Lecture 13
Cross Drainage Works
Cross drainage works are essential structures built to allow
a canal to pass over natural drains or rivers that intersect its path. These
structures are unavoidable in any canal system because canals often need to
cross various natural waterways. To reduce the number of cross drainages works
required, canals are typically designed to follow a watershed. By aligning the
canal along a watershed, it intercepts fewer natural drains, as most drains
flow away from the watershed toward the rivers on either side.
However, when a canal is initially diverted from a river, it
takes some distance before it can rise to the level of the watershed. During
this stretch, the canal often intersects several natural drains that flow from
the watershed toward the river. To address this, cross drainage works must be
constructed to carry the canal over these drains. This is why the initial reach
of the canal, before it reaches the watershed, typically requires the
construction of major cross drainage works.
Once the canal reaches the watershed, cross drainage works
are generally no longer needed because the natural drains flow away from the
watershed toward the rivers on either side. However, there may be situations
where the canal needs to temporarily deviate from the watershed to cut across a
loop or bend in the terrain. In such cases, the canal might intersect a natural
drain in the low-lying area between the watershed and the canal's path. When
this happens, a cross-drainage work must be built to allow the canal to pass
over the drain.
In the case of contour canals, which are aligned to follow
the natural contours of the land, the number of cross drainages works required
tends to be higher. This is because contour canals frequently intersect natural
drains as they follow the terrain, making cross drainage works a necessary and
frequent feature in such systems.
Cross drainage works are critical for ensuring that canals
can smoothly pass over natural drains or rivers. Their necessity depends on the
alignment of the canal, with more works required in areas where the canal
intersects natural waterways, especially before it reaches the watershed or
when it deviates from it.
Plan of crossing
Type of cross drainage works
I.
Cross drainage works carrying the canal over the
natural drain
(a) Aqueduct
(b) Syphon
aqueduct
Aqueduct
- Similar
to a bridge, but carries a canal over a natural drain instead of a road
or railway.
- Built
where the canal bed is significantly above the High Flood Level (H.F.L.)
of the drain.
- Canal
water flows through a trough supported by piers.
- Drain
water flows below the canal trough at atmospheric pressure.
- Sufficient
headway maintained between the H.F.L. of the drain and the underside of
the canal trough.
- Often
includes an inspection road alongside the trough for maintenance.
Syphon aqueduct
- Constructed
when the drain's water surface level at high flood is above the canal
bed.
- Drain
bed is depressed, with a pucca floor and barrels between piers to pass
drain water.
- Horizontal
floor of barrels has slopes at ends to connect with the drain bed on both
sides.
- Drain
water flows under pressure through barrels, which act as inverted
syphons.
- This
structure is termed a "syphon aqueduct" due to its pressurized
flow mechanism.
II.
Cross drainage works carrying the natural drain
over the canal
(a) Super
passage
(b) Syphon
(or canal syphon)
Super passage
- Super
Passage
- Similar
to a bridge, carries a natural drain over a canal.
- Reverse
of an aqueduct.
- Built
when the drain bed is well above the canal's Full Supply Level (F.S.L).
- Drain
water flows in a trough supported by piers over the canal.
- Canal
water flows under the drain at atmospheric pressure.
- Sufficient
headway maintained between canal F.S.L and the underside of the drain
trough.
- Inspection
road along the canal is not feasible.
Syphon (or canal syphon)
- A
syphon is constructed when Canal's full supply level is higher than the
drain bed.
- Similar
to a syphon aqueduct, but canal water flows through barrels under the
drain.
- Barrel
Acts as inverted syphons, enabling pressurized flow of canal water.
III.
Cross drainage works admitting the drain water
into the canal
(a) Level
crossing
(b) Inlet
or inlet and outlet
Level crossing
Level Crossing:
- Provided
when canal and drain beds are at the same level.
- Allows
drain water to pass into the canal and exit at the opposite bank.
Components:
1.
Crest:
§ Located
at the upstream junction of the canal and drain.
§ Top
of the crest is at the Full Supply Level (F.S.L.) of the canal.
2.
Drain Regulator:
§ Positioned
at the downstream junction of the drain and canal.
§ Equipped
with quick-falling shutters.
3.
Canal Cross Regulator:
§ Located
at the downstream junction of the canal and drain.
Operation:
- When
the drain is dry:
- Drain
regulator is closed.
- Canal
cross regulator is fully open, allowing uninterrupted canal flow.
- When
the drain carries water:
- Water
spills over the crest into the canal.
- Corresponding
discharge is passed through the drain regulator back into the drain.
- Canal
supplies are precisely controlled by the canal regulator.
Level crossing
Inlet or inlet and outlet
Inlet:
·
An open cut or pipe in a canal bank to admit
drain water into the canal.
·
Used for small drains crossing the canal.
·
Suitable when the drain bed level is slightly
higher or lower than the canal's Full Supply Level (F.S.L.).
·
Not suitable for drains carrying heavy silt
loads.
Inlet and Outlet System:
·
Drain water admitted into the canal via an inlet
is discharged downstream through an outlet.
·
Outlet is typically combined with other
structures requiring escape arrangements.
Features of design of cross drainage works
The important features of design of cross drainage works are
as follows.
A. Hydraulic Design
The hydraulic design involves the following aspects.
1.
Determination of maximum flood discharge and
high flood level (H.F.L.).
2.
Determination of waterway of the drain.
3.
Head loss through syphon barrels.
4.
Contraction of canal waterway or fluming.
5.
Determination of uplift pressure on the
underside of the trough (or the barrel roof).
6.
Determination of uplift pressure on the floor of
the barrel.
7.
Design of bank connections.
B. Structural Design
The structural design deals with the following aspects.
8.
Design of side walls of trough and roof and
floor of barrels.
9.
Design of piers and abutments.
10. Design
of foundations.
Steps of Design a syphon aqueduct
Step 1. Design of drain water way
Step 2. Design of canal waterway
Step 3. Design of levels at different
sections of canal trough
Step 4. Design of contraction transition
Step 5. Design of expansion transition
Step 6. Design of the trough
Step 7. Head loss through syphon barrels
Step 8. Uplift pressure on the underside of
the trough or the barrel roof
Step 9. Design of roof of barrel
Step 10. Uplift pressure on the floor of
the barrels
Step 11. Design of cutoff and protection
works for the drain floor.