Contents preface (VII) introduction 1—37

Fig. 8.10 Longitudinal section of distributary channel (Example 8.10)

Service

road

Spoil

B

Drain

bank

G.L

B/2

Canal

Section

1 m max

S

Section

e

r

v

i

c

L

e

R

Drain

o

a

L/2

d

Plan

Plan

(b) Spoil banks

(a) Borrow pits

Fig. 8.11 Borrow pits and spoil banks

314 IRRIGATION AND WATER RESOURCES ENGINEERING
On the other hand, if the excavated earth exceeds the requirement of earth material for the construction of banks and service roads, the excess earth has to be suitably disposed of. If the excess earth is not much, it can be used to widen or raise the canal banks. If the quantity of excess earth is much larger, then it is utilised to fill up local depressions in the area or deposited in spoil banks, Fig. 8.11 (b), on one or both sides of the channel. The section of spoil banks depends on the cost of the land and labour. These should be provided with good cross slopes on all sides for ensuring proper drainage. The spoil banks should be discontinuous to allow cross drainage between them.
Land Width
Land width required for the construction of a channel includes the permanent and temporary land. The permanent land extends a little beyond the outer toe of the canal banks (or the drain, if provided) on either side. If trees are to be planted along the canals, then the land acquired for this purpose should also be included in the permanent land. Planting of trees adjacent to good culturable land should be avoided as the shade of the trees would affect the plants.
In addition to the permanent land, some land along the channel is required during construction for the storage of materials and equipment, and other purposes related to the construction work. This temporary land is returned to the concerned owners after use with due compensation.

8.8. SEDIMENT DISTRIBUTION IN AN ALLUVIAL CHANNEL
The sediment distribution along a vertical is given by the Rouse equation [Eq. (7.34)]. In the channels of north India, the concentration of sediment discharge by weight near the water surface is found to be 40 to 70 per cent of the average value which is generally found at about 0.6 times depth below the water surface. The percentage of coarse sediment continuously increases towards the bed.
Lateral sediment distribution is such that there is generally greater concentration of sediment near the banks than at the centre. This is due to cross currents in a channel (Fig. 8.12).

Bottom
Current

Surface
Current
Bottom
Current

Bottom
Current

Surface
Current
Bottom
Current

Fig. 8.12 Cross currents in a channel

DESIGN OF STABLE CHANNELS

315

Surface water flowing faster than the lower layer water, tends to topple over the slower moving water and goes to the bed of the channel. At the bed, this water is deflected towards the bank. This cross current at the bed (where the concentration of coarse sediment is the greatest) from the centre towards the bank pushes the coarse sediment towards the banks. The return current is toward the centre and upwards and, hence, the lifting up of the sediment is opposed by gravity.

The tendency of silting near the banks due to cross currents can be reduced by accelerating the flow in the bank region by some means such as pitching the bank slope.
Flow along the bends in the channel is such that there is heading up of water near the outer bank due to which there is a cross current of bottom water towards the inner bank. This leads to the deposition of sediment near the inner bank. Thus, there is shallow depth on the inner bank of the bend and greater depth on the outer bank of the bend. A channel should, therefore, offtake from the outer bank if it has to draw relatively clear water.
In a bell-mouthed converging channel, the sediment concentration is maximum near the central part of the channel. An offtaking channel from such a reach would, therefore, draw comparatively less sediment.

8.9. SILTING AND BERMING OF CHANNELS
In the head reaches of a distributary channel there may exist a tendency of silting up due to one or more of the following reasons :
(i) Defective Head Regulator : A defective head regulator may make the offtaking chan-nel draw a higher percentage of coarser sediment. This increases the silt factor and sediment load of the channel and leads to the silting of the channel.
(ii) Non-regime Section : If the channel section is not able to carry its sediment load, the excess sediment is deposited on the channel bed. In due course, this deposition of sediment increases the channel slope so that the channel becomes capable of carry-ing the sediment which enters the channel. The channel may, then, continue in tem-porary equilibrium of ‘initial regime’.
(iii) Inadequate Slope : If the channel slope is less than the regime slope, the sediment will deposit on the channel bed so as to increase the slope to the regime slope. In cases where the ground slope is less than the regime slope, the entry of coarse sedi-ment into the channel should be minimised so that the silt factor of the channel sediment reduces.
(iv) Fluctuation in Supply : When the channel is running with low supplies for a long period, it gets silted up in the head reaches.
(v) Defective Outlets : If the outlets do not draw their share of sediment in proportion with the water discharge, the channel will have a tendency to silt up. This is due to the fact that downstream of the outlet, the parent channel has to carry relatively more sediment with reduced water discharge.
In the tail reaches of a channel, the discharge and velocity are both reduced. The velocities near the sides of the channel are very low and, hence, the silting takes place near the sides. The grass on the channel sides also catches the sediment. This is termed ‘berming’ of the channel and should be distinguished from the ‘berms’ which are deliberately provided in the channel section. The berming results in reduction of the channel section and, hence, rise in

