In the specific energy curve, why the angle in it is 45`?

Question

in the hydraulic jump.

Ec=critical energy.

Dc=critical depth.

Answer 1

The 45degree line is the ideal energy line.  It assumes ENERGY IN = ENERGY OUT so it is a 45 degree angle.  But, there is an energy loss through the hydraulic jump due to turbulence.  So the actual curve is asymptotic to the 45 degree line.  The line represents sub-critical flow so there is no jump, thus no energy loss.  Nothing magic about 45 degrees.  You can adjust the scales and get any angle you want.  45 is just convention.