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Storm Drains Junctions Losses Calculation

For adjoining pipes to be considered a pipe junction, the node and only two inflow pipes (a lateral and a trunk) may enter the junction. The minor loss equation for a pipe junction is in the form of the momentum equation. In Equation A the subscripts “i”, “o”, and “1” indicate the inlet, outlet, and lateral, respectively.
Equation A


  • hj = junction head loss (ft. or m)
  • Q = flow (cfs or m3/s)
  • v = velocity (fps or m/s)
  • A = cross-sectional area (sq. ft. or m2)
  • = angle in degrees of lateral with respect to centerline of outlet pipe
  • g = gravitational acceleration = 32.2 ft/s2 or 9.81 m/s2.

The above equation applies only if vo > vi and assumes that Qo = Qi + Q1.