Energy Balance Equation

The Energy Balance Equation, Equation 6‑1, relates the total energy of an upstream section (2) along a channel with the total energy of a downstream section (1). The parameters in the Energy Equation are illustrated in Figure 6‑1. Equation 6-1 now can be expanded into Equation 6-11:
EquationObject205226
Equation 6-11.
where:
  • z
     = elevation of the streambed (ft. or m)
  • d
     = depth of flow (ft. or m)
  • α = kinetic energy coefficient
  • v
     = average velocity of flow (fps or m/s)
  • h
    f
    = friction head loss from upstream to downstream (ft. or m)
  • g
     = acceleration due to gravity = 32.2 ft/ s
    2
     or 9.81 m/s
    2
    .
The energy grade line (EGL) is the line that joins the elevations of the energy head associated with a water surface profile (see Figure 6‑1).
EGL for Water Surface Profile
Figure 6-1. EGL for Water Surface Profile