River training works are defined as the structure or planning which determines the flow of water in the river through a different stage. River training works protect the way of the following water to be wider and also protect fro landslide and loss of property.
Stages of River training works
As the river flow from its origin in a mountain to sea, it passes through various stages. A river generally has the following four stages.
1) Rocky stage ( Incised stage )
It is the first stage when the river takes off from the mountain and hilly region. The flow channel is formed by degradation and cutting. Bed and bank consist of rocky strata. It is ideal for the construction of the dam.
2) Boulder stage
It is the second stage of the river. It passes from the rocky to the boulder stage. Bed and bank in this stage consist of large boulders, gravels, and shingles. River cross-section is well defined and velocity is high but less than in the straight course.
3) Trough and alluvial stage
The river in this stage flows in a zigzag manner known as the meandering. The cross-section is made up of sand and silt. The bed slope is flat and very is small at this stage. The river may be aggrading, degrading, or stable type.
4) Deltaic stage
It is the last stage of the river just before discharging into the sea. In this stage, the river gets divided into no. of small branches and forms a delta. As the river approaches the sea the channel gets silted up and the water level rises.
The importance of river training works are as follows;
- To provide a safe passage to pass flood without overflowing its banks and thus to prevent flooding of the adjoining areas.
- To prevent the river from changing its course and eroding adjoining land.
- To prevent the erosion of banks and hence improve the alignment by stabilizing the river channel.
- To keep the river transports the bed load and suspended load efficiently.
- To provide the minimum desired depth of flow required for navigation.
- To deflect the river flow away from a bank or to attract the river flow towards a bank.
Classification of river training
1) High water training works ( training for discharge )
High water training works are constructed for the purpose of quick disposal of maximum flood and to protect adjoining land against damage due to floods. It aims to provide sufficient river cross-section for stage passage of maximum flood.
2) Low water training works ( training for depth )
It is undertaken with the primary purpose of providing sufficient water depth for navigation during a low flow period. Generally, spurs are constructed to contract the width of the channel and hence to increase the depth.
3) Mean water training works ( training for sediments )
It is undertaken with the primary purpose of providing efficient disposal of sediment load and bedload thus preserving the channel in good shape. It is the most important training of the three types.
Methods of training works
Following are generally adopted methods of river training works,
A) Marginal embankments or levees
These are the earthen embankment which is constructed parallel to the river banks at some suitable distance from it. These embankment walls retain the flood water and prevent them from spreading into nearby towns. The alignment of levees should follow the normal meandering pattern of the river.
B) Guide banks
Guide banks are the earthen embankments constructed for confining the alluvial river flow within a reasonable waterway and guiding it to a hydraulic structure, such as weir, a barrage, or a bridge so that there is a straight non-tortuous approach to it.
Design of guide bank
The waterway is the actual width through which flow takes place at the structure. Generally, the length of the clear waterway provided between guide banks is taken equal to lacey’s perimeter.
2) Top level
The top level of the guide bank is kept equal to u/s total energy level plus adequate freeboard. u/s total energy level is equal to the high flood level before construction plus afflux and velocity head. Thus,
Where, free board = 1.25 to 1.5 m
3) Length of guide banks
According to gales, the length of the u/s guide bank is taken according to the river discharge as shown in table.
|Discharge||Length of u/s guide bank||Convergence towards structure|
|Less than 20,000 cumecs||1.25L||1 in 20|
|20,000 – 40,000 cumecs||1.25 L to 1.5 L||1 in 20 to 1 in 40|
|Greater than 40,000 cumecs||1.5 L||1 in 40|
The length of d/s guide banks is recommended as 0.25L for all discharges.
4) Radius of heads
a) u/s curve head
the main function of u/s curve head is to guide the flow smoothly and axially to the structure and to keep the end spans active. A safe value of radius (R) for u/s curve head is taken as, R = 0.45L ( L = width of river )
The u/s curve head is extended to subtend an angle of 1200 to 1450 at its center.
b) d/s curve head
On the d/s, the river fans out so as to attain its normal width. The radius of d/s head is kept equal to one half of the u/s head radius.
5) Cross section of bund
The top width of the guide bank should not be less than 4m. Side slope should not be steeper than 2:1. A minimum free board of 1.2 to 1.5m is generally provided.
6) Slope protection
The water face of the guide bank is protected by stone pitching to withstand erosive action of the water currents. The rear side is not pitched and is coated with a layer of earth about 0.3 to 0.6m thick.
7) Lunching apron
The stone pitching is extended beyond the toe In the form of packed stone which is called lunching apron. The lunching apron is generally laid in a width equal to 1.5 times the scour depth ‘D’ below the original bed.
The total scour depth below HFL is taken as ;’xR’ , where R is lacey’s normal scoured depth given by,
8) Groynes or spurs
A spur, spur dyke, or groyne is a structure made to project flow from a riverbank into a stream or river with the aims of deflecting the flow away from the side of the river on which the groyne is built.
Hence in this way you can design a guide bank. So, I hope this article on “river training works” remains helpful for you.
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