Orifices are regularly shaped submerged openings that are designed to control flow.

Typical applications of these devices include:

  • combined sewers where an orifice is used to divert sanitary wastewater out of the collection/conveyance system during dry weather periods and to restrict the entry of stormwater into the sanitary interceptors during periods of runoff, and

  • detention pond outlets where one or more orifices are used to drain slowly stored runoff after precipitation has ended.

In an XPSWMM link-node model, orifices are located in multilinks. A multilink may contain a single orifice or any combination orifices, weirs, pumps and conduits. An orifice may divert the flow to another pipe, a pumping station or an off-line storage tank.

Two geometric configurations are allowed:

    • a Bottom Outlet or sump orifice, and

    • a Side Outlet orifice.

The shape of the orifice may be circular or rectangular. If rectangular, the height of the opening (in feet or meters) must be entered.

The Hydraulics layer simulates both types of orifice by converting the orifice to an equivalent pipe.

The orifice pipe is assumed to be nearly flat, the invert of the discharge side being set 0.01 feet (0.003 meters) lower than the invert on the inlet side. In addition, for a bottom orifice, the pipe invert is set a distance 0.96 times the orifice diameter (0.96 time the height of a rectangular orifice) below the junction invert so that the orifice pipe is flowing full before any outflow from the junction occurs in any other pipe. For side outlet orifices, the user specifies the elevation of the  orifice invert

The Orifice Data section of the output file shows the data used for the equivalent pipe.

Orifice Data Section Output
CONDUIT NAME........................... rectangle
Upstream node........................... Pond
Downstream node......................... Riser
PIPE DIAMETER........................ 0.50
PIPE LENGTH.......................... 1000.00
MANNINGS ROUGHNESS................... 0.0025

To apply the orifice equation instead of an equivalent pipe, use the Configuration Parameter. USE_ORF_EQN

Both orifice types require Orifice Area (The orifice area, in sq feet or sq meters) and Discharge Coefficient (the orifice discharge coefficient) data be entered. An option to Vary with Time these two parameters is also provided.

To add a plot of measured time series data to the Review Results graphs use the Gauged Data option.

Vary with Time

The orifice discharge coefficient and area may be varied with time to simulate external controls. Orifice settings should not be closed "too fast" because this can cause numerical instabilities that mimic hydraulic instabilities that would occur in the model.

The orifice is defined in terms of:


First time (hours) that the orifice discharge coefficient and area change in value from the initial settings of the previous dialog box or from the previous values in this window. Time zero refers to midnight on the first day of the simulation. Increase hours past 24 for multiple day simulations.


New value for the orifice discharge coefficient to come into effect at the time shown in the adjacent cell.


New value for the orifice area (sq feet or sq meters) to come into effect at the time shown in the adjacent cell.

Orifice Area Calculator

NEW FOR 2015

Clicking the Calc. button launches the Orifice Area Calculator dialog. This dialog allows users to automatically compute and set the Orifice area in the previous dialog by entering the required parameters. 

Circular. If this shape is selected, only the diameter is required. The formula used is: 

Area = Pi * (Height / 2)2

Rectangular. If selected, both height and width are required. The formula used is: 

Area = Height * Width