Definition of cross drainage work:
Is a structure which is constructed at the crossing of a canal and a natural drain so as to dispose of a drainage water without interrupting the continuous canal supplies. A cross drainage work is generally a costly construction and must be avoided as far as possible.
In order to reduce
- The artificial canals are generally aligned along ridge line or watershed.
- diverting one drain into another.
- Changing the alignment of the canal so that it crosses below the junction of two drains.
Types of Drainage Works:
The drainage water intercepting the canal can be disposed off in either of the following ways.
By passing the canal over the drainage: This may be accomplished either through
- Aqueduct
- Syphon-aqueduct
By passing the canal below the drainage: This may be accomplished either by
- Supper passage
- Canal syphon generally called a syphon
By passing drain through the canal so that the canal water and drainage water allowed to intermingle with each other.
- A level crossing
- Inlets and outlets.
1. Aqueducts and syphon aqueducts:
In these works the canal is taken over the drain such that the drainage water runs below the canal either freely or under syphoning pressure.
When the HFL of the drain is sufficiently below the bottom of the canal, so that drainage water flows freely under gravity ,the structure is known as an aqueduct.
In this type of work, the canal water is taken across the drainage in a trough supported on piers.
Inspection road is generally provided along with the trough.
Aqueduct: An aqueduct is like a bridge except that instead of carrying a road or railway, it carries canal on its top.
2. Supper passage and canal syphon:
In these works the drain is taken over the canal such that the canal water runs below the drain either freely or under syphoning pressure.
When the FSL of the canal is sufficiently below the bottom of the drain trough, we use supper passage. Supper passage is the reverse of an aqueduct.
Inspection road cannot be provided along the canal and a separate bridge is required for the roadway. For effecting economy the canal may be flumed, but the drainage trough is never flumed.
- If the FSL of the canal is sufficiently above the bed level of the drainage trough, so that the canal flows under syphonic action under the trough, the structure is known as a canal syphon or a syphon.
- In the case of a syphon the canal bed is depressed and a ramp is provided at the exit so that the trouble of silting is minimized.
3. Level Crossing:
- An inlet is a structure constructed in order to allow the drainage water to enter the canal and get mixed with the canal water and thus to help in augmenting canal supplies.
- Such a structure is generally adopted when the drainage is small and the drain crosses the canal with its bed level equal to or slightly higher than the canal FSL.
- For the canal to remain in regime, the drain water must not admit heavy load of silt into the canal.
- When the drainage discharge is high or if the canal is small, so that the canal section cannot take the entire drainage water, an outlet some times constructed to escape out the additional discharge at suitable site.
- It also not necessary that the number of inlets and outlets should be the same.
- An inlet essentially consists of an open cut in a canal bank, suitably protected by pitching to admit the upland drainage water in to the canal.
- The beds and the sides of the canal are also pitched for a certain distance upstream and downstream of the inlet.
- Similarly the outlet is another open cut in the canal bank with bed and sides of the cut properly pitched.
Selection of Cross-Drainage Works:
- If the canal bed level is sufficiently above the HFL of drainage aqueduct is selected.
- If the canal bed level is only slightly below the HFL of the drainage and the drainage is small a syphon aqueduct is provided.
- If the FSL of the canal is sufficiently below the bed level of the drainage a supper passage is provided.
- If FSL of the canal is slightly above the bed level of the drainage and the canal is of small size canal syphon is provided.
- If the canal bed and the drainage bed are almost at the same level crossing is provided.
- When the drainage is of small size, a syphon aqueduct is will be preferred to an aqueduct as the latter involves high banks and long approaches.
- If the drainage is of large size an aqueduct is preferred.
Design considerations:
- At the site, the drainage should cross the canal alignment at right angle.
- The stream at site should be stable.
- For economical design the foundation at site should be strong and firm.
- The site should be such that long and high approaches are not required.
- The length and height of the marginal bank and guide bank for the drainage should be small.
- The water table at the site should not be high.
- As far as possible the site should be selected d/s of the confluence of two streams.
- A cross drainage work should be combined with a bridge, if required.
Types of Aqueducts according to cross section:
- The cross-section of the canal is not changed.
- The original cross –section of the canal with normal side slopes is thus retained.
- Canal wings are not required.
- It is suitable when canal width is small (less than 2m)
- The outer slopes of the canal banks are discontinued and replaced by retaining wall.
- The length of the barrel is reduced but cost of retaining wall is added.
- Suitable when width of canal is moderate(2.5 to 15m)
- The entire earth section of the canal is discontinued and replaced by concrete or masonry trough over the drainage.
- It is suitable when the width of the drainage is very large(>15m)
- The cost of trough and canal wing wall is less in comparison to the saving resulting from decreasing the length of barrel.
- The canal can be easily flumed which reduces further the length of the barrel.
- Determination of maximum flood discharge.
- Fixing the waterway requirement.
- Size of the Barrels.
- Afflux and head loss through the syphon barrels
- Fluming of the Canal
- Canal Transitions
- Design of Pucca Canal Trough
- Design of Bottom Floor.
- Foundation of cross-drainage works.
- Bank Connections
- Clearance and Free Board
- For small drain –empirical formula
- For large drain – hydrograph analysis
- Preferable formula as per the condition of area of intercept can be adopted.
2. Fixing the water way requirement for aqueduct and siphon aqueducts.
- For wide drain--- p is equal to width of drain
- No extra width is provided for piers
- Max. allowable reduction to Lacey’s perimeter is 20%
- The clear water width is checked by suitable no. of bays
- For aqueduct
- For siphon aqueduct
3. Afflux and head loss through syphon barrel
- Afflux - the rise of flood level
- Higher amount of afflux requires longer marginal banks.
4. Fluming of the Canal
Design of channel transition
- when water depth remains constant
- the equation is derived on the basis that rate of change of velocity per unit length of the transition remains constant.
- when the water depth remains constant
- The equation is formulated by experiment
- By choosing convenient value Bx; x can be computed.
- the canal trough has to be designed for taking the depth of water and the full water load.
- in addition to the above, uplift pressure is considered and design has to be done for the following condition.
6. Design of Bottom floor of aqueduct and Syphon aqueduct
- Uplift due to water table
- The maximum uplift occur when there is no water flowing in the drain and water table has risen up to the drainage bed.
- Uplift due to seepage of water may be found using Bligh’s theory
7. Design of Bank Connection
- Provide a strong connection between the masonry or concrete sides of a canal trough and earthen canal banks.
- Design of the wing should be done to resist the maximum differential earth pressure on the wing with no water in the canal.
- The foundation should be deep enough to increase the creep length and should not be placed on filled earth.
- To retain and protect earthen slopes of the canal
- To guide the drainage water entering and leaving the canal
- The foundation should be taken below the deeper anticipated scour.
- Designed to resist the maximum differential earth pressure.
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