Dams and Spillways MCQ Quiz in বাংলা - Objective Question with Answer for Dams and Spillways - বিনামূল্যে ডাউনলোড করুন [PDF]

Straight Glacis Fall

• It consists of a straight sloping glacis provided with a crest so that hydraulic jump may occur. A water cushion is provided on the downstream side to dissipate the energy of flowing water.
• The sloping glacis is constructed with cement concrete. Curtain walls and toe walls are provided on the upstream and downstream side.
• The energy dissipation on the straight glacis remains incomplete due to vertical component of velocity unaffected.

Montague Type Fall

• In this type of fall, the straight sloping glacis is modified by giving parabolic shape which is known as Montague profile. Taking “0” as the origin, the Montague profile is given by the equation,

\(x = v\sqrt {\frac{{4y}}{g}} + y\)

• The main body of the fall is constructed with cement concrete. Toe walls and curtain walls are same as in the case of straight sloping glacis. The bed protection by stone pitching is also same.
• Curved glacis is difficult to construct and hence costlier.

Khosla’s Theory: 

It states that the seepage of water does not creep along the bottom contour. It moves along a set of streamlines. In order to calculate the uplift pressure and exit gradient theory takes into account the flow pattern impermeable base of the hydraulic structure.

The seepage water exerts a force at each point in the direction of flow and tangential to the streamlines. This tangential force has the maximum disturbing tendency at the exit end, because the direction of this force at the exit point is vertically upward, and hence full force acts as its upward component. For the soil grain to remain stable, the submerged weight of soil grain should be more than this upward force. This force at any point is proportional to the gradient of the pressure of water at that point. This gradient of the pressure of water at the exit end is called the exit gradient.

It has been determined that for a standard form consisting of a floor length (b) with a vertical cut-off of depth (d), the exit gradient at its downstream end is given by

\(G = \frac{H}{d} \times \frac{1}{{\pi \sqrt \lambda }}\)

Where,  H is the maximum seepage head.

\(\lambda = \;\frac{{1 + \sqrt {1 + {\alpha ^2}} }}{2}\)

And

α = b/d

It is clear from above that, exist gradient depend on the following:

1. b/d ratio

2. H/d ratio

Explanation:

Chute blocks:

• These are triangular blocks on top base as horizontal.
• These are installed at the toe of the spillway just upstream at the end of stilling basin
• They act like a serrated device at the entrance to the stilling basin.
• These blocks stabilize the jump, improve ump performance, decrease the length of a hydraulic jump, and are used as auxiliary devices.

​Chute blocks act as auxiliary devices in the spillway.

Concept:

• Scouring action or scour holes in a dam project can endanger downstream structures such as bridges, culverts, pipelines, and other infrastructure.
• The force of water flowing through a scour hole can erode the soil or rock around these structures, compromising their stability and potentially causing them to fail.
• This can result in serious damage to property, loss of life, and disruption to essential services.
• River headworks refer to the structures and facilities located at the upstream end of a canal system.
• They are designed to control the flow of water into the canal, and to divert water from a river or other source into the canal network

Concept

For an elementary profile of a dam.

Base width for no tension at the base is given by 

\(\frac{{\rm{b}}}{{\rm{H}}} = \frac{1}{{\sqrt {\left( {{\rm{G}} - {\rm{K}}} \right)} }}\)

Where,

H = Height of water level on the upstream end

γw = Unit weight of water

Where G = Specific gravity of the material

K = Coefficient of uplift pressure

Calculation:

Given, 

b = 35 m, H = 50 m, G = 2.5

\(\frac{{\rm{35}}}{{\rm{50}}} = \frac{1}{{\sqrt {\left( {{\rm{2.65}} - {\rm{K}}} \right)} }}\) ⇒ \(1.428 = {{\sqrt {\left( {{\rm{2.65}} - {\rm{K}}} \right)} }}\)

Khosla’s Theory: 

It states that the seepage of water does not creep along the bottom contour. It moves along a set of streamlines. In order to calculate the uplift pressure and exit gradient theory takes into account the flow pattern impermeable base of the hydraulic structure.

The seepage water exerts a force at each point in the direction of flow and tangential to the streamlines. This tangential force has the maximum disturbing tendency at the exit end, because the direction of this force at the exit point is vertically upward, and hence full force acts as its upward component. For the soil grain to remain stable, the submerged weight of soil grain should be more than this upward force. This force at any point is proportional to the gradient of the pressure of water at that point. This gradient of the pressure of water at the exit end is called the exit gradient.

It has been determined that for a standard form consisting of a floor length (b) with a vertical cut-off of depth (d), the exit gradient at its downstream end is given by

\(G = \frac{H}{d} \times \frac{1}{{\pi \sqrt \lambda }}\)

Where,  H is the maximum seepage head.

