Contents preface (VII) introduction 1—37

13.11.2. Sediment Ejector
Sediment ejectors too take the advantage of the concentration distribution in a vertical by ejecting the near-bed water layers having the largest sediment concentration from the canal at a suitable location downstream of the head regulator. The approach channel upstream of the ejector should, preferably, be straight since a curved approach disturbs the uniform distribution of flow and sediment concentration across the channel in front of the ejector. The approach channel can be designed so that the suspended particles may move to lower layers. This will improve the efficiency of the ejector. The ejector should neither be too near nor too far from the head regulator. If the ejector is located too near the regulator, the residual turbulence may keep most of the sediment particles in suspension and, thus, prevent their ejection to the desired extent. If the ejector is sited too far downstream of the regulator, the sediment may get deposited between the regulator and the ejector and, thus, reduce the channel capacity. Besides, a longer reach (between the regulator and the ejector) will have to be wide enough to carry larger amount of discharge on account of the escape discharge required at the ejector.
A schematic diagram of the tunnel-type sediment ejector is shown in Fig. 13.15. The main components of an ejector include a diaphragm, tunnels, control structure, and an outfall channel. The diaphragm is so shaped that it causes least disturbance to the sediment distribution in the bottom layers of flow upstream of the ejector. Diaphragm level is fixed keeping in view the desired sediment size to be ejected, upstream and downstream bed levels of canal, size of tunnels, and the thickness of diaphragm. The lower side of the upstream end of the diaphragm is bell-mouthed. The canal bed is depressed below the ejector to facilitate further ejection of sediment. The ejector spans the entire width of the canal and is divided into a number of main tunnels which, in turn, are subdivided with turning vanes which gradually converge so as to accelerate the escaping flow. The width of ejector tunnels may be varied in order to keep the discharges in all the tunnels to be approximately the same. Generally, 10 to 20 per cent of the full supply discharge of the canal is adequate to remove the desired size and amount of sediment as well as for flushing individual channels of the ejector. The tunnel dimensions at the entry and exit should be such that the resulting flow velocities would be adequate to carry the sediments of the desired size. In addition, the sub-tunnels should be contracted such that the exit velocities further increase by 10-15 per cent and should be in the range of 2.5 to 6 m/s depending on the size of the sediment to be ejected. The depth of tunnels should be kept about 1.8 to 2.2 m to facilitate inspection and repair. The ejector discharge is controlled by regulator gates. The outflow from the ejector is led to a natural drainage through an outfall channel which is designed to have a self-cleaning velocity. Sufficient drop between the full supply level of the outfall channel at its tail end and the normal high flood level of the natural stream is necessary for efficient functioning of the channel. The present design practice for sediment ejectors is also empirical with tunnel height being kept 20-25% of the depth of flow with an escape discharge of about 20 per cent of the full supply discharge of the canal downstream of the ejector. It is, therefore, essential that the proposed design be always model-tested before construction.

CANAL HEADWORKS

Y		Edge of
Main canal
	diaphragm	Origin of
	Splay
		ellipse
			Transition for wider
Flow		approach channel	Diaphragm
	X-Axis
			X-Axis
			Y-Axis
Canal bed
	Approach channel
		Y-Axis
Edge of		Pier nose