Contents preface (VII) introduction 1—37

Example 8.10 Design the first 5 km of a distributary channel which takes off from a branch the bed of which is at 224.0 m and the water depth in the branch is 2.0 m. The channel is to be designed for Rabi irrigation of intensity 30 per cent. Outlet discharge factor is 1800 hectares/m³/s.

CCA	= 70 per cent of GCA
Evaporation and seepage losses	= 2.5 m^3/s/10^6 sq.m
Critical velocity ratio	= 1.05
Manning’s n	= 0.023
Silt factor, f1	= 1.0

310		IRRIGATION AND WATER RESOURCES ENGINEERING
Losses downstream of 5 km section	= 0.28 m^3/s
Field application efficiency	= 65%
Provide slope close of Lacey’s slope
	Distance from head of canal in km		GCA (hectares)
	0			20,800
	1			18,850
	2			15,990
	3			12,610
	4			10,725
	5			9,230

Ground Levels

Dist. (km)	RL (m)	Dist. (km)	RL(m)
0.0	225.00	2.6	223.20
0.2	225.00	2.8	223.05
0.4	225.15	3.0	222.90
0.6	225.10	3.2	222.80
0.8	225.05	3.4	222.65
1.0	224.95	3.6	222.50
1.2	224.25	3.8	222.55
1.4	224.10	4.0	222.40
1.6	224.05	4.2	222.30
1.8	224.00	4.4	222.25
2.0	223.95	4.6	222.20
2.2	223.35	4.8	222.15
2.4	223.20	5.0	222.00

Solution: Calculations for this problem have been shown in Table 8.10. Columns 1, 2 and 7 are as per the given data. CCA (col. 3) is obtained by multiplying GCA with given factor of 0.7. CCA (col. 3) is multiplied by the intensity of irrigation (30 per cent, i.e., 0.3) to obtain the area to be irrigated for Rabi crop (col. 4). If intensities of irrigation for Kharif crops and sugarcane crop (or some other major crop) are also known, then corresponding areas to be irrigated for Kharif crop (col. 5) and sugarcane or other crop (col. 6) can also be similarly obtained. For known outlet discharge factors for Rabi, Kharif and Sugarcane (or other) crops, corresponding outlet discharges can be calculated by dividing the area to be irrigated with the corresponding outlet discharge factor and also the field application efficiency. Maximum of these outlet discharges is entered in col. 10 of Table 8.10.
Now a step-by-step method is followed from the tail end to determine channel dimensions in different reaches.
To the outlet discharge below 5.0 km (col. 10) is added the given losses downstream of 5 km section (i.e., 0.28 m³/s) entered in col. 12 to obtain the total discharge (col. 13).
Irrigation channels are designed for a discharge which is 10 per cent more than the total discharge required so that the channels can carry increased supplies in times of keen demand. The design discharge has been entered in col. 14.

DESIGN OF STABLE CHANNELS

311

Channel dimensions ( i.e., S, B, and h) can now be computed using a suitable method. Bed slope has been obtained from Lacey’s equation [Eq. (8.35)]. This value of slope is modified to the nearest multiple of 2.5 and entered in col. 15. The method proposed by Ranga Raju and Misri (8) has been used to obtain B and h. Alternatively, Kennedy’s method of trial or Lacey’s equations can be used to obtain the channel dimensions.

In the present problem, since the adopted value of p is different from the value of p obtained from Fig. 8.7, the values of velocity obtained from the Manning’s equation [Eq. (8.12)], and the Kennedy’s equation [(Eq. 8.9)] should be compared. If the two values differ considerably, the channel dimensions should be revised suitably. For the present problem, the difference varies from 1.5 per cent to 6.5 per cent only for the chosen channel dimensions.
Having obtained the channel dimensions at the tail end (in this case at 5 km section), the losses in the reach between km 4 and km 5 are estimated. The water surface width at the km 5 section is 4.65 m (side slope of channel is 1/2H : 1 V). Assuming average water surface width in the reach (between km 4.0 and km 5.0) as 5.0 m, the loss in the reach is equal to 5 × 1000 × 2.5 × 10^–6 = 0.0125 m³/s (col. 11) which is added to the losses downstream of 5 km (0.28 m³/s) to obtain total losses downstream of 4 km which comes to 0.2925 m³/s (col. 12).
Channel dimensions can now be estimated for the reach between km 4.0 and km 5.0 as explained for the reach downstream of km 5.0. The average water surface width for the reach between km 4.0 and km 5.0 works out to (1/2) (4.9 + 4.65) = 4.775 m and the corresponding reach loss is 0.012 m³/s which is not much different from the assumed reach loss of 0.0125 m³/s. If the difference is large, the computations of channel dimensions may have to be revised.
Following the above procedure the channel dimensions up to the head of canal are determined. The computations have been shown in Table 8.10.
Longitudinal section is shown plotted in Fig. 8.10. Also shown in this figure are two falls which should be located keeping in view the requirements and guidelines discussed in Sec. 8.6.2.

7. 
   BORROW PITS, SPOIL BANKS, AND LAND WIDTH
   

FOR IRRIGATION CHANNEL
Borrow Pits and Spoil Banks
Although it is advisable to keep the channel in balanced earth work, it is generally not possible to do so. If the amount of earth required for filling is more than the amount of excavated earth, then the excess requirement of filling is met by digging from suitably selected areas known as borrow pits, Fig. 8.11 (a).
If unavoidable, borrow pits should be made in the bed or berms of the channel. These pits will silt up after sometime when water has flowed in the canal. The depth of the borrow pits is kept less than 1 m and the width is limited to half the bed width. The borrow pits are located centrally in the channel bed and are spaced such that the distance between adjacent borrow pits is at least half the length of the borrow pits. Borrow pits may be similarly located in wide berms of a channel.
If the material from internal borrow pits is not sufficient to meet the requirement then extra material is taken from external borrow pits which should be about 5 to 10 m away from the toe of the canal bank. These should not be deeper than 0.3 m and should always be connected to a drain.
Table 8.10 Schedule of area statistics and channel dimensions (Example 8.10)

Distance		GCA			CCA			Area to be irrigated					Outlet discharge factor
from head
of canal									Rabi			Kharif		Sugarcane	Rabi		Kharif			Sugarcane
																	or other									or other
																	major crop										crop
km		ha				ha	ha				ha			ha	ha/m^3/s		ha/m^3/s		ha/m^3/s
1			2			3		4				5			6	7			8					9
0			20800		14560		4368					–				–	1800			–					–
1			18850		13195		3958.5				–				–	1800			–					–
2			15990		11193		3357.9				–				–	1800			–					–
3			12610		8827		2648.1				–				–	1800			–					–
4			10725		7507.5		2252.25				–				–	1800			–					–
5			9230		6461		1938.3				–				–	1800			–					–