Hydraulic Structures: Fourth Edition

bottom ones. On entering the depressed area of the canal bed, the

bottom sediment-laden layers are diverted by the curved vanes

towards the escape chamber. The design should be such that the

entry disturbances are minimal; the streamlined vane passages accel-

erate the flow through them, thus avoiding deposition.

2. Vortex tube type ejector.The vortex tube ejector (Fig. 9.23) consists
   of a pipe with a slit along its top, placed across the bottom of the
   canal at an angle of around 30°–90° to the direction of flow.
   The vortex motion within the tube draws the sediment into it, and he
   wall velocities along the tube eventually eject the sediment at its dis-
   charge end. A properly designed vortex tube ejector can be more
   efficient than any other conventional ejector, with less water loss.

9.2.4 Settling basin

The settling basin is a device placed on the canal downstream of its head
regulator for the removal of sediment load which cannot be trapped by the
conventional excluders or ejectors. It consists of an enlarged section of the
channel where the flow velocity is sufficiently low so that the fine sediment
settles on the bed (Fig. 9.24). The settled sediment is removed by sluicing,
flushing or dredging.
The following equation may be used to design a settling basin:

WW 0 e^ wsx/q (4.13)

whereWis the weight of sediment leaving the basin, W 0 is the weight of
sediment entering the basin, wsis the fall velocity of a sediment particle
(equation (8.18)), qis the discharge per metre width of the basin and xis
the length of the settling basin. Alternatively

xcDsV/ws (9.23)

INTAKES 399

Fig. 9.23 Vortex tube type silt ejector