Weir
 

Water must usually flow through an opening or notch (in a few cases the flow passes over the entire edge of the weir).  The theory of flow measurement by means of the weir involves the release, or change, of energy from its potential state to energy of motion.  As the water approaches the weir, a static head will develop because the weir causes a restriction.  This head (ha) can be related to the volume (Q) of water passing through the channel or partially filled pipe.  Specific equations can be developed to determine flow for any of the weirs, which may be used in the treatment plant.  As flow increases, the velocity of the flow over the weir increases and the head developed upstream of the weir increases.  This relationship between head, velocity, and flow exists until the capacity of the weir is exceeded.