The desilting canal with its flushing sluices may be omitted if the sedi-
ment load which would settle in the settling basin is negligible; however,
smaller grain size sediment (silt) is always likely to enter the canal, and
maintenance of minimum velocities in the canal is essential to avoid its
silting up.
9.2.2 Location and alignment of an intake
The river reach upstream of the intake should be well established with
stable banks. As the bottom layers of the flow around a bend are swept
towards its inside (convex) bank (see Section 8.3), it is obvious that the
best location for an intake (to avoid bedload entry) is the outer (concave)
bank, with the intake located towards the downstream end of the bend.
This choice of location from the sediment exclusion point of view is not
always possible, and other considerations such as the pond (command)
levels and their variations, navigation hazards, and location of the diver-
sion structure, pump/power house, and outfalls must be considered.
An offtake at 90° to the main flow is the least desirable one. The
structure should be aligned to produce a suitable curvature of flow into the
intake, and a diversion angle of around 30°–45° is usually recommended to
produce this effect; in addition, an artificial bend (Fig. 9.17), a groyne
island (Fig. 9.18) or guide vanes (Fig. 9.19) may be designed to cause the
required curvature of flow (see Avery, 1989). Model tests are desirable in
deciding on the location and alignment of any major intake structure
(Novak and Cˇábelka, 1981).
The entrance losses at an intake depend upon the change in direction
of the flow (entering the intake), the extent of contraction and the type of
trash rack provided at the inlet. They are expressed in terms of the veloc-
ity head as KV^2 /2g.
The entrance loss due to a change in direction of flow (intake at an
anglewith the main stream) is given by
∆hV^2 /2g V^20 /2g (9.19)
whereV 0 is the velocity of the main stream at the inlet, and is around 0.4
for90° and 0.8 for 30°.
In the case of the inlet having a sill constructed with curved abut-
ments and piers, the head loss, ∆hc, is given by
∆hc0.3V^2 /2g. (9.20)
394 DIVERSION WORKS
