Cross Drainage Works

When irrigation channels have to cross streams or drains in an uneven country, the works necessary to dispose of these drains are called cross drainage works

or Cross Drainage Work (CDW) or drainage crossing is structure constructed to negotiate an irrigation canal over, below or at same level of drainage or another canal.

Why and How to Avoid a Cross Drainage Works?

Because it is generally a very costly item, and should be avoided as far as possible by (i) Diverting one stream into another, or (ii) Changing the alignment of the canal so

that it crosses below the junction of two streams.

Classification of CDW

Canal and Drainage discharging at same Level

Discharging Drain into Canal

CROSS DRAINAGE WORKS

Drainage over the CanalCanal over the Drainage

AqueductSyphon Aqueduct

InletLevel Crossing

Without Mixing of Water

With Mixing of Water

Super Passage

SyphonSuper-passage

Canal Syphon

Types of Cross Drainage Works

Depending upon the relative levels and discharges, cross drainage works may be of the following type:

1. Cross Drainage Works Carrying Canal over the Drainage

In this type of Cross Drainage Work the canal is carried over the natural drain.

Following are the structures under this type of Cross Drainage Works

I. AqueductII. Syphon aqueduct

(a) Plan of Crossing

(b) Aqueduct

(c) Syphon aqueduct

Types of Cross Drainage Works (continued)

2. Cross Drainage Works Carrying Drainage over the Canal

In this type CDW drainage is carried over the canalFollowing are the structures under this type of Cross

Drainage Works

I. Super Passage.II. Canal Syphon

(a) Culvert Length and Length of Aqueduct

(b) Type I Aqueduct

(c) Type II Aqueduct

(d) Type III Aqueduct

Types of Cross Drainage Works (continued)

3. Cross Drainage Works Admitting the Drainage Water into the Canal

In this type of work, the canal water and the drainage water are permitted to intermingle with each other. Following are the structures under this type of Cross Drainage Works:

I. Level crossing II. Inlet and outlet

Factors Governing the Selection of Suitable Type of Cross Drainage Works

FSL of the Canal vis-à-vis HFL of the Drainage Topography of Terrain Regime of Drainage Foundation Strata Ratio of Discharge Head Loss Subsoil Water Table Economical Aspects Canal Alignment Availability of soil for making embankments

for canal and drainage

Aqueduct

Aqueduct

An aqueduct is defined as drainage crossing in which the canal is carried over the drainage channel and the bottom of the canal trough, or the covering over the drainage openings, is above the high flood level in the drainage channel.

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

Depression of canal or drainage bed level is not involved

Types of AqueductThe aqueduct is broadly classified into the following

three types, depending on the arrangement of canal passing over the drainage

1.Barrel Type Aqueduct2.Arch Type Aqueduct3.Trough Type Aqueduct

I. Barrel Type Aqueduct It is suitable for a small drainage channel as no canal transitions are required

Types of Aqueduct (continued)

2. Arch Type AqueductIt consists of a trough for the canal supported on arches, which may be of masonry, or reinforced cement concrete. Alternatively, it may be described as an arched bridge with a platform at canal bed level and provided with two solid masonry retaining walls or earthen banks to contain the water flowing through the canal

Types of Aqueduct (continued)

3. Trough Type Aqueduct It is the form of aqueduct in which the earthen banks of the canal are discontinued over the aqueduct and the canal duly flumed is carried in a masonry or concrete trough, box, barrel, pipe.

Syphon Aqueduct

Syphon aqueducts are those in which the bed of the stream is depressed when it passes under the canal trough, the stream water flows under pressure below the canal

Fig.3.1 Syphon Aqueduct

Types of Syphon Aqueduct

1. Type I of Syphon Aqueduct2. Type II of Syphon Aqueduct3. Type III of Syphon Aqueduct

LEVEL CROSSING

A level crossing is a type of cross-darning work which is provided when the canal and the draining are at the same level. There is an intermixing of the canal water and the darning water.

LEVEL CROSSING

Level Crossing consists of the following four types of regulator

1. Canal Inlet Regulator2. Canal Outlet Regulator3. Drainage Inlet Regulator 4. Drainage Outlet Regulator

Super passage

Super passage is defined as a drainage crossing in which the drainage channel is carried in a trough over the canal normally with full supply level of the canal below the bottom of the trough. It is reverse of an aqueduct’s it is generally adopted when the high flood level for the drainage is higher than the full supply level of the canal.

INLET AND OUTLET

Inlet Inlet when a small drainage crosses the canal, an inlet may be provided to allow the drainage water in to the canal. The inlet usually consists of an opening in masonry at the canal bank.

Types of Inlet

Flush inlet When the bed level of the drainage is at or slightly higher then the designed F.S.L of the canal, the inlet provided is called Flush inlet or free fall inlet.

Weir inletWhen bad level of the drainage is slightly lower than F.S.L of the canal, the inlet is provided is called weir inlet

Types of Inlet

Stepped Weir InletWhen the bed level of the drainage is much higher than the FSL of the canal, a stepped weir inlet is provided.

INLET AND OUTLET

Out let An out is small structure constructed through the banks of the canal to remove excess later from the canal. An outlet is usually constructed d/s of the inlet .but it is not necessary for each inlet; there should be a separate outlet.

Design Features of Cross Drainage Works

Following the some of the important features of design of cross drainage works:

(A) Hydraulic Design

1. Determination of maximum flood discharge and the high flood level (H.F.L).2. Fixation of waterway of the drain.3. Construction of canal waterway (for type III aqueducts)4. Head loss through syphon barrels.5. Determination of uplift pressure on the roof of trough.6. Determination of uplift pressure on the floor.7. Design of bank connections

Design Features of Cross Drainage Works (continued)

(B) Structural Design

1. Design of the cross-section of the syphon barrels and the aqueduct trough.2. Design of piers and abutments.3. Design of foundations.