Orifice Open and Close Speed an the Target Setting

Orifice Open and Close Speed and the Target Setting

In SWMM 5 there is an orifice parameter called setting which opens or closes the orifice opening by modifying the depth of the orifice.  The setting is  based either on a RTC rule of the orifice or the Flap Gate condition of the orifice and can be between 0 and 1.  Closed is 0; Open is 1.  The difference is that the target setting is what the setting should be based on the condition of the Flap Gate or the RTC Rules and the setting is the value actually used in the model.

The open and close speed of the orifice modifies the orifice setting by changing the orifice setting based on the open and closing speed using the equation:

New Orifice Setting = Old Orifice Setting + (Target Setting – Orifice Setting) * Time Step / Orifice Open and Close Speed

If your target setting and the current orifice setting are both 1 or 0 then the orifice Open and Close option does not change the orifice setting.  New Setting equals Old Setting in that case.  If the target and  setting are out of phase then the Open and Close Option will function correctly.  For example, if the Open and Close Speed is 1 hour then the orifice setting will open and close in a one hour period.  The table shown below shows how the orifice setting changes as a function of the speed and the difference between the target and orifice settings.   The setting starts out open but the target says closed – the orifice then closes over a 1 hour period.  At one hour the target setting is 1 and the orifice will then open over a one hour period.

Setting               Target

Days         Hours

0              00:00:00    1.00                  0.00

0              00:15:00    0.74                  0.00

0              00:30:00    0.50                  0.00

0              00:45:00    0.25                  0.00

0              01:00:00    0.00                  0.00

0              01:15:00    0.25                  1.00

0              01:30:00    0.50                  1.00

0              01:45:00    0.75                  1.00

0              02:00:00    1.00                  1.00

0              02:15:00    0.75                  0.00

0              02:30:00    0.50                  0.00

0              02:45:00    0.25                  0.00

0              03:00:00    0.00                  0.00

0              03:15:00    0.00                  0.00

0              03:30:00    0.00                  0.00

0              03:45:00    0.00                  0.00

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