Floods and River Training MCQ Quiz - Objective Question with Answer for Floods and River Training - Download Free PDF

Explanation:

CWC Guidelines for Aqueducts Design Flood Period

As per the Central Water Commission (CWC) guidelines, the design flood for aqueducts should be selected based on a specified time period. The design flood is a critical parameter in ensuring the structural integrity and safety of aqueducts during flood events.

​As per CWC guidelines to select floods:

(a) For spillways for major and medium projects with storage more than 60 Mm³ recommended design flood is either probable maximum flood or flood frequency method with return period of 1000 years.

(b) For permanent barrages and minor dams with capacity less than 60 Mm³ recommended design flood is standard probable flood (or) flood with time period of 100 years whichever is higher.

(c) Aqueducts is recommended for a design flood for time period of 50 years.

Concept:

Levees

Levees construction is a method of river training. They are constructed to protect the riverbanks from flooding. It is basically an embankment constructed along the river so that water remains contained in between the levees during flooding.

Effects of levees construction:

• It reduces the width of the river stream, so flow velocity is increased.
• It contains the flood water in between the levees only, which increases the discharge.
• It increases the water surface elevation during flooding.
• It decreases the flood storage as maximum flood water becomes the part of discharge.

Groynes / Spurs: 

• These are the embankment type structures constructed transverse to the river flow.
• They extend into the river from the bank and may also be called transverse dykes.
• They are constructed to protect the bank by deflecting the current away from the bank.
• They train the river to flow along a desired course by attracting, deflecting or repelling the flow.

Explanation:

• Flood Plain Zoning (Non-Structural Measure)

  • This involves land-use planning to restrict construction in flood-prone areas.
  • It is a policy-based approach, not a physical structure.

• Detention Reservoirs (Structural Measure)

  • These are artificial reservoirs designed to temporarily store excess floodwater.
  • Since it involves physical construction, it is a structural measure.

• Channel Improvement (Structural Measure)

  • Modifying or enlarging river channels to improve water flow and reduce flooding.
  • It involves physical construction, making it a structural measure.

• Levees (Structural Measure)

  • Embankments built along rivers to prevent floodwaters from spilling over.
  • It is a physical flood barrier, making it a structural measure.

Explanation:

River Training Works

River training works are constructed for several reasons but the fundamental objective is to restrict the horizontal movement of the river channel along a certain alignment.

The measures are adopted to train the river because rivers in alluvial flood plains are prone to meandering and hence cause a lot of damage to life and property.

Additional InformationMeandering

• Meandering is considered a wave phenomenon.
• A meander is formed when moving water in a stream erodes the outer banks and widens its valley and the inner part of the river has less energy and deposits what it is carrying.
• A meandering channel tends to be deeper towards the concave bank and progressively gets shallower towards the convex bank.

Explanation:

A mass curve for a typical stream is usually shown for a 6-year period. This is because the 6-year period is a common period for hydrological analysis. It is long enough to capture the variability of the streamflow, but it is not so long that the data becomes too difficult to manage.

Temporary spurs or bunds are temporary structures constructed every year after floods. A bund is a structure made to project flow from a riverbank into a stream or river with the aim of deflecting the flow away from the side of the river on which the groyne is built.

Important Points:

Bunds are temporary in nature whereas Weirs and barrages are permanent in nature.

In a weir, the water overflows the weir, but in a dam, the water overflows through a special place called a spillway. Weirs have traditionally been used to create mill ponds.

Explanation:

Falls and drops are made when the natural ground is steeper than bed slope of channel.

Types of falls:-

1. Ogee falls

2. Rapids

3. Trapezoidal notch falls

4. Syphon well drops

5. Sarda type falls or simple verticle drop

6. Straight glacis falls

7. Montague type falls

8. Inglis falls or Baffle falls

Ordinary falls results in the deep cutting of bed strata as the water hits the bed surface.

Sarda type fall is the combination of small-sized falls resulting in gradual energy dissipation of the water without hydraulic jump formation. Such combination is adopted for weaker strata as the depth of cutting is minimized. It is also economical to build to sarda type fall.

Concept:

Groynes or Spurs:

• A groyne is a rigid hydraulic structure built either from the shore (in case of seas) or bank (in case of rivers) in order to dissipate the wave energy or to protect the banks from erosion by trapping the sediments.
• They are perpendicular or slightly oblique to the shoreline.

Guide banks:

• Guide banks are meant for guiding and confining the flow in a reasonable waterway at the site of the structure.
• The design of the guide banks is based on the theory developed by Mr. Bells. Hence, guide banks are also known as Bell’s bunds.
• The guide banks are constructed parallel or approximately parallel to the direction of flow. They extend both upstream and downstream of the abutments of the hydraulic structure.
• The guide banks may be provided on either side of the hydraulic structure or on one side as required.

Levees:

• It is a low ridge or earthen embankment built along the edges of a stream or river channel to prevent flooding of the adjacent land. 
• Levees are usually embankments of dirt built wide enough so that they will not collapse or be eroded when saturated with moisture from rivers running at unusually high levels.

