Contents preface (VII) introduction 1—37

13.12. RIVER TRAINING FOR CANAL HEADWORKS
River training structures for canal headworks are required for the following purposes (13): (i) To prevent outflanking of the structure,
(ii) To minimise possible cross-flow through the barrage or weir which may endanger the structure and protection works.
(iii) To prevent flooding of the riverine lands upstream of the barrages and weirs, and
(iv) To provide favourable curvature of flow at the head regulator from the consideration of entry of sediment into the canal.
The following types of river training structures are usually provided for weirs (13): (i) Guide banks,

(ii) Approach embankments, (iii) Afflux embankments, and (iv) Groynes or spurs.

The purpose of guide banks is to narrow down and restrict the course of a river so that the river flows centrally through the weir constructed across it without damaging the structure and its approaches. The alignment of guide banks should be determined such that the pattern of flow at the head regulator induces favourable curvature of flow minimising sediment entry into the canal system (19). More details of guide banks have been included in Chapter 12 and a typical sketch of river training structures provided at weirs and salient details of guide banks are given in Figs. 13.21 and 13.22. While constructing weirs on alluvial rivers, the natural waterway is restricted from economic and flow considerations, and the unbridged width of river is blocked by means of approach embankments. Depending upon the distance between the guide bank and the extent of the alluvial belt, the river may form either one or two meander loops (Fig. 13.21). The approach embankments, aligned with the weir axis, should extend up to a point beyond the range of the worst anticipated loop (19).
Afflux embankments are earthen embankments which extend from both the abutments (or the approach embankments) and are connected on the upstream to the ground above the affluxed highest flood level (or to flood embankments, if existing).

474 IRRIGATION AND WATER RESOURCES ENGINEERING

embankment

Approach

embankment		River flow	Afflux
	embankment
Afflux		Single meander
	loop
t			Launching	Double
				apron	meander loop
			1.0 R Max.	1.0 to 1.5 L
		Launching	30°
		apron		Launching
		0.5 R Max.	Radius
			L	apron
				0.5 R Max.
					Launching apron 1.0 R Max.
				Approach embankment
	Canal			0.25 to 0.4 L
			L = Total waterway for weir

R = Lacey s regime scour depth

Fig. 13.21 Typical layout of river training structures for canal headworks

45° to 55°	Multi radii
	curve
	90°
1.0to1.5L

Riverflow

L
Single radius curve

120° to

145°
R₁ = 0.45P

0.4L to 0.25

Multi radii curve

0.3 to

0.5 R₁

	1
	R	30°	45°
1.25L	0.25
a		60°
			to 40°	to
1.0to
0.25			0.4L	b
to

° to 60°

3. 
   to 0.5 R₁
   

1. 
   Straight guide bank
   

Single radius

flow

curve

45° to 60°

River

0.3 to

0.5 R1

Equation of

top line

x

2

y

2

a

2^+

2 = 1

a

b

L

b

Origin

0.3 to 0.5 R1

45° to 60°

(b) Elliptical guide bank

Fig. 13.22 Geometrical shape of guide banks

CANAL HEADWORKS

475

A top width of about 6 to 9 m and freeboard of 1.0 to 1.5 m above the highest flood level (for a 1-in-500 years flood) are usually provided for these river training works (19). Besides, stone pitching, launching aprons, etc. are also provided in the usual manner as described in Chapter 12.

EXERCISES

1. 
   Differentiate between a weir and a barrage. Describe the functions of different parts of canal headworks.
   

2. 
   A barrage is to be constructed across a river having a high flood discharge of 7000 m³/s. The average bed level of the river is 290.00 m and the HFL before construction is 293.80 m. The canal taking off from the river has a full supply discharge of 200 m³/s. Propose suitable values for
   

(i) Crest levels and waterway of undersluice and weir portions.

(ii) Bottom levels of the upstream and downstream sheet piles for weir portion from scour considerations. Also design the downstream launching apron for the undersluices.
Adopt Lacey’s silt factor as unity and permissible afflux as 1.0 m.

3. 
   A river carries a high flood discharge of 16000 m³/s with its average bed level at 200.0 m. A canal carrying 200 m³/s is to take off from the headworks. The full supply level of the canal at its head is 203.0 m. The high flood level before construction is 205.7 m and Lacey’s silt factor is equal to unity. Fix suitable values for the waterway and crest levels of weir, undersluices and canal head regulator. Make provision for a silt excluder. Assume suitably any other data, if required.
   