\(\lambda = \;\frac{{1 + \sqrt {1 + {\alpha ^2}} }}{2}\)

And

α = b/d

It is clear from above that, exist gradient depend on the following:

1. b/d ratio

2. H/d ratio

Concept:

Ogee (Overflow) Spillway:

• This type comprises a control weir, which is ogee or 'S' shaped.
• The ogee shape conforms closely to the profile of aerated lower nappe and falling from a sharp-crested weir.
• The upper curve at the crest may be made either broader or sharper than the nappe.
• A broader curve will support the sheet and hydrostatic pressures will occur along the contact surface.
• The support sheet thus creates a backwater effect and reduces the coefficient of discharge.
• The sharper crest on the other hand creates negative pressure, increases the effective head, and thereby discharge.
• This type of spillway should be constructed in the Nalla itself as far as possible

Concept:

Failures of earthen embankment dams are grouped into three categories:

Hydraulic Failure:

• They occur from the uncontrolled flow of water over and adjacent to the embankment and due to the erosive action of water on the embankment slopes.

Various hydraulic failures are:

i) Overtopping:

(ii) Wave erosion: 

• Notching of the upstream face by wave action that reduces the embankment cross section thickness and weakens embankment material.

(iii) Toe erosion: 

• Erosion of downstream toe of the earth slope caused by misdirected spillway outlet discharge.

(iv) Gullying: 

• Rainfall erosion of embankment slopes. Also caused by traffic from people and vehicles.

Additional Information

Seepage Failure: 

There are two types of seepage occurring in the earthen dams, controlled and uncontrolled. Controlled seepage does not do any harm to the dams, however, uncontrolled seepage may lead to the failure of dams

There are three types of failures observed due to uncontrolled seepage:

• Piping Through Foundations: Sometimes, when highly permeable cavities or fissures or strata of coarse sand or gravel are present in the foundation of the dam, water may start seeping at a huge rate through them. This concentrated flow at a higher gradient may erode the soil which ultimately leads to failure
• Piping Through The Dam: When the concentrated flow channels get developed in the body of the dam, the soil may be removed in the same manner as in foundation piping, leading to the formation of hollows in the dam body, and subsequent subsidence of the dam. Piping through the dam body generally gets developed near the pipe conduits passing through the dam body.
• Sloughing of Downstream Toe: The process of failure due to sloughing starts when the downstream toe becomes saturated and gets eroded, producing a small slump or a miniature slide.

Structural Failure: 

Structural Failures of Earthen Dams are listed below:

• Foundation slide
• Slide in embankments

Explanation:

An Arch Dam

It is a curve-shaped solid wall that is generally built with cement concrete. It is preferred where river valleys are formed.

The following classification of an arch dam depends on its angle, radius, and curvature:

Constant Radius Arch Dam

• Constant Radius Arch dam, in which the outer side curve of the arch is built with constant radii at all elevations. At the same time, the inner curve of the arch is constructed with decreasing radius from top to bottom of the dam.
• The outer side curve of an arch is called an extrados while the inner curve is called an intrados.

Variable Radius Arch Dam

• Variable radius arch dam, as in the name itself the radii of both extrados (outer side) and intrados inner side) curves are varied with respect to elevation.
• The radius is maximum at the top of the dam and minimum at its base. 

Constant Angle Arch Dam

• A constant angle arch dam is a dam in which the central angle of every arch ring is constant throughout the height of an arch dam.
• It is obtained by the modification of a variable radius arch dam.
• Safe and best central angle 133⁰ 34’ is used as the central angle in this case. 

Double Curvature Arch Dam

•  the Double curvature arch dam, in which curvature is provided not only in horizontal direction but also in the vertical direction.
• It means the cross-section of double curvature also looks like a curve. 

Explanation:

Farakka Barrage project

• It is a major irrigation project in India that is constructed across the Ganges River.
• It is located in the state of West Bengal.
• The project aims to divert water from the Ganges River to the Hooghly River for the purpose of maintaining water levels and navigation in the Hooghly River, as well as for irrigation and water supply in the region.

Additional Information

• Mahanadi and Odisha:

  • Mahanadi River: Major river flowing through Chhattisgarh and Odisha.
  • Hirakud Dam: Located in Odisha; one of the longest dams in the world, used for flood control, irrigation, and power generation.

• Gandak and Bihar:

  • Gandak River: Flows through Nepal and India, joining the Ganges in Bihar.
  • Gandak Barrage: Located near Valmikinagar, Bihar; used for irrigation in Bihar and Uttar Pradesh.

• Sutlej and Himachal Pradesh:

  • Sutlej River: Part of the Indus river system, originating in Tibet and flowing through Himachal Pradesh and Punjab.
  • Bhakra Dam: Located in Himachal Pradesh; one of the highest gravity dams, providing irrigation, drinking water, and hydroelectric power.