TABLE OF CONTENTS Bridges over streams and rivers pose challenging hydraulics problems. The flow constrictions caused by bridge abutments and piers give rise to additional energy losses due to the contraction of the flow lines at the upstream side and expansion of the flow lines on the downstream side of the bridge. Unlike the smooth and gradual channel contraction problems studied in Chapters 2 and 4, the flow constriction due to a bridge is abrupt. Therefore the resulting energy losses can be significant, and they need to be taken into account when the energy method is employed to calculate the water surface profiles through the bridge. If the momentum approach is used, then the external forces, such as the drag forces due to the bridge piers, need to be considered.
Another problem concerning hydraulic engineers is scour at bridges. By scour, we refer to erosion caused by water on the soil surrounding the bridge abutments and piers. Excessive scour during floods is a common cause of bridge failure. Therefore, it is important to estimate the scour depths accurately when designing new bridges or for evaluating the vulnerability of existing bridges.
The major portion of the discussion in the subsequent sections follows the procedures adopted in the Hydraulic Engineering Center River Analysis System (HEC-RAS) of the US Army Corps of Engineers (2002).
As we recall from Chapter 4, in the standard step method for gradually-varied flow calculations, the flow depths are determined at selected sections. The flow...
