Orifice
Calculations
An orifice is usually modeled as an outlet device that
controls the discharge from a pond. HydroCAD
supports
numerous orifice configurations, as shown at the right.
Discharge equations and other details for each type of orifice 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
orifice 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.
Setting up an orifice
A single screen (show at right) is used to define any type of orifice.
For most applications you should select the option to "Use Weir Flow at low
head", which will automatically transition to orifice flow as the head
increases.
To model multiple openings in a grate, enter the dimensions of an individual
opening, and set the number of rows and columns. For a vertical grate,
enter the vertical spacing between the rows of openings.
For a horizontal grate, you also have the option to set the overall grate
dimensions. This will check for possible weir flow around the perimeter of
the grate, which may restrict the flow at very low heads.
When using an orifice to model the top of a riser, set the "Routing" so the
orifice flow goes to the riser outlet. Details here.
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 an orifice, 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. Failing to define enough storage will result in an
overfilled storage condition, and a warning message will be issued.
