HydroCAD® Stormwater Modeling - Since 1986
Modeling a Skimmer or Floating Outlet
Certain outlet control devices are designed to float on the surface of a pond, such that a constant head is maintained over a weir or orifice, resulting in a constant discharge regardless of water surface elevation. These devices are easily modeled by using the "Constant Flow/Skimmer" outlet added in HydroCAD-10. This option also allows the user to specify a "phase-in depth" that is required for the full design flow to develop.
Although earlier versions of HydroCAD can model these devices using the procedure described below, the process is greatly simplified with the new outlet. Update details here.
Modeling a skimmer with HydroCAD 9.1 and earlier
1) Edit the desired pond, select the Outlets tab, and create a "Special" outlet. (This may be the only outlet, or may be combined with an overflow weir, culvert, or any other arrangement of outlets.)
2) On the Special outlet screen, set the "Invert Elevation" to the elevation where you expect flow to begin. This will generally be somewhat higher than the bottom of the pond, since a certain depth is usually required to float the device and submerge the outlet.
3) Enter the following values in the head/discharge table:
Line Head Q
Line #1, in conjunction with the invert elevation, establishes the level at which the outlet starts to discharge. When the water reaches line #2 (1/10 of a foot higher) full flow is achieved through the outlet. Line #3 is used to prevent extrapolation of the discharge at higher heads.
Note: If you're using HydroCAD 7.1 or later, you can omit line #3 and deselect the "Extrapolate" option instead.
After creating the special outlet, OK the pond description, open a report window, and examine the stage-discharge plot. If you've set up the device correctly, the plot will show no discharge up to the invert, and then a sudden jump to the full discharge rate. The rate should then remain constant as the level increases.
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