Stream Junction Data

Stream junctions are defined as locations where two or more streams come together or split apart. Junction data consists of reach lengths across the junction and tributary angles (only if the momentum equation is selected). Reach lengths across the junction are entered in the Junction Data editor. This allows for the lengths across very complicated confluences (e.g., flow splits) to be accommodated. An example of this is shown in the figure below.

As shown in the figure above, using downstream reach lengths, for the last cross section in Reach 1, would not adequately describe the lengths across the junction. It is therefore necessary to describe lengths across junctions in the Junction Data editor. For the example shown in the figure above, two lengths would be entered. These lengths should represent the average distance that the water will travel from the last cross section in Reach 1 to the first cross section of the respective reaches.

In general, the cross sections that bound a junction should be placed as close together as possible. This will minimize the error in the calculation of energy losses across the junction.
In HEC-RAS, for steady flow hydraulic computations, a junction can be modeled by either the energy equation (

of "Theoretical Basis for One-Dimensional and Two-Dimensional Hydrodynamic Calculations") or the momentum equation. The energy equation does not take into account the angle of any tributary coming in or leaving the main stream, while the momentum equation does. In most cases, the amount of energy loss due to the angle of the tributary flow is not significant, and using the energy equation to model the junction is more than adequate. However, there are situations where the angle of the tributary can cause significant energy losses. In these situations it would be more appropriate to use the momentum approach. When the momentum approach is selected, an angle for all tributaries of the main stem must be entered. A detailed description of how junction calculations are made can be found in "Overview of Optional Capabilities" of this manual.

For Unsteady flow computations, HEC-RAS has two options for the hydraulic computations at a junction. The default option is a very simple assumption that the water surface computed at the downstream side of a flow combining junction, is used for the cross sections just upstream of the junction. If this is not a good assumption (such as for steeper river systems), there is an option to perform an energy balance across the junction in order to compute the upstream water surface elevations.