Just trying to help. It seems it's a formula.
http://www.upg-ploiesti.ro/curs_multimedia/conv13/23.htm

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Note added at 43 mins (2007-11-24 18:02:41 GMT) Post-grading
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Q2 is also called "hydraulic jump"
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Environmental ServicesTech ForumScattergraph Principles and PracticeHydraulic Jump
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Hydraulic Jump
This scattergraph is from a site that experiences a hydraulic jump. This condition occurs when flow transitions from supercritical to subcritical flow. In this example, the hydraulic jump is located upstream from the monitor at lower flow rates, and the subcritical side of the hydraulic jump is observed (Q1). As the flow rate increases, the hydraulic jump is pushed through the monitoring location, and the supercritical side of the hydraulic jump is observed (Q2). Flow monitors can operate well in either subcritical or supercritical conditions, but accuracy may deteriorate during the transition. Therefore, a hydraulic jump should be avoided, if possible.



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