On the depression of the filament of maximum velocity in a stream flowing through an open channel

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When water flows with sinuous motion through a circular pipe the resistance introduced by the solid boundaries reduces the velocity of axial flow as the sides are approached, this velocity being greatest at the centre and least at the sides, as indicated by the transverse velocity curve of fig. 1. When flow takes place through a closed rectangular pipe, the same effect is noticed, the transverse velocity curves and the curves joining points of equal velocity, or the contours of equal velocity in a cross-section, being much as shown in fig. 2. Here, again, the maximum velocity is found at the centre of the pipe. From analogy with this latter case it might be expected that when flow takes place through an open rectangular flume such as would be obtained by taking the portion of the rectangular pipe of fig. 2 below the level AA', or indeed through an open channel of any ordinary section, the filament of maximum velocity would be found in the water surface and in the centre of the stream.