Understanding Supercritical Flow: Q^2/g^1/3

[fonseh]

Homework Statement

How to show that the dE/dy = 0 , yc will become (Q^2 / g)^1/3 ?
http://www.brighthubengineering.com...nnel-flow-basics-2-supercritical-flow/#imgn_0

Homework Equations

The Attempt at a Solution

E = y + (V^2) /2g , dE / dy = 1 ... why yc will become (Q^2 / g)^1/3 ?[/B]

 

[haruspex]

See eqn 8 at https://en.m.wikipedia.org/wiki/Energy–depth_relationship_in_a_rectangular_channel