At a condition of zero flow through the channel, the liquid level will be at or below the crest
(lowest point on the opening) of the weir. As liquid begins to flow through the channel, it must spill
over the crest of the weir in order to get past the weir and continue downstream in the channel.
In order for this to happen, the level of the liquid upstream of the weir must rise above the weir’s
crest height. This height of liquid upstream of the weir represents a hydrostatic pressure, much the
same as liquid heights in piezometer tubes represent pressures in a liquid flowstream through an
enclosed pipe (see section 2.9.14 for examples of this). The height of liquid above the crest of a weir