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CANAL CROSS DRAINAGE WORKS/STRUCTURES

Necessity of Cross Drainage Works/Structures

A cross drainage work (also called CD work) is a structure built on a canal where it is crosses

a natural drainage, such as a stream or a river. Sometimes, a cross-drainage work is required

when the canal crosses another canal. The cross-drainage work is required to dispose of the

drainage water so that the canal supply remains uninterrupted. A cross-drainage work is also

called as drainage crossing. The canal at a cross-drainage work is generally taken either over

or below the drainage. However, it can also be at the same level as the drainage.

The canals are, preferably, aligned on the watershed so that there are no drainage

crossings. However, it is not possible to avoid the drainages in the initial reach of a main

canal because it takes off from a diversion headworks (or storage works) located on a river

which is a valley. The canal, therefore, requires a certain distance before it can mount the

watershed (or ridge). In this initial reach, the canal is usually a contour canal and it intercepts

a number of natural drainages flowing from the watershed to the river. After the canal has

mounted the watershed, no cross-drainage work will normally be required, because all the

drainage originate from the watershed and flow away from it. However, in some cases, it may

be necessary for the canal to leave the watershed and flow away from it. It may be necessary

for the canal to leave the watershed for a short distance where the watershed takes a sudden

small loop and it is not possible to align the canal along the loop. In that case, the canal

intercepts the drainages which carry the water of the pocket between the canal and the

watershed and hence the cross-drainage works are required.

A cross-drainage work is an expensive structure and should be avoided as far as

possible. The number of cross-drainage works can be reduced to some extent by changing the

alignment of the canal. However, it may increase the length and hence the cost of the canal.

Sometimes it is possible to reduce the number of cross-drainage works by diverting the small

drainages into large drainages or by constructing the cross-drainages work below the

confluence of two drainages by shifting the alignment. However, the suitability of the site for

the construction of the structure should also be considered while deciding the location of the

cross-drainage works.

Types of CD Works

Depending upon the relative positions

of the canal and the drainage, the cross-

drainage works may be broadly

classified into 3 categories. In each

category, there are further sub-types:

1. Canal over the drainage

• Aqueduct

• Syphon aqueduct

2. Canal below the drainage

• Superpassage

• Canal syphon

3. Canal at the same level as drainage

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• Level crossing

• Inlet

• Inlet and outlet

1. Canal over the drainage

(i) Aqueduct: An aqueduct (also called an

ordinary aqueduct) is a structure in which

the canal flows over the drainage and the

flow of the drainage in the barrel is open

channel flow. An aqueduct is similar to an

ordinary road bridge (or railway bridge)

across drainage, but in this case, the canal is

taken over the drainage instead of a road (or

a railway). The canal is taken over the

drainage in a trough supported over the

piers constructed on the drainage bed. An

aqueduct is provided when the canal bed level is higher than the H.F.L. of the drainage.

[Note: In the case of an aqueduct, the term culvert is commonly used for the barrel.]

(ii) Syphon aqueduct: In a

syphon aqueduct also the

canal is taken over the

drainage, but the flow in

the barrel of the drainage is

pipe flow. A syphon

aqueduct is constructed

when the H.F.L. of the

drainage is higher than the

canal bed level. When

sufficient level difference is

not available between the

canal bed and the H.F.L. of the drainage to pass the drainage water, the bed of the drainage

may be depressed below its normal bed level. The drainage is provided with an impervious

floor at the crossing and thus a barrel is formed between the piers to pass the drainage water

under pressure. These barrels actually form an inverted syphon and not syphon. However, in

the common usage, the term syphon is generally used.

2. Canal below the drainage

(i) Superpassage: In a superpassage, the canal is

taken below the drainage and flow in the canal is

open channel flow. A superpassage is thus reverse of

an aqueduct. A superpassage is required when the

canal F.S.L. is below the drainage bed level. In this

case, the drainage water is taken in a trough

supported over the piers constructed on the canal bed.

(ii) Canal syphon: A canal syphon (or simply a

syphon) is a structure in which the canal is taken

below the drainage and the flow in the barrel of the canal is pipe flow. It is thus the reverse of

a syphon aqueduct. A canal syphon is constructed when the F.S.L. of the canal is above the

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drainage bed level. Because some

loss of head invariably occurs when

the canal flows through the barrel of

the canal syphon, the command of the

canal is reduced. Moreover, there

may be silting problem in the barrel.

As far as possible, a canal syphon

should be avoided.

3. Canal at the same level as the drainage

(i) Level crossing: A level crossing is provided when the canal and the drainage are

practically at the same level. In a level crossing, the drainage water is admitted into the canal

at one bank and is taken out at the

opposite bank. A level crossing

usually consists of a crest wall

provided across the drainage on the

upstream of the junction with its

crest level at the F.S.L. of the

canal. The drainage water passes

over the crest and enters the canal

whenever the water level in the

drainage rises above the F.S.L. of

the canal. There is a drainage

regulator on the drainage at the d/s

of the junction and a cross-

regulator on the canal at the d/s of

the junction for regulating the

outflows. A level crossing is

provided on the canal when it is more or less at the same level as the drainage and there is a

large discharge in the drainage for a short duration. The main disadvantage of a level crossing

is that an operator is required to regulate the discharge.

(ii) Inlet: An inlet alone is sometimes provided when the drainage is very small with a very

low discharge and it does not bring heavy silt load. Of course, it increases the discharge in the

canal, which is absorbed in the space provided as the free board above the F.S.L.

(iii) Inlet and outlets: An inlet-outlet structure is provided when the drainage and the canal

are almost at the same level, and the discharge in the drainage is small. The drainage water is

admitted into the canal at a suitable site where the drainage bed is at the F.S.L. of the canal.

The excess water is discharged out the canal through an outlet provided on the canal at some

distance downstream of the junction. An outlet is usually combined with some other masonry

work where an arrangement for removing the excess water is even otherwise required.

Selection of a Suitable Site of Cross Drainage Work

The following points should be considered while selecting the site of a cross-drainage work:

1. At the site, the drainage should cross the canal alignment at right angles. Such a site

provides good flow conditions and also the cost of the structure is usually a minimum.

2. The stream at the site should be stable and should have stable banks.

3. For economical design and construction of foundations, a firm and strong sub-stratum

should exit below the bed of the drainage at a reasonable depth.

