Multi-Event Links
A link may be used for a number of purposes, such as importing hydrograph data from a file. This feature is most often used to interconnect several sections of the same project. In this application, the data is communicated between the projects by way by an external hydrograph file ("HCE" file), which is managed automatically by the software.
However, an HCE file may also be used for other applications in which the file is created using data from another source. Such applications require the user to understand the exact format of the file, including the ability to incorporate data from multiple events within a single file. This feature can be used to:
 | Define a base-flow that changes automatically for each event |
| Automatically import a different off-site hydrographs for each event |
Sample Multi-Event file
Below is a simple HCE file that defines a hydrograph for two different rainfall events. Each [hydrograph] section defines a separate hydrograph. Note that both hydrographs have the same Name, but each one provides data for a separate Event.
FileUnits=English
Comment=Multi-Event Import example
[hydrograph]
Name=Off-Site Inflow
Event=2-Year
Area=566280
t0=0
dt=3600
Flow= 0 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 0
[hydrograph]
Name=Off-Site Inflow
Event=5-Year
Area=566280
t0=0
dt=3600
Flow= 0 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 0
The following data is defined within each hydrograph:
to (tee-zero) is the starting time for each hydrograph, in seconds. This may be zero, or some larger value. This parameter is comparable to the beginning of the calculation time span for each project, although the two values do not have to be the same. (e.g. HydroCAD can automatically handle dissimilar hydrographs.)
dt (dee-tee) is the time increment between successive flow values, in seconds. This is comparable to the time increment used within each project, but does not have to be the same.
Flow values appear on one or more lines, separated by spaces. The first value defines the flow at time to. The second value defines the flow at time to+dt, etc. Flow values may be in CFS or m³/sec, depending on the FileUnits parameter at the beginning of the file. When defining the flow, note that a complete multi-point hydrograph is required. A single flow value will define the flow at only one point in time, and the flow will be zero at all other times.
Area is the watershed area represented by this hydrograph. This value is optional. If present, it allows HydroCAD to report volumes as an equivalent depth, as well as the standard volumetric representation. The value may be in square-feet or square meters, depending on the FileUnits parameter at the beginning of the file.
Defining a constant flow
Although a link will usually be used to import a time-varying flow (a hydrograph), there may be cases were the flow should be constant at all times.
| For HydroCAD 7.13 and later, a constant flow may be defined by
using a single flow value and omitting the dt parameter.
If desired, you can cause the flow to start at a specified by using the
appropriate to value. If you use to=0 (or omit the line
entirely) the flow will start at time zero. |
| For earlier versions of HydroCAD, a constant flow can only be specified by defining a complete multi-point hydrograph, using the same value for each flow parameter. For example, to define a constant flow over ten hours you could use: |
t0=0
dt=3600
Flow= 2 2 2 2 2 2 2 2 2 2 2The hydrograph starts at time zero and uses a time increment of 1 hour (3600 seconds). Note that the Flow line contains eleven values for hour 0,1,2...9,10
Creating your own file
You may create your own HCE file by following the sample format shown above. You may use any text editor that is able to create a plain-text file. Make sure you save the file with an .HCE extension, otherwise HydroCAD will not recognize the file.
To be sure you use the correct formatting, you may want to use this sample HCE file as a template. You will also find additional details in the LinkTest file that is installed in your HydroCAD\Projects folder.
Using your multi-event file
To use your data in a HydroCAD project, create a link node and use the "File" option to select your HCE file. The outflow of the link will consist of your specified hydrograph, and may be routed just like any other node.
Important: Whenever you use a multi-event link, the associated project must contain rainfall events with the exact same event names used in the data file. The events must be defined on the Rainfall tab of the calculation settings screen.
Demonstration multi-event link
You may download the following files to demonstrate the operation of the multi-event link capability. Right click each file and select the "Save" option, placing both files in any desired folder.
After saving both file, click (or double-click) the project file to open it in HydroCAD. Then double-click the link to import the data and see the resulting hydrograph. Whenever you change the event selection (using the drop-down list on the tool bar) the corresponding hydrograph will be automatically imported from the file.