Cross Drainage Work

Conveyance Structures for Canals

The canal has to carry the water across the water bodies as well as across artificial obstacles like railway line or roads.

The main structures of a canal system for conveyance of canal flow and control of water levels are as follows-

Pipe conduits (or culverts) and inverted syphons to carry flow under railways and highways.
Aqueducts, syphon aqueducts, super-passage, canal siphon or level crossings across natural drainage courses or other depressions.
Transitions at changes in cross sections

1.1. Structures for crossing canals across roads and railway lines

These are structural elements to convey canal water under roads or railway lines. For small roads, carrying relatively less traffic, the pipe conduit is sufficient. A general view of the pipe conduit is shown in the figure below.

The inverted syphons are structures for canal water conveyance below roads, railway lines and other structures. The longitudinal profile is not exactly in a straight line and the central portion seen to sag beneath the object to be crossed. The inverted syphon, therefore, is provided where the water level in the canal is about the same as the level of the obstruction.

1.2. Structures for crossing canals across natural streams (cross drainage works)

These structural elements are required for conveying the canals across natural drainage. For larger streams and river, where the cost of diversion becomes costlier than providing a separate cross-drainage work, individual structures to cross the canal across the stream is provided.

The notations used in the figures are as follows –

(a)CBL: Canal Bed Level;

(b) SBL: Stream Bed Level;

(d) HFL: Stream High Flood Level

Case 1: Canal bed level is higher than stream high flood level:

Normal canal section is carried across the natural stream in the form of a bridge resting on piers and foundations. This type of structure is called as aqueduct.

In this case, the flow is not under pressure, that is, it has a free surface exposed to atmospheric pressure.

Case 2: HFL of the natural stream goes above the bottom level of the canal, then the flow in the natural watercourse would be pressured and the sections are modified to form which are known as syphon aqueducts.

Case 3: SBL is above FSL

If the canal water flows with a free surface, that is, without touching the bottom of the trough, it is called a super-passage

Case 4: FSL is above SBL and below HFL

When the canal passes below the trough as a pressure flow, then it is termed as a syphon or a canal syphon

Case 5: Canal water at the same level as a natural stream

Following Structures may be used-

Level Crossing - A structure in which the water of the stream is allowed to flow into the canal from one side and allowed to leave from the other, known as a level crossing.
Inlet & Outlet - It allows the entry of the stream water into the canal through an opening called inlet and water exits through another opening called outlet. The canal bed and sides are suitably pitched for protection.

REGULATING STRUCTURES FOR CANALS

Canal Falls - to lower the water level of the canal by controlling the bed grade (control of bed grade)

Cross & Head Regulators - to head up water in the parent channel & to divert some of it through an off take channel (control of FSL)

Canal escapes- to allow release of excess water from the canal system (control of discharge)

Canal Outlets or Modules

Canal outlets or Modules are structures at the head a water mine or field channel. The supply canal is usually under the control of an irrigation authority under the State government. Since an outlet is a link connecting the government owned supply channel and the cultivator owned field channel, the requirements should satisfy the needs of both the groups.

Since equitable distribution of the canal supplies is dependent on the outlets, it must not only pass a known and constant quantity of water, but must also be able to measure the released water satisfactorily.

It is essential to design an outlet in such a way that it is reliable and be also robust enough such that it is not easily tampered with.

The cost of an outlet structure should be low and should work efficiently with a small working head, since a larger working head would require higher water level in the parent channel resulting in high cost of the distribution system.

Discharge through an outlet is usually less than 0.085 cumecs.

TYPES OF OUTLETS/MODULES

(a) Non-modular outlets

These outlets operate in such a way that the flow passing through them is a function of the difference in water levels of the distributing channel and the water course. Hence, a variation in either affects the discharge.
Ex - submerged Pipe Outlet. Open Sluice Outlet.

(b) Semi-modular outlets

The discharge through these outlets depend on the water level of the distributing channel but is independent of the watercourse so long as the working head required for their working is available.
The discharge through such outlet will therefore depend on the water level in the distributary.

Ex - Kennedy's Gauge Module or Venturi Flume, Crump’s adjustable proportional Module(APM), Open Flume Outlet, Orifice semi-module outlet

(c) Rigid Modules/Modular outlets

The discharge through modular outlets is independent of the water levels in the distributing channel and the watercourse, within reasonable working limits. This type of outlets may or may not be equipped with moving parts.
Though modular outlets, like the Gibb's module Khanna's Module, Foote's module have been designed and implemented earlier, they are not very common in the present Indian irrigation engineering scenario.

RIVER TRAINING & PROTECTION WORKS

River training implies certain measures to be adopted on a river to stabilize the river channel along a certain alignment with a certain cross section.

The river training works are required to achieve the following;

Prevent out flanking of the structure
It protects the river bank
Prevent flooding by the river lands upstream
It contracts a river channel to improve its depth
It trains the flow along a certain course

NOTE:

MORPHOLOGY OF A RIVER

River/Stream morphology describes the shape of river channels and how they change in shape with direction with respect to time.

THALWEG of a river – a thalweg or talweg is the line of lowest elevation within a Valley or watercourse.

CLASSIFICATION OF R.T. WORKS

High Water Training

It is undertaken with the purpose of providing the efficient disposal of the maximum floods & there by protecting against damage due to floods.

It is also called as training for discharge.

Low Water Training

It is undertaken with the purpose of providing sufficient depth for navigation during the tow water season.

It is usually concerned with contraction of width

It is also known as training for depth.
Mean Water Training

It is undertaken to provide efficient disposal of bed & suspended sediments and thereby protecting the river channel in designed shape.

It is also known as training for sediments.

GROYNES/SPURS

Groynes are structures constructed transverse to the river flow and extend from the bank into river up-to a limit.

This form of R.T. works performs one or more functions such as:

Training the river along the desired course to reduce the concentration of flow at the point of attack,

creating a low flow for silting up the area in the vicinity, and

Protecting the bank by keeping the flow away from it.

TYPES OF GROYNES

Groynes are classified on the basis of function

Repelling/Reflecting Spur,
Deflecting Spur,
Attracting Spur
T-Shaped (Denehey), Hockey (Or Burma) Type, Kinked Type, Etc.

MEANDERING OF RIVERS

A meandering type of river flows in consecutive curves of reverse order connected with short strait called as crossing.

Sinuosity or Tortuosity

It is the ratio of the Actual length (along the curve) to the Meander Length (along straight line) between the end points of the curve of a meandering river.

Effect of Meandering

The meandering action increases the length of the stream or river and tends to reduce the slope.

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