Selection of a Suitable Site and Type of Cross Drainage Work
The following points should be considered while selecting the site of a cross-drainage work:
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:
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 super-passage should be preferred to a canal siphon. 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 super-passage 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.
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.
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 De-watering 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 finalized after fixing the sites of the major cross-drainage works.