HydroCAD® Stormwater Modeling - Since 1986
This document discusses general issues about the use of a "link" within HydroCAD.
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, or time multiplier.
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.
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.)
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.
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.
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).
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.
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.
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