Estimation on afflux of bridge on plain rivers related to blockage ratio and Froude number

Abstract

The construction of hydraulic structures such as abutments and piers in natural waterways contributes to the contraction of river channels and the upstream water level rise. The maximum increase in the water level (afflux) is considered to be a major flood risk after a new bridge is built, and the accurate estimation of it is always an important work in designing a bridge construction. However, systematic studies on the prediction of the backwater effects of piers considering the blockage ratios within 10% are rarely reported. In this paper, a 2D hydrodynamic model was numerically simulated to investigate the effect of blockage ratios in the range of 2% - 9% on the amount and location of afflux. The distribution of depth-averaged flow velocities and water level changes around bridge piers was also discussed. Results indicate that the development and attenuation of backwater effects along the river channel falls naturally into 4 regions, and as the blockage ratio gets greater, afflux increases with its location moving farther from the zone strongly influenced by the bridge piers. It can be concluded that a blockage ratio of more than 7% should be one of the key concerns to river engineers in evaluating the capacity of river channels in plains to carry flood flows. On this basis, this article presented a parametric optimization formula based on blockage ratio and Froude number to estimate afflux of rectangle bridge piers with relatively low blockage ratios on plan rivers, and to provide effective assistance in hydraulic calculation of bridge design.