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Definition of cross drainage work:

Is a structure which is constructed at the crossing of a canal and a natural drain so as to dispose of a drainage water without interrupting the continuous canal supplies. A cross drainage work is generally a costly construction and must be avoided as far as possible.

In order to reduce

  • The artificial canals are generally aligned along ridge line or watershed.
  • diverting one drain into another.
  • Changing the alignment of the canal so that it crosses below the  junction of two drains.

Types of Drainage Works:

The drainage water intercepting the canal can be disposed off in  either of the following ways.

By passing the canal over the drainage: This may be accomplished either through

  • Aqueduct
  • Syphon-aqueduct

By passing the canal below the drainage: This may be accomplished either by

  • Supper passage
  • Canal syphon generally called a syphon  

By passing drain through the canal so that the canal water and drainage water allowed to intermingle with each other.

  • A level crossing
  • Inlets and outlets.      

1. Aqueducts and syphon aqueducts:

In these works the canal is taken over the drain such that the drainage water runs below the canal either freely or under syphoning pressure.

When the HFL of the drain is sufficiently below the bottom of the canal, so that drainage water flows freely under gravity ,the structure is known as an aqueduct.

In this type of work, the canal water is taken across the drainage in a trough supported on piers.

Inspection road is generally provided along with the trough.

Aqueduct: An aqueduct is like a bridge except that instead of carrying a road or railway, it carries canal on its top.

Syphon aqueducts: however, if the HFL of the drain is higher than the canal bed and the water passes through the aqueduct under syphonic action, the structure is known as syphon aqueduct. In the case of syphon aqueduct, the drain bed is generally depressed and provided with pucca floor.

On the upstream side, the drainage bed may be joined to the pucca floor either by a vertical drop (when drop is of the order of 1m) or by a glacis (slope of 3:1). The downstream rising slope should not be steeper than 5:1.

2. Supper passage and canal syphon:

In these works the drain is taken over the canal such that the canal water runs below the drain either freely or under syphoning pressure.

When the FSL of the canal is sufficiently below the bottom of the drain trough, we use supper passage. Supper passage is the reverse of an aqueduct.

Inspection road cannot be provided along the canal and a separate bridge is required for the roadway. For effecting economy the canal may be flumed, but the drainage trough is never flumed.


  • If the FSL of the canal is sufficiently above the bed level of the drainage trough, so that the canal flows under syphonic action under the trough, the structure is known as a canal syphon or a syphon.
  • In the case of a syphon the canal bed is depressed and a ramp is provided at the exit so that the trouble of silting is minimized.

3. Level Crossing:

In this type of cross drainage work, the canal water and drain water are allowed to intermingle with each other.

This is provided when large canal and huge drainage (such as stream or a river) approach each other practically at the same level.

A regulator is provided; across the drainage just on the downstream side of the crossing so as to control the discharge passage at the out going canal to control the discharge into the canal.
sometimes at the incoming canal.


4. Inlets and Outlets:
  • An inlet is a structure constructed in order to allow the drainage water to enter the canal and get mixed with the canal water and thus to help in augmenting canal supplies.
  • Such a structure is generally adopted when the drainage is small and the drain crosses the canal with its bed level equal to or slightly higher than the canal FSL.
  • For the canal to remain in regime, the drain water must not admit heavy load of silt into the canal.
  • When the drainage discharge is high or if the canal is small, so that the canal section cannot take the entire drainage water, an outlet some times constructed to escape out the additional discharge at suitable site.
  • It also not necessary that the number of inlets and outlets should be the same.
  • An inlet essentially consists of an open cut in a canal bank, suitably protected by pitching to admit the upland drainage water in to the canal.
  • The beds and the sides of the canal are also pitched for a certain distance upstream and downstream of the inlet.
  • Similarly the outlet is another open cut in the canal bank with bed and sides of the cut properly pitched.

Selection of Cross-Drainage Works:

Relative levels and discharges: The relative levels and discharges of the canal and of the drainage mainly affect type of cross drainage works.
  1. If the canal bed level is sufficiently above the HFL of drainage aqueduct is selected.
  2. If the canal bed level is only slightly below the HFL of  the drainage and the drainage is small a syphon aqueduct is provided.
  3. If the FSL of the canal is sufficiently below the bed level of the drainage a supper passage is provided.
  4. If FSL of the canal is slightly above the bed level of the drainage and the canal is of small size canal syphon is provided.
  5. If the canal bed and the drainage bed are almost at the same level crossing is provided.
Performance: As far as possible the structure having an open channel flow should be preferred. Therefore aqueduct is preferred to a syphon aqueduct.

Provision of road: Aqueduct is better than a supper passage b/c in the former, a road bridge can easily be provided along with the canal trough at small extra cost.

Size of drainage: 
  • When the drainage is of small size, a syphon aqueduct is will be preferred to an aqueduct as the latter involves high banks and long approaches.
  • If the drainage is of large size an aqueduct is preferred.
Cost of earth work: The type of cross drainage work which doesn’t involve large quantity of earth work is advantageous.

Foundation: The type of cross drainage work should be selected depending on the foundation available at the site.

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

Over all cost: The overall cost of the canal banks and the cross drainage works, including maintenance cost should be minimum.

