When critical depth (d

c

) is higher than uniform depth (d

u

), the slope is steep. The conduit may flow completely full (pressure flow) or partly full (free surface flow). The free surface flow may be supercritical or subcritical depending on tailwater conditions.

When critical depth is lower than uniform depth, the slope is termed mild. Pressure flow or free surface flow may occur. Free surface flow is most likely to be subcritical within the conduit.

The shape of the free water surface is dependent on whether the conduit slope is steep or mild and on the tailwater conditions. The described in Chapter 7 accommodates the differences in water surface shape.