Floods and River Training MCQ Quiz in मल्याळम - Objective Question with Answer for Floods and River Training - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Silt ejectors are the devices that extract silt from the canal water when silt water entering into the off-taking channel. These works are constructed on the bed of the canal (downstream of head regulator).

Different type of silt ejectors includes:

1) Silt vanes or King’s vanes

2) Gibb’s Groyne wall

3) Skimming platforms

Sluice escapes are the devices that are used to remove extra water over HFL (high flood level) of a canal through a sluice and they are not used as silt ejectors.

∴ The correct answer is (i), (ii) and (iii).

Explanation:

Statement 1: correct

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.

Causes of meandering:

1. Extra turbulence generated by excess river sediment during floods. 

2. The increase in bed slope tends to increase the width of the 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 inflow. 

Statement 2: Incorrect

Groynes: 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.

Repelling groynes points towards the upstream at an angle of 10° to 30° to the line perpendicular to the bank.

Attracting groynes points downstream in the direction of flow.

Concept-

Arch dam

An Arch dam is a solid dam made of concrete that is curved upstream in the plan. The arch dam is designed so that the force of the water against it, known as hydrostatic pressure, presses against the arch, compressing and strengthening the structure as it pushes into its foundation or abutments.

An arch dam is most suitable for narrow gorges or canyons with steep walls of stable rock to support the structure and stresses. Since they are thinner than any other dam type, they require much less construction material, making them economical and practical in remote areas.

Since in a variable radius arch dam, the center of the various arch rings at different elevations, do not lie on the same vertical line, it is also known as a variable center arch dam.

Such dams are preferred for V-shaped valleys whereas constant radius arch dams may be provided for U-shaped valleys.

Earth fill dam

Earth fill dams are made up mostly of compacted earth. Most embankment dams have a zone in the middle, called the core, made of low permeability material, a permeable part growing gradually outward called a filter on the two sides covering the core, and the shell on the upstream and downstream heels. The core is usually made of clayey soils to stop water passing through the dam.

Barrage-

The only difference between a weir and a barrage is of gates that is the flow in barrage is regulated by gates and that in weirs, by its crest height. Barrages are costlier than weirs.

Overflow dam-

The dam designed for raising the water level of rivers or for creating a reservoir.

It permits the overflow of water during the passage of excess (flash-flood) discharges over the entire length of the dam crest or through drain openings.

Concept:

River Training:

River training means adopting various measures on a river to stabilize the river channel along a certain alignment for the given/planned cross-sectional dimensions.

Objectives:

• Provides a safe passage to flood discharges without overflowing to the bank for protection of the cultivated or inhabited area.
• Prevent outflanking of structures like a bridge etc.
• Protects the banks from erosion and improves alignment by stabilizing the river channel.
• Deflect river away from the bank which it might be attacking.
• To provide a sufficient draft for navigation as well as a good course for it
• Ensure efficient disposal of sediment load.

The river training can be classified as follows:

1. High water training: It aims at providing sufficient river c/s for safe passage of maximum flood and is concerned with making the adjoining area flood proof, by construction of dykes or leaves, etc.

2. Lower water training: Its primary purpose is to provide sufficient water depth for navigation during lower water periods. It may be accomplished by concentrating and enhancing the flow in the desired channel by closing other channels.

3. Medium water training: It aims at efficient disposal of suspended load and bed thus preserving the channel in good shape.

Canal drop:

It is a structure constructed across a channel to permit lowering down of water level in order to dissipate the surplus energy possessed by the falling water which may otherwise scour the bed and banks of the channel.

Canal Escape:

During floods/rains, when the canal carries water beyond its full capacity, water is released through Canal Escapes.

Canal Cross regulators:

It is provided to admit water into the off taking canal and to regulate the supplies into the canal ( Depth of flow). It also regulated the discharge to be passed into the canal and controls the silt entry into the canal.

Canal outlets:

A canal outlet or a module is a small structure built at the head of the watercourse so as to connect it with a minor or a distributary channel. In other words, canal outlets are devices to regulate the flow of water from a bigger channel into a smaller one.

Concept:

Guide bank:

Guide bank is a hydraulic structure across an alluvial river are prevents the outflanking of the structure by the changing course of the stream.

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.

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. 

Concepts:

In order to find an appropriate location for the head work on the river, the river is divided into four stages. These four stages are:

1. Mountainous stage: The main features of this stage are:

• It is a first stage of the river course.
• In this stage the river flows through steep valleys in the hills.
• The river bed in generally composed of rocks.
• The river water is very clear in this stage.

2. Boulder stage: The main features of this stage are:

• The river bed is mixture of sand, gravel and boulders.
• There exists good subsoil water flow as river bed is very porous.
•  Rivers in this stage have straighter courses with wide bed and non-submersible banks.

3. Alluvial plain: The main features of this stage are:

• It is also called through stage.
• It forms new land by depositing its excess load during the flood season.
• The slope of the bed is flatter (1 in 1000 to 1 in 2000).
• The rivers in this stage generally meander.

4. Delta stage: The main features of this stage are:

• When the river is about to end its course by joining the sea it may split up into number of channels or branches.  These branches are called delta.
• The slope of bed is very flat (1 in 5000 to 1 in 10,000).
• The flat slope results low velocities due to which the channel gets silted up.

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 started to be a meandering river.

Meander parameters:

• Meander length- It is the axial length of one mender, i.e. tangential distance between the corresponding points of the 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 to the direct axial length.

For a meandering alluvial river of width W in flood plain, the meander length is about 6W.

Concept:

Avulsion:

• It is a natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain.
• Avulsions are primarily features of aggrading floodplains.
•  Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain.

​Annual flood:

• It is the highest momentary peak discharge at the respective point of observation in river flow, which is equaled or exceeded once every year.
• The highest flow at a point on a stream during any particular calendar year or water year.

Base flood:

• A flood having a recurrence interval that has one ­percent chance of being equaled or exceeded during any given year (1-­percent-­annual-­chance flood). 

Backwater:

• If a river has developed one or more alternative courses in its evolution, one channel is usually designated the main course, and secondary channels may be termed backwaters.
• The main river course will usually have the fastest stream and will likely be the main navigation route; backwaters may be shallower and flow more slowly.