Contents preface (VII) introduction 1—37

11.2.2. Structures for a Carrier Channel Underneath a Natural Stream
The structures falling under this category are superpassages and siphons. The maintenance of such structures is relatively difficult as these are not easily accessible.
A superpassage [Fig. 11.5] is like an aqueduct, but carries the stream over the canal. The canal FSL is lower than the underside of the stream trough and, hence, the canal water flows with a free surface.

382 IRRIGATION AND WATER RESOURCES ENGINEERING
Foot bridge

Stream

H.F.L.

U/S F.S.L.	U/S .L.S.F
Flow	Canal
Loose stone	Loose stone
protection	protection

Solid apron	Solid apron	Pervious
Pervious protection		protection

Fig. 11.5 Typical cross-section of a superpassage (1)
A siphon [Fig. 11.6] carries the canal water under pressure through barrels below the stream trough. For siphoning small discharges, precast RCC pipes will be economical. For siphoning higher discharges, horse-shoe-shaped rectangular or circular barrels, either single or multiple, are adopted. Roofs of rectangular barrels are, at times, arch-shaped for economy. For discharges under high pressures, circular or horse-shoe-shaped barrels are more suitable.




A

U/Sbedwidth

Transition wall

B

p

a

Cut-off

Water line

Stream

q

b

c

X

Joints

X

45°

r

Boulder

width

c

Canal

CC blocks

protection

bed

d

A

s

e

c

D/S

f

Splay 1 in 5

t

Splay 1 in 3

B

(Cut-off abcdef on U/S and pqrstu on D/S)

u

PLAN

Stream

(Showingdetailsofreduction

incut-offdepths)

U/S transition

than 1 in 3

Joints

steeper

D/S transition

Cut-off

Top of bank

Face wall

Top of

Top of

Face wall

CC

transition wall riprap

Water level

Section

Water

Stream

300 to 750

Boulders

CC blocks

seal

mm min

Smooth

curve

Cut-off

Slope not steeper

Slope not

Partition

than 1 in 4

walls of

barrel

Section AA

Partition

Rubber water stops at

top and bottom may be

walls of

provided if numbers of

barrel

barrel are large

Section BB

Fig. 11.6 Profile of a typical canal siphon (1)

CROSS-DRAINAGE STRUCTURES

383

11.2.3. Structures for Carrier Channel Crossing a Natural Stream at the Same Level
Structures falling under this category are level crossings and inlets. Inlets are, at times, combined with escapes. When the canal and the stream meet each other at practically the same level, a level crossing [Fig. 11.7] is provided. Level crossings involve intermixing of the canal and stream waters. They are usually provided when a large-sized canal crosses a large stream which carries a large discharge during high floods, and when siphoning of either of the two is prohibitive on considerations of economy and non-permissibility of head loss through siphon barrels (1). A barrier with its top at the canal FSL is constructed across the stream and at the upstream end of the junction. The regulators are provided across the stream and canal at the downstream junctions of the level crossing. These regulators control the flow into the canal and stream downstream of the crossing. This type of arrangement is also useful in augmenting the canal supplies with the stream discharge.

			Side	Downstream loose apron
			pitching	Downstream inverted filter
				Downstream impervious
				floor pier
	Toe wall
				(D)
Incomingcanal	Looseapron	Invertedfilter	Upstreamimperviousfloor	Sheet pile
Cut-off
					Impervious floor
					(A)
					(B)	Pier
				Upstream impervious floor

Inverted filter

Upstream loose apron
Toe wall
Stream

Toe wall
Side pitching

Flare 2.5:1
Abutment	Side pitching
Invertedfilter
	Looseapron	Outgoingcanal	Siltejector
	Pier

(C)	Downstream
	impervious
	floor
	Sheet pile
	Regulator
	axis
	Toe wall
	Abutment
	Flare 2:1

1. 
   Canal inlet regulator
   
2. 
   Stream inlet regulator
   
3. 
   Canal outlet regulator
   
4. 
   Stream outlet regulator
   

Fig. 11.7 Typical layout plan of a level crossing (1)
When the stream is dry, the stream regulator is kept closed and the canal regulator is opened so that the canal water flows in the canal itself without interruption. When the stream is bringing water, it mixes with the canal water and the stream regulator is used to dispose of that part of the stream water which is not used to augment the canal supply.
If a small and relatively sediment-free stream meets the canal at practically the same level (or its bed level is higher than the canal FSL) then an inlet is provided. An inlet is a structure consisting of an opening in a canal bank, suitably protected, to admit upland stream water into the canal. Inlets are constructed only if the stream discharge is too small and does

384 IRRIGATION AND WATER RESOURCES ENGINEERING
not carry large quantity of sediment. Inlets do not have a regulator and, hence, the stream bed should be higher than the canal FSL. An inlet consists of a fall or a pitched slope confined within wing walls to guide the stream water into the canal. An inlet simply allows the stream water to be taken into the canal. If the stream water so taken into the canal is appreciable in quantity, it is allowed to flow out at a suitable site downstream (along the canal) of the inlet. The outlet is generally combined with some other structure for economic reasons, but at times only an inlet (and no outlet) is provided.
Sometimes a small stream is diverted into a larger stream and a cross-drainage structure for the combined discharge is provided at a suitable site.