316 IRRIGATION AND WATER RESOURCES ENGINEERING
water levels. To improve the channel condition in the tail reaches, sometimes berm cutting may have to be carried out.

EXERCISES

1. 
   What are the ‘true regime’ conditions in an alluvial channel as stipulated by Lacey ?
   

2. 
   A trapezoidal channel is 5.0 km long and has a trapezoidal cross-section with the side slopes of
   

5. 
   H : 1V. Up to the first 2 km, the bed-width of the channel is 4.90 m and the depth of flow is
   

5. 
   m. Thereafter, the bed-width and the depth are 4.0 m and 0.885 m, respectively, right up to the downstream end of the channel. Estimate the total seepage loss through the channel if the rate of seepage loss is 3 m³/s per million square metres of the water surface area exposed.
   

3. 
   A stable channel is to be designed for a discharge of 40 m³/s and the silt factor of unity. Calculate the dimensions of the channel using Lacey’s regime equations. What would be the bed-width of this channel if it were to be designed on the basis of Kennedy’s method with critical velocity ratio equal to unity and the ratio of bed-width to depth of flow the same as obtained from Lacey’s method.
   

3. 
   For the design of an irrigation channel through a flat ground it has been decided to avail fully
   

the available slope of 25 cm/km, and adopt Lacey’s method of design by restricting the discharge. Design the channel taking the average size of bed material as 0.0005 m and side slope of 0.5H : 1V.

5. 
   Design an irrigation channel to carry 200 m³/s of flow with a bed load concentration of 100 ppm by weight. The average grain diameter of the bed material is 1.0 mm. Use Lacey’s method and a suitable bed load equation for the design. Side slope of the channel is 0.5H : 1V.
   

REFERENCES

1. 
   Lane, EW, Design of Stable Channels, Trans. ASCE, Vol. 120, 1955, pp.1234-1279.
   

2. 
   French, RH, Open Channel Hydraulics, McGraw-Hill Book Company, Singapore, 1986.
   

3. 
   Chow, VT, Open Channel Hydraulics, McGraw-Hill Book Company, New York, 1959.
   

4. 
   Arora, AK, Suspended Sediment Transport in Rigid Open Channels, Ph. D. Thesis, University of Roorkee, Roorkee, India, 1983.
   

5. 
   Ranga Raju, KG, Flow through Open Channels, Tata McGraw-Hill Publishing Company Ltd., New Delhi, 1986.
   

6. 
   Kennedy, RG, The Prevention of Silting in Irrigation Canals, Paper No. 2826, Proc. ICE (Lon-don), Vol. 119, 1895.
   

7. 
   Lane, EW, Stable Channels in Erodible Material, Trans. ASCE, Vol. 102, 1937.
   

8. 
   Ranga Raju, KG and RL Misri, Simplified Canal Design Procedure Using Kennedy’s Theory, CBIP J. of Irrigation and Power (India), April 1979.
   

9. 
   Lindley, ES, Regime Channels, Proc. Punjab Engg. Congress, Vol. 7 1919.
   

10. 
    Woods, FW, A New Hydraulic Formula, The Engineer, June, 1927.
    

11. 
    Lacey, G, Regime Flow in Incoherent Alluvium, CBIP Publication no. 20, 1939.
    

12. 
    Grade, RJ and KG Ranga Raju, Mechanics of Sediment Transportation and Alluvial Stream Problems, New Age International Publishers, New Delhi 1985.
    

13. 
    Ranga Raju, KG, KR Dhandapani and DM Kondap, Effect of Sediment Load on Stable Canal Dimensions, J. of Waterways and Harbours Division. Proc. ASCE, Vol. 103, 1977.
    

14. 
    Engelund, F and E Hansen, A Monograph on Sediment Transport in Alluvial Streams, Teknisk Forlag, Denmark, Jan. 1967.
    

15. 
    Bharat Singh, Fundamentals of Irrigation Engineering, Nem Chand and Bros., Roorkee, 1988.