This section of the manual discusses how to perform stream junction analyses for steady flow water surface profile calculations. For 1D Unsteady Flow analyses, see "Modeling Bridges" of the HEC-RAS user's manual.

Stream junctions can be modeled in two different ways within HEC-RAS. The default method is an energy based solution. This method solves for water surfaces across the junction by performing standard step backwater and forewater calculations through the junction. The method does not account for the angle of any of the tributary flows. Because most streams are highly subcritical flow, the influence of the tributary flow angle is often insignificant. If the angle of the tributary plays an important role in influencing the water surface around the junction, then the user should switch to the alternative method available in HEC-RAS, which is a momentum based method. The momentum based method is a one dimensional formulation of the momentum equation, but the angles of the tributaries are used to evaluate the forces associated with the tributary flows. There are six possible flow conditions that HEC-RAS can handle at a junction:

  1. Subcritical flow - flow combining
  2. Subcritical flow - flow split
  3. Supercritical flow - flow combining
  4. Supercritical flow- flow split
  5. Mixed flow regime - flow combining
  6. Mixed flow regime - flow split

The most common situations are the subcritical flow cases (1) and (2). The following is a discussion of how the energy method and the momentum based method are applied to these six flow cases.