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4. The site should be such that long and high approaches of the canal are not required.

5. The length and height of the marginal banks and guide banks for the drainage should be

small.

6. In the case of an aqueduct, sufficient headway should be available between the canal

trough and the high flood level of the drainage.

7. The water table at the site should not be high, because it will create dewatering problems

for laying foundations.

8. As far as possible, the site should be selected d/s of the confluence of two streams, thereby

avoiding the necessity of construction of two cross-drainage works.

9. The possibility of diverting one stream into another stream upstream of the canal crossing

should also be considered and adopted, if found feasible and economical.

10. A cross-drainage work should be combined with a bridge, if required. If necessary, the

bridge site can be shifted to the cross-drainage work or vice versa. The cost of the combined

structure is usually less. Moreover, the marginal banks and guide banks required for the river

training can be used as the approaches for the village roads.

Selection of a Suitable Type of Cross Drainage Work

The following factors should be considered while selecting the most suitable type of the

cross-drainage work.

1. Relative levels and discharges: The relative levels and discharges of the canal and of the

drainage mainly affect type of cross-drainage work required. The following are the broad

outlines: (i) If the canal bed level is sufficiently above the H.F.L. of the drainage, an aqueduct

is selected. (ii) If the F.S.L. of the canal is sufficiently below the bed level of the drainage, a

superpassage is provided. (iii) If the canal bed level is only slightly below the H.F.L. of the

drainage, and the drainage is small, a syphon aqueduct is provided. If necessary, the drainage

bed is depressed below the canal. (iv) If the F.S.L. of the canal is slightly above the bed level

of the drainage and the canal is of small size, a canal syphon is provided. (v) If the canal bed

and the drainage bed are almost at the same level, a level crossing is provided when the

discharge in the drainage is large, and an inlet-outlet structure is provided when the discharge

in the drainage is small. However, the relative levels of the canal and the drainage can be

altered to some extent by changing the canal alignment to have another crossing. In that case,

the most suitable type of the cross-drainage work will be selected depending upon the levels

at the changed crossing.

2. Performance: As far as possible, the structure having an open channel flow should be

preferred to the structure having a pipe flow. Therefore, an aqueduct should be preferred to a

syphon aqueduct. Likewise, a superpassage should be preferred to a canal syphon. In the case

of a syphon aqueduct and a canal syphon, silting problems usually occur at the crossing.

Moreover, in the case of a canal syphon, there is considerable loss of command due to loss of

head in the canal. The performance of inlet-outlet structures is not good and should be

avoided.

3. Provision of road: An aqueduct is better than a superpassage because in the former, a road

bridge can easily be provided along with the canal trough at a small extra cost, whereas in the

latter, a separate road bridge is required.

4. Size of drainage: When the drainage is of small size, a syphon aqueduct will be preferred

to an aqueduct as the latter involves high banks and long approaches. However, if the

drainage is of large size, an aqueduct is preferred.

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5. Cost of earthwork: The type of cross-drainage work which does not involve a large

quantity of earthwork of the canal should be preferred.

6. Foundation: The type of cross-drainage work should be selected depending upon the

foundation available at the site of work.

7. Material of construction: Suitable types of material of construction in sufficient quantity

should be available near the site for the type of cross-drainage work selected. Moreover, the

soil in sufficient quantity should be available for constructing the canal banks if the structure

requires long and high canal banks.

8. Cost of construction: The cost of construction of cross-drainage work should not be

excessive. The overall cost of the canal banks and the cross-drainage work, including

maintenance cost, should be a minimum.

9. Permissible loss of head: Sometimes, the type of cross-drainage is selected considering the

permissible loss of head. For example, if the head loss cannot be permitted in a canal at the

site of cross-drainage, a canal syphon is ruled out.

10. Subsoil water table: If the subsoil water table is high, the types of cross-drainage which

requires excessive excavation should be avoided, as it would involve dewatering problems.

11. Canal alignment: The canal alignment is sometimes changed to achieve a better type of

cross-drainage work. By changing the alignment, the type of cross-drainage can be altered.

The canal alignment is generally finalised after fixing the sites of the major cross-drainage

works.

General Design Requirements for CD Works

1. Data

For preparing the design of a cross drainage structure, the following specified hydraulic data

should be made available.

(a) Canal - Full supply discharge, Q; Bed width; Full supply depth; Water surface slope; Bed

level; Bed slope; Full supply level; Top of bank level; Cross section of canal showing Natural

Ground Level; Subsoil water level; and Nature of bed material and value of ‘n’ (rugosity

coefficient in Manning’s formula).

(b) Drainage Channel - Extent and nature of drainage area (catchment area); Maximum

annual rainfall and the period (years) of data; Maximum intensity of rainfall with year;

Maximum observed flood discharge at the site; Maximum flood level; Water surface slope;

Site plan of proposed crossing including contours; Log of borehole or trial pit data; Type of

bed load of drainage channel; Longitudinal section of the stream for suitable distance

upstream and downstream of the canal depending upon site conditions; Cross section of the

drainage channel for a distance 100 m to 300 m upstream and downstream, at intervals of 10

m to 50 m; Waterways provided in road and railway bridges or other hydraulic structures on

the drainage channel; Spring water level at the crossing site in May and October; and Silt

factor.

2. Design flood

The design discharge of the drainage should be selected considering various factors such as

the size of the drainage, the size of canal, importance of the canal and type of the cross-

drainage work. The following are the broad guidelines for estimating the design discharge.

1. For very large cross-drainage works where the failure of the structure may lead to

disruption of canal supplies over a long period, the design flood should be taken equal to the

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standard project flood (S.P.F.).

2. For moderate type of structures, the waterway is usually determined for the flood of 50

years recurrence interval but for the foundation and free board, the flood of 100 years

recurrence interval is taken.

3. For small cross-drainage works, the design flood is usually taken as 10 to 25 years flood,

and an increased afflux is also considered.

4. For important structures, an additional margin of safety is usually provided in the design of

foundation and free board fixation to the take care of unexpected large floods by increasing

the design discharge, depending upon the area of catchment.