Design considerations:

  • At the site, the drainage should cross the canal alignment at right angle. 
  • The stream at site should be stable.
  • For economical design the foundation at site should be strong and firm.
  • The site should be such that long and high approaches are not required.
  • The length and height of the marginal bank and guide bank for the drainage should be small.
  • The water table at the site should not be high.
  • As far as possible the site should be selected d/s of the confluence of two streams.
  • A cross drainage work should be combined with a bridge, if required.

Types of Aqueducts according to cross section:

Type I aqueduct:
  • The cross-section of the canal is not changed.
  • The original cross –section of the canal with normal side slopes is thus retained.
  • Canal wings are not required.
  • It is suitable when canal width is small (less than 2m)

Type II aqueduct:
  • The outer slopes of the canal banks are discontinued and replaced by retaining wall.
  • The length of the barrel is reduced but cost of retaining wall is added.
  • Suitable when width of canal is moderate(2.5 to 15m)

Type III aqueduct:
  • The entire earth section of the canal is discontinued and replaced by concrete or masonry trough over the drainage.
  • It is suitable when the width of the drainage is very large(>15m)
  • The cost of trough and canal wing wall is less in comparison to the saving resulting from decreasing the length of barrel.
  • The canal can be easily flumed which reduces further the length of the barrel.
Design Considerations:
The following steps may be involved in the design of cross drainage works.
  • Determination of maximum flood discharge.
  • Fixing the waterway requirement.
  • Size of the Barrels.
  • Afflux and head loss through the syphon barrels
  • Fluming of the Canal
  • Canal Transitions 
  • Design of Pucca Canal Trough
  • Design of Bottom Floor.
  • Foundation of cross-drainage works.
  • Bank Connections
  • Clearance and Free Board
The design procedure for aqueduct and siphon aqueduct is done in the same way with that of super passage and siphon except interchanging the canal and drain each other.

1. Determination of max flood discharge 
  • For small drain –empirical  formula
  • For large   drain – hydrograph analysis
  • Preferable formula as per the condition of area of intercept can be adopted.

2. Fixing the water way requirement  for  aqueduct and siphon aqueducts.

P = 4.75√Q ----Lacey’s formula
Where, P----wetted perimeter (m) and Q----total discharge (m3 /s) 
  • For  wide drain--- p is equal to width of drain
  • No extra width is provided for piers
  • Max. allowable reduction to Lacey’s perimeter is 20%
  • The clear water width is checked by suitable no. of bays
#Size of the barrel 
After having fixed the waterway width and no. of compartments, the height of drain barrel is fixed as follows.
  • For aqueduct 
The difference between HFL and drainage bed level. 
  • For siphon  aqueduct
By using permissible velocity 
A=Q/V-----permissible
h=A/water way width 

3. Afflux and head loss through syphon barrel

  • Afflux - the rise of flood level
  • Higher amount of afflux requires longer marginal banks.
h = [1+f1+f2L/R]V2/2g - Va/2g
The above formula is Unwin’s formula. Where,
h-head loss
f1- coefficient of head loss at entry
f2- coefficient of head loss due to surface friction through the barrel
f2 =a(1+b/R) 
a and b are constants and depend on barrel surface material.  

The u/s HFL of the drain will get headed up by an amount equal to head loss, afflux. 
By permitting high afflux the x-sectional area can be reduced, but it results increase in the cost of guide bank, marginal bund and down stream protection.

4. Fluming of the Canal

The contraction in the water way canal reduces the length of barrel or width of the aqueduct.
The maximum fluming is governed by the velocity(subcritical) to avoid hydraulic jump, economy and permissible loss of head.
The greater is the fluming, the greater is the length of transition.
An economic balance should be done to decide the extent of fluming.

The transition between normal and flumed canal section has to be smooth to avoid for motion of eddies and sudden transition. 

Design of channel transition

i) Mitra’s Hyperbolic transition method
  • when water depth remains constant
  • the equation is derived on the basis that rate of change of velocity per unit length of the transition remains constant.
ii) Chaturvedi’s-Semi cubical parabolic transition
  • when the water depth remains constant
  • The equation is formulated by experiment
  • By choosing convenient value Bx; x can be computed.
5. Design of Pucca Canal trough 

For an aqueduct:
  • the canal trough has to be designed for taking the depth of water and the full water load.
For Syphon aqueduct:
  • in addition to the above, uplift pressure is considered and design has to be done for the following condition.

6. Design of Bottom floor of aqueduct and Syphon aqueduct

  • Uplift due to water table
  • The maximum uplift occur when there is no water flowing in the drain and water table has risen up to the drainage bed.
  • Uplift due to seepage of water may be found using Bligh’s theory

7. Design of Bank Connection 

i) Canal wing or land wing
  • Provide a strong connection between the masonry or concrete sides of a canal trough and earthen canal banks.
  • Design of the wing should be done to resist the maximum differential earth pressure on the wing with no water in the canal.
  • The foundation should be deep enough to increase the creep length and should not be placed on filled earth. 
ii) Drainage wings or water wings
  • To retain and protect earthen slopes of the canal
  • To guide the drainage water entering and leaving the canal
  • The foundation should be taken below the deeper anticipated scour.
  • Designed to resist the maximum differential earth pressure.