As mentioned previously, there are four types of external boundary conditions that can be applied to the outer boundary of 2D Flow Areas.  These boundary condition types are: flow hydrograph, stage hydrograph, normal depth, and rating curve.  External boundary condition locations can be added to the geometry either in RAS Mapper, or in the Geometric Data editor.  The following explains how to do this from the Geometric Data editor.  To use RAS Mapper to do this, please review the HEC-RAS Mapper User’s Manual.

To add external boundary conditions to a 2D flow area, open the Geometry Data editor and select the tool (button) called SA/2D Area BC Lines (Figure 4-1).  Once the SA/2D Area BC Lines tool has been selected, the user can draw a line along the outer boundary of the 2D Area to establish the location of the boundary condition.  To create the external boundary condition, click the left mouse button one time at the location along the outside perimeter of the 2D Area where the boundary condition should start.  Next, add points by single clicking along the perimeter, then double click to end the boundary condition line at the location where it should end.  Once the user double clicks to end the boundary condition line, an HEC-RAS window will open and ask the user to enter a name for the new boundary condition.  In the example shown in Figure 4-1, two 2D flow area boundary condition lines were entered at the right hand side of the 2D flow area.  These boundary condition locations were given the name “DSNormalDepth1” and “DSNormalDepth2”; however, the user can enter any name desired.

The user can add any number of external boundary conditions to a 2D flow area.  For example, HEC-RAS allows one or more locations where a Flow Hydrograph boundary condition (or other types) can be connected to a single 2D flow area.  The user can also have one or more Stage Hydrographs linked to the same 2D flow area.  The user can have Rating Curves and Normal Depth boundary conditions at multiple locations to allow flow to leave the 2D area.

WARNING!

Two different external boundary conditions cannot be attached to the same cell face.  The user must start or end at the adjacent cell face.


Figure 4-1. Example of adding an External 2D flow area boundary condition location.
 

Once all of the 2D flow area boundary conditions have been identified (drawn with the SA/2D Area BC Lines tool), the boundary condition type and the boundary condition data must be entered (from the Unsteady Flow Data editor).  The Unsteady Flow Data editor is where the user selects the boundary condition type and enters the data for the created boundary condition (Figure 4-2).


Figure 4-2. Example of adding external boundary conditions directly to a 2D flow area.
 

As shown in Figure 4-2, the Select Location in table then select Boundary Condition Type lower table on the Boundary Conditions tab will contain all of the 2D flow area Boundary Condition locations that were entered in the Geometric Data editor (Figure 4-1).  To enter a 2D flow area boundary condition, select the appropriate field in the Boundary Condition column (Figure 4-2) for a particular location, then select the boundary condition type from the active Boundary Conditions Types (Figure 4-2) at the top of the window.  For example, when a 2D flow area is selected, there are only four types of boundary conditions available:  Stage Hydrograph, Flow Hydrograph, Rating Curve, and Normal Depth.  For the example shown in Figure 4-1, two boundary condition lines were established for the 2D flow area.  These two boundary conditions lines were added to allow flow to leave the 2D flow area using the Normal Depth (Manning’s equation) boundary condition method.  Boundary condition lines can also be placed along other parts of the 2D flow area to allow flow to enter.  In this case, the Flow Hydrograph (to bring flow directly into the 2D Area), or the Stage Hydrograph boundary condition type can be used. 

The following four sections (Flow Hydrograph, Stage Hydrograph, Normal Depth, and Rating Curve) describes the information that is required for each boundary condition type connected directly to a 2D flow area.

Flow Hydrograph

A flow hydrograph is generally used to bring flow into a 2D flow area; however, it can also be used to take flow out (negative flow values).  The required data for this boundary condition type is:

  1. Flow hydrograph (discharge, Q, versus time)
  2. Energy Slope (for computing Normal Depth)

The Energy Slope is used to compute a Normal Depth from the given flow rate and the cross-section data (underlying terrain data) along the Boundary Condition Line for each computational time step.   A flow distribution in the cross section is then computed (based on the normal depth water surface and the conveyance in the cross section underneath the 2D Boundary Condition line) and this flow distribution is used to appropriately distribute the flow to the cells along the boundary condition line that are wet.  At any given time step, only a portion of the boundary condition line may be wet, thus only the cells in which the water surface elevation is higher than their outer boundary face terrain will receive water.  However, if the computed Normal Depth water surface is higher than all the boundary face elevation data along the Boundary Condition Line, then all the cells will receive water based on a conveyance weighting approach.

Stage Hydrograph

A Stage Hydrograph can be used to bring flow into or take flow out of a 2D flow area.  If the water surface elevation in the Stage Hydrograph is higher than the cell water surface elevation (or dry elevation), flow will go into the 2D cells.  When the water surface elevation of the Stage Hydrograph is lower than the water surface in the 2D flow area, flow will go out of the 2D area.  If a cell is dry and the stage boundary condition is lower than the 2D flow area cell minimum elevation, then flow will not transfer.  The flow is computed on a per cell basis.  For instance, if the Stage Hydrograph water surface is higher than the water surface of some of the 2D boundary cells and lower than the water surface of other 2D boundary cells, then water will simultaneously enter and exit the Stage Hydrograph boundary.  The Stage Hydrograph boundary condition also has an option called “Use Initial Stage.”  When this option is turned on, the first stage in the hydrograph will be used to fill the 2D area as an initial condition.  Specifically, the filling starts at the boundary condition and then progressively fills any cell to that water surface if the cell is hydraulically connected.  Then the filling stops when the water surface elevation of the first stage hydrograph is lower than the water surface of the cell. 

In HEC-RAS Version 6.3 the option is available to specify spatially variable stage hydrographs along boundary condition lines. This new feature is useful for nesting HEC-RAS within other models. This feature is described in detail here. 

Normal Depth

The Normal Depth boundary condition can only be used to take flow out of a 2D flow area.  When using the Normal Depth boundary condition, the user is required to enter a friction slope for that area, which is also required for a 1D cross section location.  The friction slope can be based on the land slope in the vicinity of the 2D flow area boundary condition line. The Friction Slope is used within Manning’s equation to compute a Normal Depth water surface elevation for each given flow, based on the cross section underneath the 2D Boundary Condition line.  Just like the Stage Hydrograph boundary, the Normal Depth boundary is computed on a per cell basis.

Rating Curve

The Rating Curve option can only be used to take flow out of a 2D flow area.  The user is required to enter a stage (water surface elevation) versus flow relationship (rating curve) for this option.  The rating curve is also applied on a per cell basis.