= Hydraulic control
A <hydraulic control> is a critical section where a <shallow water> flow passes smoothly between <subcritical flow> and <supercritical flow>. One long-wave <characteristic speed> relative to the bed vanishes there. Regularity supplies an extra relation between discharge, depth and channel geometry, allowing the control to set the discharge or upstream state. A critical depth alone is not sufficient for a regular transcritical solution: the channel forcing must also be compatible there.
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