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HydroCAD Stormwater Modeling - Since 1986

Broad-crested Weir Compound Weir Check Dams

Click for complete self-training materialsWeir Calculations

A weir is usually modeled as an outlet device that controls the discharge from a pond.  HydroCAD provides numerous weir options, as shown at the right.

Discharge equations and other details for each type of weir are provided in the Reference Manual and the help system.  The quickest way to access this information is to click the Help button on the applicable weir screen.

To model more complex outlet control structures, you can combine several basic devices to create a compound outlet.  A common example is a riser structure, which combines weir, orifice, and culvert flow characteristics to model a single pond outlet.

Common mistakes

As with any outlet device, it's essential that you define all of the available pond storage that will be used as the water flows through the outlet.  Even if you are only expecting one inch of head over a weir, your must specify how much storage is represented by that inch.  It could be a few cubic-feet or acre-feet, depending on your situation.  And you cannot get an accurate routing without that information.

This doesn't mean that you need to "make the pond bigger" or specify "fictitious" storage.  Just provide an accurate description of the volume upstream of the weir, using an imaginary wall above the weir to delineate the storage.  This is the pond volume that is being controlled by the weir outlet.

Failing to define enough storage will result in an overfilled storage condition, and a warning message will be issued.

For further information:

Broad-crested weirs (and how they compare to sharp-crested)
Compound weirs (such as notched weirs)
Compound outlets (such as risers)

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