Introduction:
In irrigation projects, the construction of main canals, branch canals, and distributaries often requires crossing natural drainages such as rivers, streams, and nallahs within the project area. These crossings are inevitable, necessitating the construction of appropriate structures known as cross-drainage works. These works enable the smooth flow of water in both the canal and the drainage, ensuring uninterrupted water supply and efficient drainage disposal.
Importance of Cross-Drainage Works:
Cross-drainage works play a crucial role in maintaining uninterrupted canal supply by effectively managing drainage water. Typically, the canal is either elevated or placed below the drainage level, although in some cases, they can be at the same level. Canals are generally aligned on watersheds to avoid drainage crossings. However, in the initial stretch of a main canal, it is often necessary to intercept several natural drainages flowing from the watershed to the river. Once the canal crosses the watershed, the need for cross-drainage works diminishes as the drainage originates from the watershed and flows away. However, in certain situations, when the canal deviates from the watershed, cross-drainage works become necessary to intercept drainages and ensure proper water flow.
Types of Cross-Drainage Works:
Cross-drainage works can be classified into three categories based on the relative positions of the canal and the drainage:
1. Canal over the drainage:
(a) Aqueduct: An aqueduct is a structure where the canal flows over the drainage, while the drainage flows below in an open channel. Similar to a road or railway bridge, the canal is carried over the drainage using a canal trough supported by piers. Aqueducts are constructed when the canal bed level is higher than the High Flood Level (H.F.L.) of the drainage.
(b) Syphon aqueduct: In a syphon aqueduct, the canal is taken over the drainage, but the drainage water flows under syphonic action through a pipe. Syphon aqueducts are built when the H.F.L. of the drainage exceeds the canal bed level. By depressing the drainage bed or constructing an impervious floor at the crossing, a barrel is formed between piers to allow the drainage water to pass under pressure. Syphon aqueducts are preferred over aqueducts, despite being costlier.
2. Canal below the drainage:
(a) Superpassage: A superpassage involves taking the canal below the drainage, with open channel flow in the canal. This is the reverse of an aqueduct. A superpassage is required when the Full Supply Level (F.S.L.) of the canal is lower than the drainage bed level. The drainage water is channeled through a trough supported by piers constructed on the canal bed, allowing gravity-driven flow with atmospheric pressure.
(b) Canal syphon: In a canal syphon, the canal is taken below the drainage, and the canal water flows under symphonic action without atmospheric pressure. It is the opposite of a syphon aqueduct. Canal syphons are constructed when the F.S.L. of the canal is above the drainage bed level, although they should be avoided due to head loss and potential silting issues.
3. Canal at the same level as drainage:
(a) Level crossing: A level crossing is established when the canal and the drainage are practically at the same level. In this arrangement, the drainage water is allowed to enter the canal at one bank and is discharged at the opposite bank. A crest wall across the drainage, with its crest level matching the F.S.L. of the canal, facilitates the passage of drainage water into the canal. Drainage and canal outflows are regulated.
(b) Inlet and outlet: An inlet-outlet structure is provided when the drainage and the canal are almost at the same level, and the discharge in the drainage is small. The drainage water is admitted into the canal at a suitable site where the drainage bed is at the F.S.L. of the Canal. The excess water is discharged out the canal through an outlet provided on the canal at some distance downstream of junction. There are many disadvantages in use of inlet and outlet structure, because the drainage may pollute canal water and also the bank erosion may take place causing the deterioration of the canal structure so that maintenance costs are high. Hence, this type of structure is rarely constructed.
Selection of a suitable type of cross-drainage work:
The following points should be considered while selecting the most suitable type of cross – drainage work:
1. Relative levels and discharges: The relative levels and discharges of the canal and of the drainage mainly affect the type of cross – drainage work required. The following are the broad outlines:
If the canal bed level is sufficiently above the H.F.L of the drainage, an aqueduct may be provided.
If the F.S.L. of the canal is sufficiently below the bed level of the drainage, a super-passage is provided.
If the canal bed level is only slightly below the H.F.L. of the drainage, and the drainage is small, a syphon aqueduct is provided.
If the F.S.L. of the canal is slightly above the bed level of the drainage and the canal is of small size, a canal syphon is provided.
If the canal bed and the drainage bed are almost at the same level, a level crossing is provided when the discharge in the drainage is large, and an inlet-outlet structure is provi1ded when the discharge in the drainage is small.
2. Performance: As far as possible, the structure having an open channel flow should be preferred to the structure having pipe flow. Therefore, an aqueduct should be preferred to a syphon aqueduct. Similarly, a super-passage should be preferred to a canal syphon. The performance of inlet-outlet structures is not good and should be avoided.
3. Provision of road: A 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 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 should be preferred.
6. Foundation: The type of cross-drainage work should be selected depending upon the foundation available at the site.
7. Material of construction: Suitable types of material of construction in sufficient quantity should be available near the site for the particular type of cross – drainage work selected.
8. Cost of construction and overall cost: The cost of construction of cross-drainage work should not be
9. Subsoil water table: If the subsoil water table is high, the types of cross – drainage works which require deep excavation should be avoided.
10. Permissible loss of head: The cross-drainage works should be selected based on the permissible loss of head. Where the head loss cannot be permitted, a canal syphon should be avoided.
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 work can be altered. The canal alignment is generally finalized after fixing the sites of the major cross – drainage works.
Selection of site of a cross-drainage work:
The following points should be considered while selecting the site of a cross-drainage work:
1. At the site, the drainage should cross the canal alignment at right angles. Such a site provides good flow conditions and also the cost of the structure is usually a minimum.
2. The stream at the site should be stable and should have stable banks.
3. For economical design and construction of foundations, a firm and strong sub-stratum should exist below the bed of the drainage at a reasonable depth.
4. The site should be such that long and high approaches of the canal are not required.
5. The length and height of the marginal banks and guide banks for the drainage should be small.
6. In the case of an aqueduct, sufficient headway should be available between the canal trough and the H.F.L of the drainage.
7. The water table at the site should not be high, because it can create dewatering problems for laying foundations.
8. As far as possible, the site should be selected downstream of the confluence of two streams, thereby avoiding the necessity of construction of two structures.
9. The possibility of diverting one stream into another stream upstream of the canal crossing should be considered, if found feasible.
10. A cross-drainage work should be combined with a bridge, if required. If necessary, the bridge site can be shifted to a cross-drainage structure or vice-versa.