Weir
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)
