The discussions of bridge design assume normal lengths and alignments perpendicular to the flow at flood stage even though the low flow streambed may be at a skew to the bridge. In actuality, many bridges are skewed to some degree which causes the hydraulic length of the bridge to be longer than the plan width. For example, a 60-foot wide bridge perpendicular to the stream has a hydraulic length of 60 feet. The same bridge at a 30º skew to the stream has a hydraulic length of 69.3 feet.

Hydraulic programs do not automatically account for the skew. The program operator is required to specifically account for the skew by putting in the correct hydraulic length.