Imagery

Velocity distribution within circular culverts is always heterogenous.
Note the range in velocities and the zone of favorable velocities near the culvert wall and water surface (see Behlke et al. 1994).
Average and maximum velocity may be quite different from the velocity where fish swim.
Strict conservation with fish passage analysis may result in many false negatives (predicting no passage where passage exists) so strive to be accurate and know which parts of the problem are sensitive to errors and which are not.

See: The Bad and The Ugly