Contents preface (VII) introduction 1—37

Table 8.6 Side slopes for unlined channel

Type of soil		Side slopes (H : V)
Loose sand to average sandy soil	1.5	: 1 to 2	:	1 (in cutting)
	2	:	1 to 3	:	1 (in filling)
Sandy loam and black cotton soil	1	: 1 to 1.5	: 1 (in cutting)
	2	:	1 (in filling)
Gravel	1	: 1 to 2 : 1
Murum or hard soil	0.75	:	1 to 1.5		: 1
Rock	0.25	: 1 to 0.5		: 1

DESIGN OF STABLE CHANNELS

307

8.6.4.2. Berms
A berm is a narrow horizontal strip of land between the inner toe of the bank and the top edge of cutting. Berms between water section and inner bank slopes are required along the channels where bank materials are susceptible to sloughing. Berms slope towards water section to facilitate drainage.
Because of irregular changes in the ground level and regular changes in the channel bed level, the depth of cutting d₁ and the depth of filling, (d₂ – d₁) (Fig. 8.9) would vary. This will cause variation in the horizontal distance between the bed and top of the bank (i.e., between X and Y). If r₁ (H) : 1(V) is the side slope in cutting and r₂( H) : 1(V) is the inner side slope of embankment, then the berm width is kept equal to (r₂ – r₁) d₁. This will ensure that the horizontal distance between the bed and top of the bank (i.e., between X and Y) will remain constant. As can be seen from Fig. 8.9, this distance is equal to r₁d₁ + ( r₂ – r₁) d₁ + r₂(d₂ – d₁) which equals r₂d₂. Since the total height from the bed to top of the bank, d₂, remains practically constant, the horizontal distance between X and Y (i.e., r₂d₂) also remains constant if the berm width is equal to (r₂ – r₁)d₁.
r₂ d₂

Y

FSL
r₂ : 1

d₂

h
^d1 r₁ : 1

X
Fig. 8.9 Berm
For usual side slopes of 1.5 (H) : 1 (V) (in filling) and 1(H) : 1(V) (in cutting), a berm of width equal to half the depth of cutting will make the horizontal distance between the bed and top of the bank (i.e., XY) equal to 1.5 times the height of the top of the bank with respect to the channel bed.
As a result of silting on berms, an impervious lining is formed on the banks. This helps in reduction of seepage losses. In addition, the berms protect the banks against breaches and the eroding action of waves. These berms break the flow of rain water down the bank slope and, thus, prevent guttering. An additional berm may also be provided in channel sections which are in deep cutting.
8.6.4.3. Freeboard
Freeboard is the vertical distance from the water surface at full supply level to the top of bank. Freeboard provides the margin of safety against overtopping of the banks due to sudden rise in the water surface of a channel on account of improper operation of gates at the head regulator, accidents in operation, wave action, land slides, and inflow during heavy rainfall. The excessive growth of vegetation or accumulation of sediment deposits may also result in the gradual rise of water surface levels above the design levels. Design of channels should specify adequate freeboards to prevent overtopping of the banks during sudden rises in water surface. Adequate freeboard would depend on dimensions of the flow section, flow condition, bank material, method of construction of banks, and resulting damage due to failure of banks.

308 IRRIGATION AND WATER RESOURCES ENGINEERING
Freeboard in unlined channels vary from about 0.3 m in small channels to about 2 m in large channels. For channels of intermediate size, freeboard is sometimes estimated by adding 0.3 m to one quarter of the flow depth.
The Central Water Commission of India (CWC) has recommended the value of freeboard as given in Table 8.7.