[jarvus lunt]

I studied this before.

For 8-1

H = Zu - Zsp

For 8-2

H = Zu - Zd

For 8-1 the Head is the energy grade minus the SPILLWAY CREST.

For 8-2 the Head is the energy grade minus the DOWNSTREAM WATER SURFACE.

The program switches from 8-1 to 8-2 at 0.67 submergence. The 3 comes from 1 / (1-0.67) at the transition point.