3. Waterway

Waterway for a cross drainage work is fixed from hydraulic and economic considerations

with particular reference to: a) design flood, b) topography of the site, c) existing and

proposed section and slope of the drainage channel in the vicinity of the crossing, d)

permissible afflux, and e) construction and maintenance aspects. In plains, the drainage

channels are generally in alluvium and the waterway usually provided in works without rigid

floor is about sixty to eighty percent of the perimeter, given by Lacey’s formula QCP =

where C = a coefficient varying from 4.5 to 6.3 according to local conditions, the usual value

adopted being 4.8 for regime channel.

NOTES

1. When flow carries abrasive materials with it, the permissible values may be further

reduced by 25%.

2. Hard steel troweling, power floating, smooth surface finish and continuous long

curing can have higher abrasion resistance, and higher velocities than that given in

this table can be permitted, for surface using cement.

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The value of wetted perimeter obtained is the total waterway between the two faces of

the abutments. In works with rigid floors, however, waterway can be further flumed within

the permissible limits of velocity negotiated through the available ventages. Ordinarily such

velocities should be limited to the values given in Table. For sub-mountainous and

mountainous terrains with flashy flows, the waterway is provided within the width of the

existing stream. Where the slope of the natural drainage channel is quite steep suitable

methods may be adopted to bring the velocity within the desired limits. The minimum

dimension of openings should be such as to permit, as far as possible, manual clearing of

deposits therein.

4. Free board

Free board in the case of canals is the difference between the F.S.L. of the canal to the top

level of banks or the formation level of guide banks. In the case of a drainage, the free board

is the difference between the H.F.L., including afflux, and the top of the embankment or

guide banks. A minimum free board of 0·6 m is usually provided. However, it should be

increased suitably for large discharges or wherever heavy wave action is anticipated.

5. Canal Transitions

A canal is flumed to reduce the length of barrels (or culverts) in an aqueduct (or a syphon

aqueduct). Fluming Ratio - Except when dictated by conditions particular to a specific

structure, a fluming ratio less than seventy percent may not be adopted. For the purpose of

computing the fluming ratio of canal, the width at mid depth may be taken as one hundred

percent. In drainage channel when the course is undefined, a fluming ratio from seventy to

ninety percent of the Lacey’s waterway may be adopted.

Suitable canal transitions are provided on the u/s and d/s of the flumed section. A

channel transition is a gradual change in the cross-section of the channel that produces a

change of flow from one uniform state to another. This change in flow occurs over the length

of transition. A transition avoids excessive energy loss, eliminates cross-currents and

turbulence and thus provides safety to the structure. Transition walls as seen in plan, should

at their ends turn nearly at right angles to the flow in the channel and should extend for a

minimum length of 0.6 m into the earth bank. Suitable pitching may be provided to the

slopes, beyond the Transition end. In a transition, varied flow (non-uniform flow) occurs, and

the accelerating or decelerating forces are more predominant than the frictional resistance.

Besides aqueducts and syphon aqueducts, fluming of canal is also sometimes done at

superpassages and canal syphons, falls, regulators and bridges to reduce the cost of the

structure. In all these cases, transitions are provided to minimise the head loss and reduce the

maintenance cost. Fluming can be done by either of the following two methods.

• By reducing the width of the channel without varying the depth of the channel.

• By reducing the depth of the channel with or without varying the width. However,

generally the width is also varied.

The transitions may be classified into two types:

1. Contraction transitions 2. Expansion transitions

1. Contraction transitions: In contraction transitions, the cross-sectional area is gradually

reduced. The design of contraction transition is relatively easy. In this case, the accelerating

forces tend to counterbalance the boundary shear and consequently the losses are small.

Unless the velocity is very high or the contraction too severe, any suitable streamlined shape

like a bell-mouth or a cylindrical quadrant of transition can be provided as a contraction

transition. The flow through a contraction transition is always stable because of favourable

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pressure gradient. The length of transition depends upon the degree of contraction adopted,

the approach condition, the type of structure and the permissible head loss. An average splay

of 2: 1 is usually provided. However, for important high-velocity flumes, an average splay of

3 : 1 to 4 : 1 will give better flow conditions. On the other hand, for low velocity, unimportant

structures, the splay may be even 1 : 1. The contraction transition should be tangential to the

walls at the throat of flume where velocity is high.

2. Expansion transitions: In expansion transitions, the cross-sectional area is gradually

increased. In this case, the decelerating forces tend to increase the boundary layer effect and

the losses are more. The pressure gradient is positive and hence unfavourable. The boundary

flow is unstable and may result in flow separation from the boundary and lead to turbulence

and eddies. Moreover, the high intensity of shear at the separation surface produces

appreciable circulation and rollers may be formed. These rollers are the main sources of head

loss associated with such flows, since the main flow has to infuse the energy to sustain them.

Therefore, expansions transitions should effect the change more gradually as compared to

that in contraction transitions. The exact laws governing the complex flow in expanding

transitions are not known. Simplifications are usually done to arrive at approximate solutions.

Providing very long transitions is quite costly. A splay of 3 : 1 is usually adopted. Sometimes,

splay of 4 : 1 and 5 : 1 are adopted in the cause of high velocity flumes. In addition,

separation control devices, such as splitter vanes, bed deflectors, sills, baffles, etc. are also

sometimes used for minimising the separation of flow.

Methods of Design of Transitions: The following methods are commonly used for the

design of transitions:

1. Chaturvedi's method

2. Mitra's method

3. Hind's method

In the first two methods, it is assumed that the depth remains constant and the width varies;

whereas in the third method, both the depth and width are varied.

1. Chaturvedi's method: Chaturvedi used the two dimensional approach to analyse the flow.

He assumed that the lateral velocity is a function of the depth but the longitudinal velocity

remains constant. He demonstrated that for a hyperbolic expansion, the lateral acceleration of

water is positive and increases with width. On the basis of experimental studies, he proposed

the following equations for a semi-cubical transition

of constant depth

( )( )5.15.15.1

5.1

1 xf

fc

cfBB

BB

BLx −

−=

where x is the distance from the throat, Lf is the

length of transition flume, Bc is the normal width of

the channel, Bf is the width of throat and Bx is the

width of transition at a distance x from the throat.

2. Mitra's method: Mitra proposed a hyperbolic

transition, based on the following two assumptions:

1. The rate of change of velocity along the

length of transition remains constant.

2. The depth of flow remains constant.

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An equation for the shape of transition can be derived these assumptions. According to the

first assumptions

f

cfxf

L

VV

x

VV −=

−

where Vf and Vc are velocities in the flume and channel respectively; and Vx is the velocity in

the transition at a distance x from the throat.

