Flow Redistribution at Bridge Contractions in Compound Channel for Extreme Hydrological Events and Implications for Sediment Scour

Abstract

This study investigates the flood flow characteristics in a compound channel subject to both lateral contraction (caused by bridge abutment/embankment) and vertical contraction (caused by bridge deck submergence) at bridge sites. Three abutment setback distances and three pressure flow types [free surface (FS), submerged orifice (SO) and overtopping (OT)] are tested. The results show that turbulence structures in the approach channel remained the same irrespective of downstream obstruction. However, lesser abutment setback from the main channel led to greater flow acceleration as the flow approached the bridge section, where SO flow had the highest flow intensity and FS flow had the lowest. At a bridge contraction, the distributions of flow velocity, turbulence intensity, and Reynolds shear stress are significantly affected by the type of pressure flow and the extent of contraction. Enclosed counterclockwise secondary circulating flows in the main channel may occur for a combination of long or medium setback abutments and a FS flow, while other conditions usually feature open-ended upward flows due to flow relief. The strong downslope flow component detected at the main channel bank has a noticeable bank erosion capability. The bed shear stress, which is an indicator of sediment scour, is highlighted by apparent peak zones at the main channel bank and abutment toe. Finally, clear relationships between turbulence intensity, unit discharge ratio, and lateral contraction length are found. In general, normalized turbulence intensities on the floodplain and in the main channel can be used to assess the contribution of macroturbulence to the final bed topography after scour.