Isotachs or isovels are lines linking points of constant velocity, and as such, are used to indicate velocity variation within a given section of the flow field. If Figure 1 represents the cross-section of an open channel flow, the isotachs (indicated by the dotted lines) highlight the velocity reduction at the boundaries of the flow and the occurrence of the maximum velocity (point A) some distance beneath the water surface.
Heat & Mass Transfer, and Fluids Engineering