Hydraulic Design of Highway Culverts - DOT On-Line Publications

Critical depth is used when the tailwater is less than critical depth and the tailwater depth is
used when tailwater is greater than critical depth but below the top of the barrel. The total barrel
area is used when the tailwater exceeds the top of the barrel.
B. Performance Curves
Performance curves are representations of flow rate versus headwater depth or elevation for a
given flow control device, such as a weir, an orifice, or a culvert. A weir constricts open channel
flow so that the flow passes through critical depth just upstream of the weir. An orifice is a flow
control device, fully submerged on the upstream side, through which the flow passes.
Performance curves and equations for these two basic types of flow control devices are shown
in Figure III-15.
Figure III-15--Performance Curves and Equations for Weirs and Orifices
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