Flood walls:

• When no space is available for the construction of a levee or when it is not suitable to construct the levee due to local site conditions, then a flood wall is constructed as local flood hazard prevention for some important area situated at a low level.
• There are masonry or concrete walls constructed just on the river bank.

Concept:

Levees

Levees construction is a method of river training. They are constructed to protect the riverbanks from flooding. It is basically an embankment constructed along the river so that water remains contained in between the levees during flooding.

Effects of levees construction:

• It reduces the width of the river stream, so flow velocity is increased.
• It contains the flood water in between the levees only, which increases the discharge.
• It increases the water surface elevation during flooding.
• It decreases the flood storage as maximum flood water becomes the part of discharge.

Groynes / Spurs: 

• These are the embankment type structures constructed transverse to the river flow.
• They extend into the river from the bank and may also be called transverse dykes.
• They are constructed to protect the bank by deflecting the current away from the bank.
• They train the river to flow along a desired course by attracting, deflecting or repelling the flow.

Explanation:

Guide banks are meant for guiding and confining the flow in a reasonable waterway at the site of the structure i.e., channelize the flow of the river

Guide Banks or Bell’s Bunds:

• Rivers in flood plains submerge very large areas during flood periods.
• Naturally, when some structure is to be constructed across such a river (for example, bridge, weir, etc.), it is very expensive to construct the work spanning the whole width of the river.
• To economies, some training work may be constructed to confine the flow of water within a reasonable waterway. 

Concept:

Meandering River:

• The formation of successive bends of reverse order may lead to the formation of a complete S curve called Meanders.
• When consecutive curves of reverse order connected with short straight reaches called crossings are developed in a river reach, the river is stated to be a Meandering river.

Meandering of a river is due to erodibility of the bed and banks of streams.

Causes:

1. Extra turbulence generated by excess of river sediment during floods. When the silt charge is in excess of the quantity required for stability, the river starts building up its slope by depositing the silt on its bed.

2. The increase in bed slope tends to increase the width of channel, if the banks are not resistant. The banks are attacked by river water and in the process one bank is likely to be attacked slightly more than the other, causing a slight deviation in flow. This slight deviation from the uniform axial flow, helps in improving more and more flow towards one bank than towards the other. This process continues causing once bank convex and another concave. Hence, increasing slope is also a reason for meandering.

Important Points

Meander parameters:

Meander Length: It is the axial length of one mender, i.e. tangential distance between the corresponding points of meander.

Meander Belt: It is the distance between the outer edges of clockwise and anticlockwise loops.

Meander Ratio: It is the ratio of meander belt to meander length.

Tortuosity: It is the ratio of length along the channel (actual length) to the direct axial length.

Crossings or cross-over: It is the shortest reaches of the river, connecting two consecutive clockwise and anticlockwise loops.

Concept:

Groynes or spurs: 

• These are constructed transverse to the river flow extending from the bank into the river.
• This form of river training works perform 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 slack flow for silting up the area in the vicinity and protecting the bank by keeping the flow away from it.
• Each groyne can protect only a certain length and so the primary factor governing the spacing between adjacent groynes is their lengths. Generally, a spacing of 2 to 2.5 times the length of groynes at convex banks and equal to the length at concave banks is adopted.

Additional Information

Groynes or spurs are classified as -

1.  Permeable or impermeable
2.  Submerged or non-submerged
3.  Attracting, deflecting repelling and sedimenting and
4. T-shaped (Denehey), hockey (or Burma) type, kinked type, etc. 

The probability of exceedance means that probability that flood of certain magnitude will be equalled or exceeded in a particular year during its entire design life.

Probability of exceedance, \(P = \frac{1}{T};\) where T is return period

\({\rm{P}} = \frac{1}{{25}} = 0.04{\rm{\;or\;}}4{\rm{\% }}\)

Explanation

Launching Apron:

• To protect the face of the guide bank at the river bed level a thick stone cover is laid on the bed. It is called an apron. When the scour undermines the river bed the apron comes down or launches to cover the face of the scour. Hence it is called Launching apron also.
• Heavy scour of the river bed at curved heads and shanks of guide banks can cause undermining of stone pitching thereby resulting in failure of guide banks. Such a failure can be prevented by providing a launching apron beyond the toe of guide banks.
• The width of the launching apron is generally taken as 1.5D.

Concept:

Silt regulation works are required to preformed to control the erosion along the banks.

Two popular silt regulation works are as silt excluders and ejectors.

Silt excluders silt control device consisting of a number of rectangular tunnels - provided parallel to the axis of head regulator and terminating near the under sluiced weir - to allow the clearer water to flow through the head regulator. Silt excluders are constructed on the bed of the river, upstream of the head regulator.

Silt ejectors also called silt extractors, and it extracts the silt from the canal water after the silted water has travelled a certain distance in the off-take canal. Silt ejectors are constructed on the bed of the canal and a little distance downstream from the head regulator.

These works are, therefore, constructed on the bed of the canal, and little distance downstream from the head regulator.