Aqueduct - Cross Drainage works

Aqueduct || Cross Drainage Works || Types and functions

Aqueduct is a cross drainage structure constructed to channel water from the rivers or stream to the distribution points. But modern engineering aqueduct is a cross drainage system constitute of pipes, ditches, canals, tunnels, and supporting structures constructed/laid to convey water from a source to the main distribution points. Aqueducts play and important role in maintaining the strategic flow between drainage water with canal water.

Cross drainage works is a modern day aqueduct constructed to intercept a drain crossing or canal crossing to prevent the water from mixing between these sources. This article is about the types of cross drainage works and there features.

Cross drainage system/Aqueduct – Need

The cross drainage system is an expensive structure and has to be avoided. But there may be situations when the following conditions are encountered and the cross drainage is to be provided.

  • The cross drainage design can minimise the discharge velocity at the intersection point.
  • The ideal condition of aligning the canal without intersecting the drainage is not possible due to increase in the length and difficulty in construction. 
  • A canal is set between head work and the main watershed. The water supply is not intervened by providing cross drainage work.

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Aqueduct /Drainage type selection

The primary factors for drainage works are relative bed levels, water levels, canal and drainage discharge. However, the choice of drainage types can be summarised based on

  • Alignment of the canal
  • Discharge from canal and drainage 
  • Capacity of Foundation 
  • Economic condition
  • Canal head loss
  • The water level of canal and drainage

Must Read : Controlled Flooding – 6 types – Free Flooding – Basin Flooding

Aqueduct – Cross Drainage works – Types

The cross drainage works are broadly classified into three types as shown below

Type 1 – irrigation canal passes over the drainage

  • Aqueduct
  • Syphon aqueduct

Type 2 – Drainage passes over the irrigation canal

  • Super passage
  • Canal syphon

Type 3 – Drainage and canal intersection each other of the same level

  • Level crossing
  • Inlet and Outlet

Type 1 – irrigation canal passes over the drainage

The structures comming in this category are

  • Aqueduct
  • Siphon Aqueduct


An aqueduct is a cross drainage work, which is used when the bed level of the canal is above drainage bed level as shown in the figure. The canal water flows from upstream to downstream freely under gravity. The canal trough is rested on a series of piers

Cross Drainage works - Aqueduct

The canal water level is known as full supply level or FSL and drainage water level is referred as High Flood Level or HFL which is below the canal bed level.

The shape of the canal is a rectangular trough or trapezoidal trough and is similar to a bridge, railway or roadway. 

Advantages of Aqueduct

  • They are utilized for irrigational purposes and water supply.
  • It is held over piers, made of stone or reinforced concrete.
  • The freeboard of 0.5 m is implemented.
  • The section of the trough is determined by FSL and the height of the section is determined by HFL. 
  • An inspection road is given on the sides of the trough. 

Syphon Aqueduct

In the syphon aqueduct, the canal bed level is below the full supply level. The water flows from upstream to downstream through aqueduct barrels following syphonic action. A sloping apron is provided on both sides to depress the canal level. 

In a syphon aqueduct, canal water is carrier above the drainage but the high flood level (HFL) of drainage is above the canal trough. The drainage water flows under syphonic action and there is no presence of atmospheric pressure in the natural drain.

Syphonic aqueducts are more often constructed and better preferred than simple Aqueduct, though costlier.

syphon Aqueduct
Syphon Aqueduct
  • The section of the trough is determined by the canal water level.
  • Cut off walls are provided at both ends. 
  • The sloping apron is built by using stone or cement concrete. 
  • The bottom of the siphon aqueduct is impervious.
  • The atmospheric pressure is not taken into account. 
  • They are more prefered than an aqueduct.
  • They are expensive.

Type-2- Drainage passes over the irrigation canal

  • Super passage
  • Canal Siphon

Super Passage

The super passage is a hydraulic structure where drainage passes over the irrigation canal. They contradict the aqueduct. The water from the drainage flows through the troughs under gravity and atmospheric pressure. 

Super passage
Super Passage

The drainage trough is constructed at the road level. These are preferred when drain discharge is less compared to canal discharge. An inspection road cannot be constructed on the sides of the trough. Thus they are not available for an open investigation.

A separate bridge is equipped for the roadway. Also, a ramp is given at the doorway. It is supported by piers. 

  • To avoid scouring, boulder pitching is given at the bed and banking. 
  • The section of the drainage is concluded by the high flood level.
  • For safety, a freeboard of 1.5 m is given.
  • The concrete foundation is provided, But the depth of the foundation is determined by the availability of soil. 

Canal Syphon

Canal syphon is implemented when the drainage passes over the canal. The canal water flows under syphonic action and no atmospheric pressure is considered. Since the canal water is under drainage, the exclusion of sediments and silts is impossible. 

Canal Syphon
Canal Siphon

The inspection road cannot be provided, a separate bridge is constructed for the roadway. 

  • The selection of trough is designed based on the HFL.
  • A ramp is provided for the exit. 
  • The sloping apron is seen with stone or concrete pitching. 
  • They have a high head loss. 
  • These are opposite of syphon aqueduct.

Type 3 Drainage and canal intersection each other of the same level

  • Level Crossing
  • Inlet and outlet

Level Crossing

Level crossing is recommended when the canal level and drainage level are the same. The quality and discharge of both canal and drainage water should also be equivalent. A barrier is provided at the upstream level. A regulator is provided at the downstream side.

The components of level crossing are 

  • Crest wall
  • Drainage regulator
  • Canal regulator

The top wall of the crest is equivalent to the FSL of the canal. The crest is provided on the upstream side. 

Level Crossing
Cross Drainage Works - Aqueduct

The drainage regulator is given at the downstream side. They regulate the flow of water by an adjustable shutter. 

The Canal regulator is also kept on downstream at the crossing point. This regulator is used at the peak water supply. Thus the drainage water can be stopped behind.

In peak supply time of canal water parallel to drainage, both the regulators are opened to clear the drainage water from that of canal for certain time interval. 

Inlet and Outlets or Canal Inlet

These are provided where the channel and drainage are small. Inlet and Outlet are simple openings. The inlet allows the flow of the water. While the outlet allows the water drain. 

The drainage mixed with canal travels through the canal for certain length as shown in the fig. After that the drainage solids are sucked through and outlet provided to create suction pressure disposing to the watershed nearby.

Canal Inlet
Canal Inlet

Stone pitching is provided at the bed and banks of the drainage. The maintenance cost of the inlet and outlet system is high. But the construction cost is low. The main disadvantage of this system is that they also cause soil erosion and water pollution. 

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