In subcritical flow conditions, the backwater tails off upstream until it reaches the normal water surface. The distance upstream over which backwater occurs depends on the channel conditions and flow conditions (see the in Chapter 7). The maximum backwater tends to occur in an arc around the opening as Figure 9-2 shows. The relatively steep water surface gradient between the maximum backwater and the opening is termed the drawdown area.

In a stream channel with supercritical flow conditions a constriction such as a bridge may not affect the upstream flow conditions. However, if the constriction is severe enough, it could cause a change in flow regime such that a backwater occurs upstream of the bridge and a hydraulic jump occurs near the bridge.

As the flow becomes constricted as it moves toward the bridge opening, the velocity increases, which can result in scour along the embankment and through the bridge. At the bridge headers, intersecting velocity vectors can cause severe turbulence and eddies as shown in Figure 9-3. Piers in the waterway create additional local turbulence and vortices. Turbulence, eddying, and vortices often result in scour.