The river system schematic is required for any geometric data set within the HEC-RAS system. The schematic defines how the various river reaches, storage areas, and 2D flow areas are connected, as well as establishing a naming convention for referencing all the other data. The river system schematic is developed by drawing and connecting the various hydraulic elements of the system within the geometric data editor (see the HEC-RAS Mapper User's manual for how to layout the model schematic in a geospatial manner. See "Modeling Bridges" of the HEC-RAS User's Manual for details on how to develop the schematic from within the user interface). The user is required to develop the river system schematic before any other data can be entered.
Each river reach on the schematic is given a unique identifier. As other river reach data are entered, the data are referenced to a specific reach of the schematic. For example, each cross section must have a "River", "Reach" and "River Station" identifier. The river and reach identifiers defines which reach the cross section lives in, while the river station identifier defines where that cross section is located within the reach, with respect to the other cross sections for that reach. The connectivity of reaches is very important for the model to understand how the computations should proceed from one reach to the next. The user is required to draw each reach from upstream to downstream, in what is considered to be the positive flow direction. The connecting of reaches is considered a junction. Junctions should only be established at locations where two or more streams come together or split apart. Junctions cannot be established with a single reach flowing into another single reach. These two reaches must be combined and defined as one reach. An example river system schematic is shown in the figure below.
The example schematic shown in the figure above is for a dendritic river system. Arrows are automatically drawn on the schematic in the assumed positive flow direction. Junctions (red circles) are automatically formed as reaches are connected. As shown, the user is require to provide a river and reach identifier for each reach, as well as an identifier for each junction.
HEC-RAS has the ability to model river systems that range from a single reach model to complicated networks. A "network" model is where river reaches split apart and then come back together, forming looped systems. An example schematic of a looped stream network is shown in the figure below.
The river system schematic shown in the figure above demonstrates the ability of HEC-RAS to model flow splits as well as flow combinations. The current version of the steady flow model within HEC-RAS does not determine the amount of flow going to each reach at a flow split, unless the user turns on the split flow optimization option.