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HydroCAD Stormwater Modeling - Since 1986

Orifice Flow

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.

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