Cross Drainage Work

                            INTRODUCTION

The structures constructed at the crossing of a canal and in a natural drain in order to dispose of the natural drainage water safely without interrupting the continuous canal supply. The cross drainage work should be minimized as far as possible. This artificial alignment of the canal should be through watershed as far as possible.

              TYPES OF CROSS DRAINAGE WORKS

The cross drainage works are classified into 3 categories;

A. Cross drainage works carrying the canal over the drainage.

     1. Aqueduct:

          The canal is taken over the natural drain in such a way that water of natural coarse runs below the canal.    

                 When high flood level(HFL) of the drain is sufficiently below the bottom of the canal so that the drainage water flows freely under gravity, the structure is called an aqueduct.

  2 Syphon  Aqueduct :

       When the canal is above the natural drain and the (HFL) of the natural drain is higher than the canal bed, then siphonic action occurs. Thus the structures are called siphonic aqueduct.

       

 B. Cross drainage works carrying canal drainage over the canal.

      a. Super passage:

               The drain is taken over the canal such that canal water runs either freely then the structure is called super-passage.

      b. Canal siphon:

                               The drain is taken over the canal such that canal water runs either freely or siphonic pressure. If the canal runs under siphonic pressure then the structure. If the canal runs under siphonic pressure then the structure is called siphon.

C. Cross drainage works admitting the drainage water into the canal.

1. level crossing:

     The level crossing is constructed in the circumstances when the bed of canal and drainage are practically at the same level.

   #Advantage:

1 low initial cost.

2 Additional discharge can be supplied to the canal from drainage.

#Disadvantage:

1 Regulation of work is difficult.

2 faulty regulation of the gate may damage the canal.

3 There is additional expenditure silt clearance.

2. Inlet and outlet:

      If the admitted drainage water in the canal is discharged at a suitable site opposite to inlet or on downstream the arrangement is called inlet and outlet.

 1.2 Selection of suitable type of cross drainage works:

          Factor affecting the selection of a suitable type of cross drainage works:

1.  Relative bed and water levels of canal and drainage.

2.  Relative discharged of canal and drainage.

1.3 Considerations selecting the type of cross drainage works:

 

1. When the bed level of the canal is much above the HFL of drainage so that sufficiently headway is available for floating rubbish etc then aqueduct is the obvious choice.

2. If the bed level of drainage is well above the FSL of the canal then super-passage is provided.

3. When drainage bed is much lower but HFL drainage is higher than the bed level of the canal, then the siphon aqueduct is provided.

4. When the bed level of the canal is much more than the drainage and FSL of the canal is higher than the bed level of drainage, then we may prefer canal siphon.

5.When the drainage and canal cross each other particularly at the same level. Then the level crossing may be used.

6. If Q canal>Q drainage siphon is provided.

7. If Q canal<Q drainage siphon is provided.

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