The library

When a subcritical flow motion in a horizontal channel reaches a drop at the downstream end of the flume, the flow becomes a free-jet. An important feature of the flow is the rapid variation in pressure distribution from hydrostatic upstream of the brink to quasi-atmospheric in the free-falling nappe.
For a free overfall at the downstream end of a smooth flat rectangular channel, the ratio of the brink depth to the 'critical flow depth' equals 0.71.
At the brink of the overfall, the pressure distribution differs from hydrostatic, with the pressure being atmospheric at both the free-surface and the lower nappe. Ideal fluid flow calculations show that dc is observed at a distance of about 3-4 times dc upstream of the brink, where the vertical pressure profile starts to differ from hydrostatic. Theoretical analyses predict further the end depth ratio (0.715) that has been verified experimentally. Different end depth ratios are observed for non-rectangular channels
A free overfall may be used as a discharge meter, although the data are very sensitive of the exact location of the brink depth. Further practical applications of overfall include drop structures, stepped cascades and dropshafts.
Hubert Chanson.