HydroCAD^® Stormwater Modeling - Since 1986

Sample Pond #1

The following photos illustrate an earth-berm detention pond in a residential subdivision.  The outlet device is a riser constructed of corrugated metal pipe, topped with a trash rack and anti-vortex plate.  Side openings in the riser handle low-flow conditions, and serve to empty the pond after each storm.

To model this pond:  First define the stage-storage curve.   This is readily done by entering the area of the pond at each contour elevation, from the bottom of the pond up to the top of the berm.  For this size pond, areas are most easily entered in square-feet, rather than acres.  Since all sides of the pond are sloped, volumes are most accurately determined by conic sections, and you should specify entry by "Conic Area."

The easiest way to determine the stage-discharge curve is to describe a "compound outlet."  Compound outlets are most easily configured by working backwards from the final device, numbering the devices in that order.  In this case the final device is the horizontal culvert barrel.  This would be device number one, and must be routed to the "primary" discharge.

The next outlets we encounter are the perforations in the side of the riser.  These are placed in several rings around the riser.  Rather than modeling each hole as a separate orifice (which would require an excessive number of devices) we can define one device for each ring, and use the discharge multiplier to specify the number of openings at that elevation.

If there were three rings of side opening, we would need three device definitions.   Continuing to number in sequence, these would be devices 2, 3, and 4.  Since each of these devices flows through the culvert barrel (rather than discharging directly out of the pond) each one must be routed through the barrel.  This is done by setting the routing to device number "1", which is the culvert barrel in this example.

Finally we encounter the top of the riser.  Under low-head conditions this will behave as a sharp-crested weir, but as the head increases it will transition to orifice flow.  This is easily modeled using a horizontal orifice and selecting the option to "Use weir equation at low heads."  (Prior to HydroCAD-5 you must explicitly set up the orifice and a weir routed in series.)

When configuring the horizontal orifice, note that it must be routed through device number 1, the horizontal culvert barrel.  This is critical in order to allow for possible control by either device, and to avoid adding the flow from both devices!

After entering all the devices, the Outlets screen will look similar to the example below.  When reading this table, it is essential to distinguish between the device number (which appears in the # column) and the device routing!  In this example, we would read:

Device number 1 is a culvert, which is routed to the primary discharge.
Device number 2 is a side opening, which is routed to device number 1, the culvert.
Device number 3 is a side opening, which is routed to device number 1, the culvert.
Device number 4 is a side opening, which is routed to device number 1, the culvert.
Device number 5 is the top of the riser, which is routed to device number 1, the culvert.

The pond summary report includes a graphical representation of the device configuration, as shown in the right-hand illustration above.  Note that device 1 (the culvert barrel) is the only device routed directly to the primary discharge.  All other devices are combined and then routed through device 1.  This allows for the possibility that the culvert barrel may control under certain conditions.

As a final check, be sure to evaluate the stage-discharge curve (not just the hydrograph) for all compound outlets.  In this example the curve should show these characteristics:

1. Zero discharge until the lowest side openings are reached.
2. A small discharge as the side openings are submerged and the head increases.
3. A large increase in discharge at the top of the riser.

Depending on the specific design, you may also be able to observe points of inflection where the top of the riser transitions from weir flow to orifice flow, or where the culvert barrel begins to control.

For further details you may download the actual HydroCAD project file used in this example.  You may open the file directly in HydroCAD, or save the file on your computer and then open it.  If you have trouble with the download, please clear your browser's cache and try again.  If you don't already have HydroCAD installed on your computer, you can open the file using a copy of the free HydroCAD Sampler. 

Also see sample pond #3 which employs a concrete riser structure, and sample pond #2 which illustrates a larger flood-control pond.

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