From continuity and the second assumption

)(sayCVBVBVBQVdBVdBVdB xxccffxxccff ===⇒===

Therefore,

xxccff BCVBCVBCV ===

Substituting these values in the Equation based on the first assumption

cfffxfcffxf BLBLxBxBB

C

B

C

LB

C

B

C

x

111111−=−⇒

−=

−

Solving for Bx

( )fcfc

ffc

xBBxLB

LBBB

−−=

Mitra's transitions have been commonly used in U.P. and elsewhere and have been found to

be quite satisfactory. In these transitions, there is a positive lateral acceleration which

eliminates separation. However, these transitions are useful for constriction less than 50% i.e.

width at the throat is more than 50% of the normal width.

Disadvantages of transitions: It is very costly to provide long expanding transitions.

Sometimes transitions of shorter length are used and various separations control devices, such

as splitter vanes, bed deflector, sills, and baffles are provided to reduce separation of flow.

However, these devices increase the loss of head and should be used with caution

6. Foundations of Cross-Drainage Works

The cross-drainage works are somewhat like road and railway bridges. The load acting on

these works is mainly due to the weight of water. However, some portion of the aqueducts

and syphon aqueduct is generally used as a road bridge which should be designed for the

loads specified for the bridges. The choice of the type of super-structure depends upon the

most economical arrangement possible at a given site. A common criterion used for the

determination of the most economical span is to keep the cost of superstructure equal to that

of substructure. However, while applying this criterion, the foundation conditions should also

be considered. If the cost of individual piers and their foundations is high, the minimum

number of piers should be provided by increasing the span and vice-versa. The cost of

substructure depends upon the type of foundation and the height of piers.

7. Depth of Scour

The type of foundation usually depends upon the depth of scour. If the scour is small, shallow

foundations may be adopted. However, if the scour is excessive, deep foundations are

required. Generally, the well foundations are used as deep foundations. The normal scour

depth in alluvial streams is usually determined by Lacey's formula. The mean depth of scour

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in metres below the check/high flood level may be calculated from the equation:

( ) 3/1234.1 sfism KDd =

where Di = the discharge in cumecs per metre width. The value of Di should be the maximum

of the following: (i) the design flood divided by the effective linear waterway between

abutments or guide bunds, as the case may be. (ii) The value obtained should take into

account any concentration of flow through a portion of the waterway assessed from the study

of the cross section of the drainage channel. Such modifications of the value may not be

deemed applicable to minor cross drainage structures with overall waterway less than 60 m.

(iii) Actual observation, if any. Ksf = the silt factor for representative sample of the bed

material obtained up to the level of the deepest anticipated scour and given by the expression

md76.1 in which dm being the weighted mean diameter in millimetres. dm may be taken as

the grain size at 50% passing from grain size distribution curve. The above method of

estimating dsm is based on Lacey’s theory for regime conditions in alluvial beds.

Maximum Depth of Scour for Design of Foundation: The maximum depth of scour below the

Highest Flood Level (H.F.L.) at obstructions and configurations of the channel should be

estimated from the value of ‘dsm’ on the following basis : For the design of piers and

abutments located in a straight reach and having individual foundations without any floor

protection works

(i) In the vicinity of piers = 2.0 dsm

(ii) Near abutments = 1.27 dsm (approach retained) or 2.00 dsm (scour all around).

For the design of floor protection works, for raft foundations or shallow foundations, the

following scour values should be adopted:

(i) in a straight reach 1.27 dsm

(ii) at a moderate bend 1.50 dsm

(iii) at a severe bend 1.75 dsm

(iv) at a right angled bend 2.00 dsm.

NOTE - The values of scour depth obtained as above may be suitably modified where actual

observed data is available.

8. Bank Connection

The bank connection consists of masonry wing walls of the canal and the drainage. These are

required to connect the regular section of the canal and the drainage to the modified section at

the cross-drainage site.

Canal wing walls are provided on the upstream and downstream side of the aqueduct

to guide the canal flow and to retain the earth of the canal banks on both sides of the canal

trough. The foundation of the canal wing walls should kept on the sound natural ground. It

should not be left on the made up formation. The faces of wing walls are warped from the

natural section of the canal (usually, 1.5 : 1) to the vertical at the trough.

Drainage wing walls are provided on the upstream and downstream of the barrel (or

culvert) to guide the drainage flow and to retain the natural banks of the drainage. The wing

walls should be taken sufficiently deep into the guide banks. The wing walls should be

shaped such as to provide smooth entry and exit at the drainage barrel. As the bed of the

drainage gets scoured during floods, the foundation of the drainage wing walls should be

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taken deep below the maximum scour

depth. The wing walls of canal and drainage

are designed to withstand earth pressure due

to the dry soil above the hydraulic gradient

line and due to the submerged soil below

this line.

9. Weep holes

Weep holes are small openings in the

retaining walls, like wings (i.e. transitions

of natural stream). These are to facilitate the

drainage of backfills and avoid build up of

pressure. Weep holes may be provided

above the flow net line of zero water

pressure, under the condition of canal

flowing full and natural stream with lowest annual flow. Weep holes if provided, should have

filters with graded material suitably provided to avoid piping of earth fill behind the wall and

also to avoid choking of the holes. The provision of weep holes should be so, as to not render

the creep coefficient of seepage unsafe, and. should also not contribute to enhanced loss of

canal water.

DESIGN OF AQUEDUCTS

Types: Depending upon the cross-section of the canal over the barrel (or culvert), the

aqueducts and syphon aqueducts are classified into the following three types:

1. Type I Aqueduct: In this type of aqueduct (or syphon aqueducts), the cross-section of the

canal is not changed. The original cross-section of the canal with normal side slopes is thus

retained. The length of the barrel through which the drainage passes under the canal is a

maximum in this type of structures, because the width of the canal section is a maximum. In

this type of structures, the canal wings are not required. This type is suitable when the width

of the drainage is small (say less than 2.5 m). If the section is changed, the cost of canal

wings would be large in comparison to the saving resulting from decreasing the length of

culvert.

