ABSTRACT
The flow behaviour around channel bends is very similar, but owing to the presence of a free surface, the increase in pressure with radius produces a greater depth of water at the outer boundary. In the case of an ideal fluid the transverse surface profile is hyperbolic (Eq. (4.23)). With a real fluid there is a considerable velocity gradient adjacent to the bed and as a result the hydrostatic force in this region is greater than the centrifugal force. A secondary transverse current is thus induced which is directed inwards at the bed (Fig. 4.32). For continuity, there must exist an outward transverse current near the surface so that the resultant motion is of a spiral or helicoidal nature. This behaviour of the current in an unlined channel or river bend is conducive to the scouring
t ' sy-^h I ~ T
I K,1 i___ _______________ t \ Figure 4.32 Flow around a channel bend
and transportation of material from the bank and bed on the concave side and its deposition in the less disturbed regions both at and down stream of the bend. The transverse current at the surface is weaker and is not capable of returning the scoured material. Nevertheless it does constitute a navigational hazard since vessels tend to be carried towards the concave bank. The whole phenomenon is favourable to the accentua tion of a bend and the development of a meander.
