analysis, based on the experimental results, scouring rate formulas are suggested with the help of dimensionless, effective flow work concept according to the value of the relative sediment size. Findings in this research can be used to improve scour prediction methods that currently tend to overpredict bridge scour depth.