2. Type II Aqueduct: In this type of aqueduct (or syphon aqueduct), the outer slopes of the

canal banks are discontinued and replaced by retaining walls. Thus the length of the barrel is

reduced, but the cost of retaining wall is added to the overall cost. This type of structure is

suitable when the width of the drainage is moderate (say 2.5 m to 15 m) so that the cost of

retaining walls is less in comparison to the saving resulting from decreasing the length of

barrel.

3. Type III Aqueduct: In this type of aqueduct (or syphon. aqueduct), the entire earth section

of the canal is discontinued and replaced by a concrete or masonry trough over the drainage.

This type of structure is generally suitable when the width of the drainage is very large (say

more than 15 m), so that the cost of the trough and canal wing walls is less in comparison to

the saving resulting from decreasing the length of barrel. In this type of structure, the canal

can be easily flumed, which further reduces the length of the barrel.

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Selection of suitable type: The choice of the type of aqueduct should depend on consideration

of economy which in turn would depend mainly upon the size of the drainage to be passed in

relation to the size of the canal and the foundation strata. Over a small drainage channel, an

aqueduct of Type 1 may be suitable as NO canal transitions would be required. The savings

made due to absence of canal transitions would more than compensate the increased cost due

to the length of drainage culverts which would have larger length (across the canal). Over a

large river an aqueduct of Type 3 may be more economical as the length of drainage culverts

across the canal is small and the saving made in cost of drainage culverts would be greater

than the increased cost of canal transitions. For intermediate conditions an aqueduct of Type

2 may work out to be more

economical. However, techno-

economic studies should be

carried out to decide the exact

type of aqueduct to be

constructed. Therefore, a very

small drainage requires a type I

aqueduct, which in many cases

may be merely a pipe or a small

culvert passing under the canal.

On the other hand, over a river of

a large size type III aqueduct

would be the most economical.

For moderate size of the

drainage, type II aqueduct may be

most suitable. However, the

actual limits with regard to the

size of drainage for which one

particular type of aqueduct will

be the most suitable will vary

with local conditions and the cost

of construction. Comparative

estimates should be prepared for

finding out the most economical

type of aqueduct for a particular site.

Design Load and Structural Stability: The forces acting on the various parts of the

structure are evaluated and the worst combination of forces is taken in the design. The loads

and forces to be considered in designing aqueducts are as follows: (a) Dead load; (b) Water

load; (c) Live load; (d) Impact or dynamic effect of the live load; (e) Longitudinal forces

caused by the tractive effort or by braking force of vehicles and/or those caused by restraint

to free movement of bearings; (f) Wind load; (g) Horizontal force due to water currents; (h)

Centrifugal forces - in case the aqueduct and/or the road is curved in plan; (i) Buoyancy; (k)

Earth pressure; (1) Forces due to temperature variation; (m) Erection loads; (n) Seismic load;

and (p) Water pressure.

NOTE - (d) and (e) are applicable only in case a road bridge is provided over the aqueduct.

Wind load should not be considered simultaneously with earthquake.

Generally the following design combinations should be considered: (a) Canal empty

and stream/drain at its low water level - normal condition without earthquake; (b) Canal

running full up to its F.S.L. and stream/drain at its low water level – normal condition

without earthquake; (c) Canal empty and stream/drain at its H.F.L. without earthquake; (d)

13

Canal running full up to F.S.L. and stream/drain at its H.F.L. without earthquake; (e)

Construction condition (i) Pier is constructed and superstructure is not constructed and

stream/drain at it’s H. F. L. (design) without earthquake. (ii) Superstructure is constructed on

one side of a pier and stream/drain at it’s H.F.L (design) without earthquake. NOTE - (a) and

(b) combinations of loadings may also be checked for seismic conditions after accounting for

higher permissible stresses. For design of aqueducts the effect of earthquake forces in all the

three directions that is longitudinal (L), transverse (T) and vertical (V) should be taken into

account. The combination of these should be either T + V or L + V at a time.

Layout

The layout of the aqueduct should be so fixed that it is preferably in a straight reach of

drainage channel. The canal/carrier channel should be at right angles to the drainage channel

as far as possible. Bank connections to canal and drainage channel should be provided

depending upon the properties of the soil available in the area. Wing walls for drainage may

be provided with 2 : 1 and 3 : 1 splays on upstream and downstream side; the splay should

not be flatter than 3 : 1 and 4 : 1 respectively. Drainage wing walls should be suitably

connected to high ground. Canal transitions should preferably be provided with 2 : 1 and 3 : 1

splays on upstream and downstream side, but not flatter than 3 : 1 and 5 : 1 respectively.

However, it should be ensured that the flow follows the boundaries of the transition. The

drainage .channel shall be directed towards the structure by suitable training works like

training walls, guide banks, spurs, etc. The canal banks adjacent to the cross drainage work

should be protected by suitable protective measures such as turfing, pitching and launching

apron, wherever necessary. Uplift pressure and exit gradient caused by seepage flow from the

canal when it is running full and the drainage channel is dry, be accounted for in design. For

reducing the uplift pressure and exit gradient pucca floor should be provided for in the canal

bed in adequate lengths upstream and downstream of the work with cut-off walls at the ends.

Pucca floors of adequate lengths should be provided at either end of the barrel in the drain

with cut-off wall at the end.

Clearance for Aqueducts

The clearance for Rectangular Openings will depend upon the relative levels of the canal bed

and high flood level of the drainage channel. Values given in Table are suggested as suitable

minimum clearances (taking into account allowable afflux) for purposes of design, where

available.

Design Steps

As already discussed, an aqueduct carries the canal water over the drainage such that the

bottom of the canal trough (or the roof of the culvert or barrel) is above the H.F.L. of the

14

drainage. Before the actual design, all the available data of the canal and drainage at the

crossing site should be collected. The design discharge and H.F.L. of the drainage should be

estimated. The types of super-structure should be selected. The design procedure may be

summarised as follows:

1. Check whether proper clearance is available or not.

2. Determine the required waterway from Lacey's equation, QP 75.4= . From the

waterway, select suitable span and the number of spans.

3. Determine the canal waterway after deciding the fluming ratio and type of aqueduct.

Generally for large canals, the canal is flumed and type III aqueduct is provided.

4. Fix the dimensions of the canal trough, and the number of compartments. Generally,

an inspection road is provided over one of the compartments of the trough.

5. If the canal is flumed, deign the transitions.

6. Find the losses in transitions and canal trough and then fix levels of the TEL, WSL

and canal bed level at key points.

