Contents preface (VII) introduction 1—37

Table 8.7 Freeboard of irrigation channels as suggested by CWC

Discharge (m^3/s)	up to 0.7	0.7 to 1.4	1.4 to 8.5	over 8.5
Freeboard (m)	0.46	0.61	0.76	0.92

Alternatively, Table 8.8 may be used for the estimation of freeboard.

Table 8.8 Freeboard in irrigation channels (15)

Bed width (m)	Discharge (m^3/s)	Freeboard(m)
Less than 1.0	–	0.30
1 to 1.5	–	0.35
Greater than 1.5	Less than 3.0	0.45
–do–	3 to 30	0.60
–do–	30 to 60	0.75
– do–	Greater than 60	0.90

USBR has proposed the following formula for the estimation of freeboard F (in metre) in canals.

F = CD
where, C is a constant varying form 0.45 (for discharges up to 0.07 m³/s) to 0.76 (for discharges greater than 85 m³/s) and D is the water depth in metre.
8.6.4.4. Canal Banks
Canal banks hold water within the water section of a channel. Suitable bank dimensions of an earth channel depend on size of channels, height of water surface above natural ground, amount and nature of excavated earth available for bank construction, and need of inspection roads along the channel. Bank widths at all elevations must provide stability against water pressure at the sides of the channel section. They should also keep percolating water below ground level outside the banks and prevent piping of bank materials.
A canal bank should be of such width that there is a minimum cover of 0.5 m above the saturation line. For large embankments of major canal projects, the position of the saturation line is determined as in case of earth dams and the stability of slope is computed using principles of soil mechanics. This has been discussed in Chapter 15. The saturation line for small embankments is drawn as a straight line from the point where full supply level meets the bank. The slope of the saturation line, i.e., the hydraulic gradient, may very from 4H : 1V for relatively impermeable material (such as ordinary loam soil) to 10H : 1V for porous sand and gravel. For clayey soils, the hydraulic gradient may be steeper than 4(H) : 1(V). If the bank section does not provide a minimum cover of 0.5 m above the saturation line, a counter berm, shown in Fig. 8.8(b), is provided. Alternatively, the outer slope of the bank is flattened.

DESIGN OF STABLE CHANNELS

309

The Central Water Commission has recommended bank widths as given in Table 8.9.