Subcritical Flow Over Highway Embankments

A highway embankment, when overtopped by flood waters, is a form of broad-crested weir. As a weir, the flood discharge over the embankment is only a function of an upstream depth for free flow conditions. Free flow exists when critical depth occurs on the roadway, usually near the crown line. If a structure or vegetation downstream from the embankment controls the stage-discharge relationship, the depth of flow at the embankment may be raised sufficiently to prevent the occurrence of critical depth on the roadway. If critical depth does not occur, then subcritical (submerged) flow exists over the embankment. A method for determining the discharge under submerged flow conditions using an upstream and a downstream flow depth is presented. The method is illustrated using data collected from model highway embankments.