7. Determine the height of the piers and abutments.

8. Estimate the maximum scour and design the foundation.

9. Provide suitable bank connections.

DESIGN OF SYPHON AQUEDUCTS

Layout: The layout of syphon aqueduct shall be so fixed that the drainage channel crosses

the carrier channel preferably at right angles.

Materials: For the construction of the syphon barrels, prestressed concrete, RCC or masonry

or a combination of these may be used depending upon the availability of materials, labour

15

and relative economy.

Types of Syphon aqueducts: Syphon aqueducts may be classified into the following two

types:

1. Barrel type, using barrel for drainage waterways, and

2. Trough type, using trough section for carrier channel waterways.

The type of syphon aqueduct to be adopted shall be decided on the basis of relative economy

and restraints of carrier channel design.

Barrel Type

In barrel type of syphon aqueducts, the entire carrier channel portion may be taken as it is

over the barrel or it may be flumed to a rectangular or trapezoidal section to reduce the length

of barrels. The floor level of barrels shall be fixed in relation to the drainage bed level at the

syphon aqueduct site. The future retrogression and regrading of drainage channel depending

on outfall conditions may also be considered. The floor level is combination with ventway

shall be judiciously fixed below the existing drainage bed in such a manner that a water seal

of 1.5 times the change in velocity head, with a minimum of 150 mm, should be provided

over the crown of barrels at start to prevent air entering the barrel. At the outlet end of the

syphon, the top of the barrel may be kept slightly depressed below the normal downstream

flood level in the drainage channel. The amount of this depression may be equal to the

difference in the velocity head at the exit end of the barrel and that in the drainage channel on

the downstream side. In case of barrel of RCC box type or RCC slab with masonry walls, a

minimum cushion of 300 mm may be provided with the precaution that heavy vehicles do not

ply over the barrels unless the cover is suitably increased and the structure is properly

designed. In case of RCC pipes and circular barrels, a minimum cushion of 900 mm should

be provided. This will protect the pipes and barrels against damage by the movement of

construction equipment over them. This cover also permits any future regrading of the carrier

channel.

At the site of syphon aqueduct the drainage bed is generally depressed and provided

with PUCCA floor. On the upstream side, the drainage bed may be joined to the PUCCA

floor by a vertical drop (when drop is of the order of 1 m or less) or by a glacis of 3:1 (when

drop is more then 1 m). The downstream rising slope should not be steeper than 5:1.

Depending upon the bed level of carrier channel and HFL of drainage channel, the

barrels under the carrier channel bed portion are generally lower than the barrels at the entry

and exit. This difference is negotiated with the provision of sloping length of barrels under

carrier channel banks. The upward inclination of the barrel shall start from a point at least 1.0

m away from the end of the carrier channel bed width on either side.

The length of barrels should be fixed on the consideration of economics of increasing

barrel length with respect to reduction in length of wing walls of drainage channel and height

of breast wall and practical and economical depth of cutoff under barrels for safe exit

gradient. However the length should be sufficient to accommodate the width of service road.

The main considerations for design of syphon barrels are the following: a) It should

be safe against uplift, b) It should be strong to resist internal and external forces, c) It should

be safe against subsurface flow, and d) It should be safe against surface flow.

Safety against Uplift: The barrels shall have enough loads to resist upward buoyancy force

tending to lift it. The barrels underneath carrier channel bed are critical for checking against

uplift. The safety of barrels should be checked for the following three conditions: a) Carrier

16

channel at full supply level and drainage barrels empty, b) The drainage channel at designed

flood level and carrier channel empty, and c) Carrier channel is suddenly closed and drainage

barrel is empty thereby causing two thirds of the head corresponding to carrier channel full

supply level to act. Lesser percentage of head up to one-third of total head may be considered

in case the carrier channel section is made of relatively permeable material. The minimum

factor of safety against uplift should be 1.2 in all the three conditions.

NOTE: Full hydrostatic head from carrier channel full supply level to the drainage barrel

foundation should be taken for checking the stability. The bottom slab of barrel may be

suitably projected beyond its side walls to take advantage of the weight of the earth wedge

over the projection in counteracting the uplift forces. If the weight is taken to counteract

uplift in design computation, the earth cushion over the barrels should not be allowed to fall

below the corresponding design depth. In such cases the carrier channel bed should be

adequately protected against erosion by providing a suitable protective cover over the earth

extending 20 m upstream and downstream of the syphon. Lean concrete/random rubble

masonry over the barrel as additional weight may be provided.

Safety against Internal and External Forces: The barrels should be designed strong enough

for the dead load of the structure, earth and water loads, earth and water pressures, soil

reaction and uplift pressure and live load, if any. The combination of loads which will result

in maximum stresses shall be carefully considered. Due to the difference in loading, the

length of barrels of major structures can be divided into two portions, one under carrier

channel bed and the other under carrier channel banks for economical design. The barrels for

the purpose of transverse analysis shall be treated as a box. The box shall be analyzed by any

standard method. For the following conditions and loadings to determine the worst moments,

shear and thrust at any section: Condition (i) Carrier channel at full supply level and barrels

dry. Condition (ii) Carrier channel dry and the drainage channel at degined HFL including

afflux. Longitudinal analysis shall be made in cases where loose soil or various types of soils

are met with at the completed final level of foundation.

Safety against Sub Surface Flow: Cut-offs: The depths of cut-off shall be calculated from

scour and exit gradient considerations. Depth of cut off below the entrance and exit ends of

barrels may be provided along the width of the barrels and along the river or the drainage

channel wings up to 1.25 to 1.5 times the normal scour depths below HFL based on the site

conditions. In case of rocks the cut off shall be taken minimum 1.0 m from the sill of the

barrel into the fresh rock. The width of concrete cut off shall not be less than 0.3 m. Depth of

cut off shall be checked for safe exist gradient in accordance with Khosla’s theory for two

dimensional flow. In syphons of carrying capacity over 20 cumecs the effect of 3 dimensional

seepage flows on exit gradient shall be considered. For this, electrical analogy model testing

should be carried out and cut offs at the end of the barrels should be provided accordingly.