3. 
   Determine the heights of gates and hoisting deck levels for the undersluice and barrage portions and also for the canal head regulator of a diversion headworks. The relevant data are as follows:
   

River bed level	= 256.20 m
High flood level	= 260.60 m
Afflux	= 0.65 m
FSL of canal	= 258.40 m
Discharge intensity in the barrage portion at high flood	= 15.00 m^2/s
Discharge intensity in the undersluice portion at high flood	= 18.00 m^2/s

Provision of silt excluder is envisaged. A freeboard of 0.3 m may be assumed for gates.

5. 
   A barrage is to be constructed across a river whose high flood discharge is 8000 m³/s and whose minimum bed level is 290.0 m. A canal with full supply discharge of 200 m³/s would take off from the right bank of the headworks at an angle of 90°. The bed level and the full supply level of the canal are 290.50 and 292.5 m respectively. The stage-discharge curve for the river at the barrage site is given by
   

Stage (in metres) = 290 + 0.75 Q – 0.03 Q²

where, Q is the discharge in 1000 m³/s. The median size of the river bed material is 0.60 mm. Design the following:
(i) Weir and undersluice portions including floor thickness, sheet piles, launching apron, etc., (ii) Waterway and crest level of canal head regulator,
(iii) Guide banks, and (iv) Silt excluder.
Prepare plan layout of headworks and also the longitudinal sections through weir and undersluice portions. Assume suitably any other data, if required.
REFERENCES
1. ...... IS:7720-1975, IS Code for Criteria for Investigation, Planning and Layout for Barrages and Weirs.
2. ...... IS:6966-1973, IS Code for Criteria for Hydraulic Design of Barrages and Weirs.

476	IRRIGATION AND WATER RESOURCES ENGINEERING
3. ...... Barrages in India, CBIP Publication No. 148, CBIP, New Delhi, 1981.
4.	Bharat Singh, Fundamenatals of Irrigation Engineering, Nem Chand & Bros., 1988.

5. ...... IS:6531-1975, IS Code for Criteria for Design of Canal Head Regulators.

6. ...... IS:7880-1975, IS Code for Criteria for Hydraulic Design of Skimming Platform for Sediment Control in Offtaking Canal.
7. ...... IS 7871-1975, IS Code for Criteria for Hydraulic Design of Groyne Walls (Curved Wings) for Sediment Distribution at Offtake Points in a Canal.

8. 
   Garde, RJ and PK Pande, Use of Sediment Transport Concepts in Design of Tunnel-Type Sedi-ment Excluders, ICID Bulletin, Vol. 25, No.2, July 1976.
   

8. 
   Garde, RJ, Initiation of Motion on a Hydrodynamically Rough Surface–Critical Velocity Approach, CBIP Journal of Irrigation and Power, Vol. 27, No. 3. July 1970.
   

8. 
   Durand R, Basic Relationships of the Transportation of Solids in Pipes – Experimental Research, Proc. 5^th IAHR Congress, Minneapolis, 1953.
   

8. 
   Garde, RJ, Sediment Transport through Pipes, M.S. Thesis, Colorado Agri. and Mech. College, Fort Collins, Colorado, USA, 1956.
   

8. 
   Engelund F, Instability of Erodible Beds, J. Fluid Mech., Vol. 42, 1970.
   

13. ...... IS:6004, IS Code for Criteria for Hydraulic Design of Sediment Ejector for Irrigation and Power Channels.

14. 
    Vittal, N, GA Shivcharan Rao, Diaphragm Height of Ejector of Uniform Sediment ASCE Journal of Hydraulic Engineering, Vol. 120, No. 3, March 1994.
    

14. 
    Dobbins WE, Effect of Turbulence on Sedimentation, Trans. ASCE, Vol. 109, 1944.
    

14. 
    Camp, TR, The Effect of Turbulence on Retarding Settling, Proc. of the 2^nd Hydraulic Conference, Iowa, 1944.
    

14. 
    Sumer, MS, Settlement of Solid Particles in Open Channel Flow, JHD, Proc. ASCE, No. Hy-11, Nov. 1977.
    

14. 
    Garde, RJ, KG Ranga Raju, and AWR Sujudi, Design of Settling Basins, Journal of IAHR, Vol. 28, No. 1, 1990.
    

19. ...... IS:8408-1976, IS Code for River Training Works for Barrages and Weirs in Alluvium.