Water Resources and Environmental Engineering

Critical Depth

The depth at which a given open-channel flow has the least specific energy and a Froude number of 1.

For a given discharge, specific energy E = y + V²/(2g) is lowest at one depth, the critical depth. In a rectangular channel it is y_c = (q²/g)^(1/3), with q the discharge per unit width, and there E = 1.5·y_c.

Flow deeper than critical is subcritical, slow and controlled from downstream; shallower is supercritical, fast and controlled from upstream. Flow passes through critical depth at a free overfall or the crest of a broad weir, which is why those make good flow-measuring sections.

How much of the FE Civil exam is Water Resources and Environmental Engineering?