The safe value of exit gradient for different types of soil generally adopted can be as follows:

Clay - l in 4; Shingle - l in 4 to 5; Coarse sand – l in 5 to 6; Fine sand – l in 6 to 7. The

vertical cut off shall also be provided under river or drainage channel wings. However, the

depth of cut off may be suitably reduced under wings depending upon the length of wings,

but should be adequate from scour considerations. Ribs at suitable spacings may be provided

to increase the seepage path. A suitable filter under open jointed cement concrete blocks or

rubble should be provided along the cutoff that is u/s of upstream cut-off and d/s of

downstream cut-off. The length of filter to be provided should be 1.5 times the, scour depth

(below drainage bed). The filter should be designed in accordance with standard criteria

conforming to IS 8237. The safety of filter should also be checked against heave. Scour shall

be considered at entry and exit of syphon barrels in accordance with IS 7784 (Part 1) and

launching appron adequate to provide a cover of 0.6 to 0.9 m over the entire slope of the

17

scour shall be provided.

Trough type

In trough type of syphon aqueducts, carrier channel water is taken across drainage channel

through a trough supported on barrels or on piers/abutments raised from the drainage bed.

Bottom of the trough of carrier channel is lower than HFL of drainage channel. An

impervious floor, if necessary, with protection against surface and subsurface flow may be

provided in drainage bed. In the case of trough type syphon aqueduct, both carrier channel

transitions and drainage wings shall be provided.

The basic design features of this type shall be the same as those of aqueducts [see IS

7784 (Part 2/See l)]. However, the carrier channel trough in this type should be designed so

as to provide the dead load of the trough at least 1.2 times the upward thrust acting on it when

the drainage channel is in high floods and the carrier channel is dry. If it is not so, the trough

shall be suitably anchored to the piers. The floor of the syphon aqueduct shall be suitably

designed for the uplift pressures acting on it.

Additional Provisions

The outer slopes of carrier channel banks and drainage channel slope should be protected in

the vicinity of syphon aqueduct by pitching. For large size drainage channel, properly

designed guide banks should be provided. In major syphon aqueducts stop-log grooves in the

barrels at the upstream and downstream ends may be provided by extending the partition

walls to facilitate isolating one or more barrels for annual repairs and maintenance. Stability

of barrels shall be suitably ensured in this case. A retaining wall should be constructed over

the barrels to retain the carrier channel banks slopes over the barrels. If constructed in

reinforced cement concrete, these walls (breast wall) should be constructed monolithically

with the top slab of the barrels. Adequate anchorage of reinforcement as well as

reinforcement for proper transference of loads and moments to the top slab shall be provided.

The effect of live load, wherever applicable, shall also be considered in the design of this

wall. At the junction of this wall with barrel, a haunch of suitable size shall be provided. The

design of the wings, both for carrier channel and the drainage channel shall be carried out for

earth and water pressures calculated according to standard practice. The effect of live load

and surcharged effect, if any, shall be taken into account while designing the wings. Wing

walls shall be checked for earthquake conditions also, in seismic zones.

Drainage Wings

The length of drainage wings shall be adjusted so as to contain the slopes of carrier channel

embankments. The wings shall be provided straight or in a smooth curve giving a minimum

splay of 2:1 on upstream and 3:1 on downstream. If necessary, return wall may be provided

thereafter. The top of wings shall be kept at least 300 mm higher than the HFL of the

drainage channel. The wing wall sections shall be checked for carrier channel full and

drainage channel dry condition, considering backfill as saturated. No passive resistance shall

be considered from drainage channel side. If the foundation of wing wall requires to be taken

deeper than 1.5m from consideration of scour, a concrete cut-off of required depth shall be

provided along the upstream face of the wing wall.

Carrier Channel Section and its Fluming

The carrier channel embankment adjoining the syphon aqueduct should have adequate

provisions to avoid possibility of any breach and to minimize seepage. The outer slope of

bank should have a clear cover of 600 to 900 mm over the designed phreatic line often

referred to as hydraulic gradient. High banks (say, more than 6 m height above ground level)

18

should be checked for slope stability and normal provisions of filter and rock toe should be

made. In cases where HFL in the drainage channel is substantially higher than bed of the

carrier channel, the bank of the carrier channel should be checked for the condition when

drainage channel is in high floods and the carrier channel is dry.

Fluming ratio shall be adopted as given in IS 7784 (Part 1) keeping in view the

permissible head loss in the carrier channel, and whether the carrier channel is lined or

unlined.

Limiting Velocity

The vertical slope on approaches should not be steeper than 1 in 3 on the entry side and 1 in 4

on the exit side. The minimum permissible velocity allowed in the drainage channel may be

derived from Table. However, the velocity in the barrels shall not exceed the maximum

permissible velocity.

Afflux

The afflux to be adopted in the design should be that which would correspond to the design

flood. The afflux should be restricted to such a value that the resulting velocity does not

cause serious bed scour in the drainage or does not create submergence which cannot be

permitted.

19

Rational formulae

Broad crested weir discharge formula or orifice discharge formula depending upon the flow

conditions through the cross drainage work openings, may be applied for calculating afflux.

When the performance of the cross drainage work openings remains unaffected by the depth

downstream of the obstruction, that is, a standing wave is formed, weir formula is applicable,

otherwise the orifice formula holds good. Approximately, when the downstream depth Dd

above the crest is more than eighty percent of the upstream depth Du the weir formula does

not hold good.

a) Weir formula

2/37.1 LHCQ w=

where Q = discharge through the openings in m3/s; Cw = coefficient of discharge accounting

for losses in friction; the values may be taken as 0.94 for Narrow openings with or without

floors; as 0.96 for Wide openings with floors; and as 0.98 for Wide openings without floors;

L = Linear waterway in m; H = total energy head upstream of the obstruction in m, that is,

gVDu 22+ ; V = average velocity in the approach section worked out from the known width

of unobstructed section (W).

b) Orifice formula

( ) 2/12

0 2)1(2 gVehLDgCQ d ++=

where C0 = coefficient of discharge; g = acceleration due to gravity in m/s2; h = afflux in m, e

=a factor accounting for recovery of some velocity as potential head on emergence from the

cross drainage work openings. The value of C0 and e to be adopted from given Figures. The

afflux can be calculated knowing (a) the discharge, (b) the unobstructed width of the stream,

and (c) the average depth downstream of the cross drainage work opening.

