When flood flows encounter a restriction in the natural stream, adjustments take place in the vicinity of the restriction. The portion of flow not directly approaching the bridge opening is redirected towards the opening by the embankment. The flow contracts as it enters the bridge and then expands as it exits the bridge. Maintaining the contraction and expansion of flow and overcoming friction and disturbances associated with piers and abutments requires an exchange of energy. An increase in the depth of flow upstream of the encroachment, termed backwater, reflects this energy exchange, as shown in Figure 9-1.