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Vertical Contraction Scour at Bridges with Water Flowing Under Pressure Conditions

One of the areas of bridge scour estimation technology that hydraulic engineers, researchers, and scientists generally agree needs more understanding is what happens to the bottom of the channel under a bridge when the low chord becomes inundated and the flow through the opening changes from free surface flow to pressure flow. The vast majority of what is known about local scour in bridge openings has been derived for bridges with water flowing with a free surface. The literature shows that no previously conducted research is capable of being used to predict the component of vertical contraction scour that would be encountered under a full range of sediment transport conditions. In an attempt to fill part of this gap in the field of bridge scour estimation technology, a comprehensive program was developed to investigate pressure flow scour in bridge openings. Data were collected so that a relationship could be developed to predict the scour that would occur across the channel as a result of a vertical flow contraction. An equation is presented that can be used to estimate the magnitude of vertical contraction scour that could occur at a bridge with water flowing under pressure conditions. The equation presented has a coefficient of determination of 0.89 and has readily obtainable independent variables. In addition to the equation that can be used to estimate the magnitude of vertical deck scour that may occur, techniques and guidance that could be applied by the practicing hydraulic engineer to estimate the effects of pressure flow conditions on local scour in bridge openings are also presented.