20

Empirical formula

When the area of obstruction is not very large compared to the original unrestricted area, the

following formula gives reasonably good results:

( )( )1)/(85.170152.0 22 −+= aAVh

where A = the unobstructed sectional area drainage channel in m2, and a = sectional area of

the drainage provided in the construction in m2. If the value of V varies considerably in the

unobstructed cross section of the drainage channel, as in the case of a drainage channel which

spills over its banks, V for the purposes of this formula may be taken as the average velocity

in the main channel and correspondingly the value of A should be determined by dividing the

total discharge by V. In case of readily erodable beds, full afflux as calculated may not occur.

It may be noted that greater the afflux, the more will be submerged area on the U/S of

the syphon aqueduct. The extent of submergence which can be permitted at a particular site

will depend upon the value of the property on the upstream side and the topography of the

area. If the submergence can be increased without submergence of property, the afflux may

be kept more. Thus the waterway may be reduced and the number of barrels can be reduced.

However, the velocity in the barrel should not be permitted to increase beyond the safe limit

which is normally taken as 3 m/s for concrete barrels; otherwise abrasion of the barrel surface

will occur. The afflux is usually limited to 1 m for large catchments. In the case of a canal

syphon, the afflux occurs in the canal u/s. It can be computed using the same procedure.

Uplift Pressure on the Roof of a Barrel

As the barrel of a syphon aqueduct runs full during floods, an uplift pressure acts on the roof

of barrel (or on the under of the canal trough). The uplift pressure at any point of the barrel

can be obtained from the hydraulic gradient line H.G.L. The uplift pressure at any point is

obviously equal to the ordinate between the hydraulic gradient line and the roof of the barrel

at that point. Because of entry loss, there is a sudden drop in the HGL at that entrance. It is

followed by a gradual drop due to friction throughout the length of barrel and again there is a

21

sudden drop at the exit. The

maximum uplift occurs just

after the entry point at the

upstream end of the barrel.

While designing the trough,

the following two extreme

conditions are considered (i)

The canal trough is carrying

full discharge but the barrel

is empty; (ii) The barrel is

running full but there is no

water in the trough.

Generally, the thickness of the trough designed for downward loads is sufficient to

counterbalance the uplift pressure. In case it not sufficient, it is generally more economical to

anchor the trough to the piers and abutment than to increase the thickness of slab. The uplift

pressure on the roof of a canal syphon can be found by the same method.

Uplift Pressure on the Floor of a Syphon Aqueduct

The floor of a syphon aqueduct is subjected to an uplift pressure due to following causes:

1. Rise of water table: The maximum uplift on the bottom surface of the floor occurs when

the barrel is empty and the subsoil water table rises upto the drainage bed. The uplift pressure

is equal to the difference of the bed level of the drainage and the bottom surface of the floor.

Thus, Uplift head (h1) = Drainage bed level - bottom level of floor.

2. Seepage from canal: The maximum uplift pressure due to seepage from the canal occurs

when the canal is full and the barrel is empty. The uplift pressure due to seepage from canal

is difficult to compute because the subsurface flow is three-dimensional. Because the flow

cannot be approximated as two

dimensional, Khosla's theory cannot be

used for the estimation of uplift pressure.

Relaxation method can be used but it is

very laborious. Generally a simple

analysis based on Bligh's theory is used

for small works. Total seepage head =

Canal F.S.L. - D/s bed level of drainage.

However, for large and important works

the uplift pressures should be obtained by

electrical analogy method or by model

studies.

In order to reduce the thickness of

floor, a RCC raft may be provided as the

impervious floor. In that case, a part of the uplift pressure is resisted by the weight of floor

and the remaining part resisted by bending strength of the raft supported between the piers.

Thus the uplift pressure gets transferred to the piers and is resisted by the weight of the entire

superstructure. If the uplift pressure is very high, it can be reduced by the following methods:

• The creep length is increased by increasing the length of the impervious floor at

the bed of the canal.

• Drainage holes (or relief holes) are drilled in the floor of the barrels to release the

22

uplift pressure. Inverted filter is provided below the holes to prevent piping. These

holes are covered by flap values which open only upwards.

A depressed floor is usually required in the case of a syphon aqueduct. The floor should be

protected against scour by providing suitable sheet piles and loose apron on either side of

floor. The floor may be designed as a gravity section. However, when the thickness required

is large, a RCC raft is provided. The choice between a gravity section and a RCC raft is

usually a matter of economy. A raft is structurally superior and is usually more economical in

the case of poor foundations.

Design Steps

The design of a syphon aqueduct in many respects is similar to that of an aqueduct. However,

the design of barrel is somewhat different, as explained below

1. Design of barrel: The area of flow of the barrel is determined from the maximum

permissible velocity V, thus Cross-sectional area, A

VQA =

The maximum permissible velocity depends on the material as listed in the previous table.

This velocity is usually limited to 3 m/s. The higher velocity may cause abrasion of the barrel

surface by rolling grits. Moreover, it will cause more afflux, requiring higher and larger

marginal banks. The total required area when divided by the number of spans gives the cross-

sectional area of each opening. While designing a syphon aqueduct barrel, care shall be taken

to maintain the minimum scouring velocity required to prevent silting. The minimum

scouring velocity should be maintained for the normal floods which may occur quite

frequently so as to cause flushing of the deposited sediment in the barrels.

2. Knowing the span and the shape of opening, the height of opening can be calculated. That

is: the height of barrel is determined as h = A/Clear water way.

However, the height of barrel should not be less than 2 m so that a person can enter for

cleaning the barrel when required. If necessary, the floor of the barrel is depressed. Generally

a vertical drop in provided at inlet to join the drainage bed to the barrel. However, if the drop

is greater then 1 m, a glacis of 3: 1 is provided. On the d/s side, a ramp with a slope of 5: 1 is

provided so that silt is carried by the drainage water. The vertical distance between the d/s

bed and the underside of the trough should be not less than 1·0 m. Likewise, the distance

between the vertical drop wall and the u/s end of the pier should not be less than 1 m.

3. Compute afflux from orifice formula and then find uplift on the barrel roof. The roof of the

barrel is designed for the maximum uplift pressure which occurs when the canal is empty and

the maximum discharge occurs through the barrel.

4. The floor of the barrel is designed for the maximum uplift pressure which occurs when the

canal is full and the barrel is empty.

5. The design for canal, its fluming, u/s and d/s transitions and fixing of bed levels remain

similar to aqueduct.