Link FAQ
This document discusses general issues about the use of a "link" within
HydroCAD.
What
is a link?
A link is a multi-purpose node that can be used to perform a number of
specialized operations:
1) Sum on-screen inflows (from other nodes)
2) Enter a hydrograph manually.
3) Link a hydrograph from another project.
4) Import a hydrograph from a file.
5) Adjust a hydrograph by applying a flow threshold,
flow limit, flow multiplier, time multiplier, time limit(s), or volume limit.
6) Split an inflow into two separate hydrographs.
7) Define a tailwater elevation for an inflowing
node.
Details on each of these options is provided below.
1) Sum on-screen inflows
To sum several the outflow from several nodes, just route each outflow to the
link. The outflow from the link will be the sum of all the inflows.
Although you don't need to set any of the link parameters for this purpose, it's
a good idea to enter the node "name", so that HydroCAD knows to open a report
window when you double-click the node. (Note: You can also sum
hydrographs with any reach or pond.)
2) Enter a hydrograph manually
If you select "Enter Hydrograph Manually", you can enter a custom hydrograph
with any desired starting time and time increment. You can also
right-click the table to load certain file formats into the table.
Starting with HydroCAD 9.1, you can select a specific
rainfall event for which the hydrograph will be used. Otherwise, the
hydrograph will be generated regardless of the event selection. Starting
with HydroCAD 10.0 build 17, a link can also be used to define a different
hydrograph for each event. For
full details, click the Help button on the
Manual tab.
3) Link a hydrograph from another HydroCAD project
This option allows you to import a hydrograph from a special .HCE
file format. Although you can create these files by hand, this
option is most often used to link several project files
together.
4) Import a hydrograph from another program or
spreadsheet
There are three basic options for importing hydrograph data from another
source:
A) Format the data as an HCE file, as described above. -or-
B) Select the "Manual" option, right-click the hydrograph table, and select
"Load from file". This requires that the data be formatted with one flow
value per line, and is not recommended for a large number of flow values (over
100). -or-
C) HydroCAD 8.5 (and later) can import hydrograph data
directly from specially formatted text (.TXT) or
comma-separated-value (.CSV) files, as generated by a number of hydrology
programs. The CSV format can be used to import hydrographs with a variable
time step. For details, see the "Link Test" files installed in the
HydroCAD\Projects folder.
5) Adjust a hydrograph
The Advanced tab allows you to make special
adjustments to the link outflow.
A flow threshold causes only the portion of flow that exceeds this
value to appear in the primary outflow. The remainder of the inflow will
appear as a secondary outflow.
A flow limit will prevent the outflow from exceeding the specified
value. Any excess flow will appear at a secondary outflow.
A discharge multiplier will adjust all flow values by the specified
coefficient. A value less then one will reduce the flow, with the
remainder appearing as the secondary outflow.
A time multiplier scales any external inflow to a different duration.
Click the Help button for details.
A time lag will delay (translate) the outflow hydrograph by the
specified time. (New feature added in HydroCAD-8).
A Start Time and/or End Time can be used to retain only the
portion of the hydrograph bounded by those times. The remainder of
the inflow will appear as a secondary outflow. (New feature added in
HydroCAD-10.1).
A Slice Volume will retain the specified volume at the start of the
hydrograph. The remainder of the inflow will appear as a secondary
outflow. (New feature added in HydroCAD-10.1).
6) Split a hydrograph
You split an inflow hydrograph according to a known ratio and/or flow
threshold by using the adjustment parameters described above.
These parameters will alter the primary outflow, and the secondary outflow will
contain the remainder of the inflow hydrograph.
7) Define a tailwater elevation
In most applications, tailwater information is
generated automatically when one node is routed to another. But if the
tailwater is being generated at a point outside the HydroCAD model, you
can often use a link to define the tailwater. The
Elevation tab provides several options, including a fixed, tidal, or
tabular tailwater.
Using a link to create a tailwater condition allows inflowing ponds to adjust
their discharge in response to the water surface elevation of the link. If
the WSE elevation of the link exceeds the WSE of the pond, a reverse-head
warning will be issued. However, water will not automatically flow
from the link to the pond. The HydroCAD model will only allow flow in the
direction of the outflow arrow, as if a one-way valve were present.
Starting with HydroCAD 9.1, you can select a specific
rainfall event for which the tabular tailwater will be used.
Otherwise, the tailwater will be generated regardless of the event selection.
Starting with HydroCAD 10.0 build 17, a link can also be used to define a
different tailwater table for each event. Click the Help button for
full details.
