Internal Flow Hydrograph


The user has the option to add a flow hydrograph as an internal boundary condition.  To add an internal boundary condition line, from the Geometric Data editor, select the SA/2D Area BC Lines tool, then draw an internal line inside of the 2D Flow Area (Figure 4-3).  The internal boundary condition (BC) line can encompass one or more 2D cells.  Once the line is drawn, a name for the BC line must be entered. Figure 4-3 provides an example of a created internal BC line named “Internal BC,” as well as an example of an external BC line (labeled “edge BC”).



Figure 4-3. Example Internal BC Line for Attaching a Flow Hydrograph inside a 2D Area.
 

After an internal BC line is drawn in the Geometric Data editor, the user can open the Unsteady Flow Data editor (opened from the HEC-RAS main window by clicking the   button) to attach flow hydrographs to the internal BC lines.  Once the Unsteady Flow Data editor is open (Figure 4-4), select the Boundary Conditions tab, and view in the “Storage/2D Flow Areas” table that there is a row for the newly created internal boundary condition line(s) (e.g., 2D Interior Area BCLine: Internal BC in Figure 4‑4) that were created in the Geometric Data editor (Figure 4-3).


Figure 4-4. Unsteady Flow Data Editor with Example Internal BC Line.

As shown in Figure 4‑4, the created internal BC lines will be listed in the boundary conditions table, and will allow the user to attach a flow hydrograph to a selected BC line.  If the BC line crosses more than one cell, flow is distributed across the cells based on the percentage of the line length that crosses that cell.  For example, if a cell contains 20% of the BC line length, then the cell will receive 20% of the flow each time step.

Precipitation

This Precipitation option can be used to apply rainfall excess (rainfall minus losses due to interception and/or infiltration) directly to a 2D flow area.  To apply a precipitation boundary condition to a 2D flow area (or storage area), open the Unsteady Flow Data editor and select the button that is labeled Add Storage Area from the Add Boundary Condition Location table.  Select the 2D flow area of interest to add to the table.  Once the 2D flow area is in the Select Location in table then select Boundary Condition Type lower table, select the blank field under the Boundary Condition column (Figure 4-4), then select the Precipitation boundary condition type.  The Precipitation boundary condition type selection opens an editor that will allow users to either read the precipitation data from HEC-DSS or enter the data as a time series into a table directly.  Precipitation is applied equally to all cells within the 2D flow area.

NOTE: This is the older method for adding precipitation data into HEC-RAS.  Newer capabilities have been added to HEC-RAS Version 6.0 that allow for spatially distributed rainfall.  See the section below on “Global Boundary Conditions for further information."