HydroCADģ Stormwater Modeling - Since 1986

Using a link to model a large watershed

A link is frequently used to connect two (or more) routing diagrams that are part of a single watershed. This may occur if the watershed exceeds the capacity of your HydroCAD system and you need to break it into smaller pieces for analysis. A link allows you to automatically import a hydrograph from another diagram, in effect reconnecting the pieces. The link's outflow can then be routed on the diagram, just like any other flow.  For this purpose you should use a File Link.


We can model the following system as two smaller diagrams, using a link to connect them.

Complete project to be split

1) The first step is to decide where to split the diagram. Since each routing connection we "break" will require one link, try to break as few connections as possible. One possibility is to split the diagram between Reach 1 and Pond 1. First we create a "project" for the "upstream" portion of the watershed as shown below on the left. This project should be called something like "My Project - Part A," and ends with reach 1.

Part A: Upstream portion of project    Part B: Downstream portion of project

2) Next, turn on the automatic-export option in the upstream portion:

+ For HydroCAD 6.1 and later: Select Settings|Export and select "Unrouted Outflow Hydrographs."
+ For HydroCAD 6.0: Select Settings|Calculation and set the Export Mode to "unrouted nodes."
+ For HydroCAD 5.11 and earlier: Select
RECALC ALL to make sure all flows in this diagram are up-to-date.

3) Verify the calculation settings for the upstream portion of the project. Since rainfall information must be set independently for each section of the project, itís very helpful to define each of the applicable rainfall events.  This will allow you to select between the defined events without having to edit each of the interconnected files.  (Event definitions require HydroCAD 7.0 or later.)

4) Close and save the upstream project.  As the file is saved, HydroCAD will automatically update the calculations for all events and export the final flows for use below.

5) Create a second project for the "downstream" portion of the watershed.  We'll name it "My Project - Part B." The significant feature of the downstream diagram (shown above) is the addition of Link 2. The link is created and routed just like any other node.

+ For HydroCAD 6.0 and later: Edit the link, select "File" link, and browse to the final node of the upstream project.  Click OK.
+ For HydroCAD-5.11 and earlier: Edit the link and enter the following information:

PROJECT: My Project - Part A

(See the User's Guide for other options.)

6) Make sure the lower portion of the projects uses the exact same event definitions as the upper portion. (The easiest way to do this is to import the events from the upper project.) This will allow you to select any event in the lower project and the link will automatically import the corresponding hydrograph from the upper project. If you donít use events, youíll have to re-open the upper project each time you need to change the rainfall.

7) You're done!  When "Part B" is calculated, the final outflow from "Part A" (Reach 1) will be automatically imported and routed in "Part B". When using a link it is your responsibility to be sure that the rainfall (and other relevant settings) are the same in each diagram. (If you're using HydroCAD 3.2 or earlier, after making any changes to the upstream portion of the watershed you must select the RECALC LINKS option in the downstream project. Later versions detect link changes automatically.)

Note: Starting with HydroCAD-7, each link will automatically import data for multiple storm events.  To allow this feature to operate, you must use event definitions in each project with matching names.  When multiple events are defined, the exporting (upstream) project will export data for all events, while the link in the downstream project will automatically select the data for the current event.  This makes it possible to examine multiple events without switching back-and-forth between the upper and lower portions of the project.  (When using Rational method you are still responsible for setting the same duration in each project.)  For further details, read about multi-event links.

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