Dams and Spillways MCQ Quiz - Objective Question with Answer for Dams and Spillways - Download Free PDF

Explanation:

Equipotential line

• A line in the seepage flow field where the hydraulic head is constant.

• No water flow occurs along this line (flow is always perpendicular to it).

• Helps in plotting flow nets but is not the upstream slope itself.

Additional InformationFlow line

• A line that shows the path of water particles as they move through soil.

• Water flows along flow lines, and these are perpendicular to equipotential lines.

• Represents direction of seepage flow but not the surface slope of the dam.

Phreatic line

• The true free water surface within the dam where pore water pressure is atmospheric (zero gauge pressure).

• Separates saturated zone below from unsaturated above inside the dam.

• The upstream slope of an earth dam under steady seepage coincides with the phreatic line.

• It is a curved line starting from the reservoir water level and ending at the downstream toe, indicating the seepage surface.

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.

Explanation:

Concrete or Masonry Gravity Dam:

• The bottom portion of concrete or a masonry gravity dam is usually stepped is stepped at the base to increase the shear strength at the base and at other joints.
• It also enhances the bonds between the dam and the rock foundation.
• By ensuring a better bond between the surfaces the shear strength of these joints should be increased as much as possible.

Additional Information

Brief about Gravity dam:

• It is defined as a structure that is designed in such a way that its own weight resists external forces.
• This type of structure is most durable, solid, and requires little maintenance.

• They may be constructed of masonry or concrete.

• The thickness of the dam provides resistance to sliding.
• It can be constructed on any site where the natural foundation is strong enough to bear the weight of the dam.
• It is most suitable when the foundation is strong.
• Most of the gravity dams are solid so that no bending stress is introduced at any point.
• The failure of this dam, if any, is not sudden. It gives enough warning time before the area to the downstream side is flooded due to the damage to the structure.

Explanation:

Zoned Dam

• Zoned darns usually have a central zone of selected soil material, to form a relatively impermeable core, a transition zone along both faces of the core to prevent piping through cracks which may form in the core, and outer zones of more pervious material for stability.
• This construction is widely used in earth dams and is selected whenever suitable materials are available.
• Clay, even though highly impermeable, may not make the best core if it shrinks and swells too much.
• The most satisfactory cores are of clay mixed with sand and fine gravel

Explanation:

• A fall (like a drop structure or weir) is constructed in waterways to reduce the energy of flowing water.

• It helps in dissipating the surplus energy that the water gains due to a steep slope or sudden drop.

• This prevents erosion and damage downstream by controlling the flow velocity.

• It does not create or maintain energy but rather controls and reduces it for safe flow management.

 Additional InformationTypes of Falls 

Chute Fall

• Water flows down a steep, smooth channel (chute) into a stilling basin or a lower level.

• Used where there is a moderate drop in the channel bed.

• Energy is dissipated mainly by the turbulence generated when water hits the basin.

Shaft Fall (Vertical Drop)

• Water falls vertically through a shaft or drop structure.

• Common in areas with large vertical drops.

• Effective in dissipating energy due to free-fall and impact.

Cascade Fall

• Water flows over a series of small steps or cascades.

• Used where the slope is gentle but the total drop is significant.

• Energy is dissipated gradually over multiple small falls.

Screw Fall

• Water flows down a helical or spiral channel.

• Less common, used where space is limited.

• Provides controlled energy dissipation through friction and turbulence.

Explanation:

Different types of spillways are as follows:

Concept:

The maximum compressive stresses occur at the heel (mostly during reservoir empty condition) or at the toe (at reservoir full condition) and on planes normal to the face of the dam.

∴ For reservoir empty condition maximum compressive force will be at the heel.

∴ For reservoir full condition maximum compressive force will be at the toe of the dam.

The figure below is the pressure distribution diagram for reservoir full condition:

Explanation:

The Discharge over an Ogee Spillway is given by 

\(Q = C \times {L_e} \times H_e^{\frac{3}{2}}\)

Where,

C is the Coefficient of discharge

L_e is the length of the spillway crest

H_e is the Total head above the crest 

Concept:

For no tension Criteria:

\({\rm{B}} = \frac{{\rm{H}}}{{\sqrt {{\rm{S}} - {\rm{C}}} }}\)

Where C = 1, B = Base width of dam, H = Height of dam

No Sliding Criteria:

\({\rm{\;B}} = \frac{{\rm{H}}}{{{\rm{\mu }}\left( {{\rm{S}} - {\rm{C}}} \right)}}\)

Where B’ = Minimum base width for no sliding criteria and S = Specific gravity of material of dam

Calculation:

For C = 0

H = b × √s

Explanation:

Stream/silt load:

• Stream load is a geologic term referring to the solid matter carried by a stream.
• Erosion and bed shear stress continuously remove the particles which are transported by water either in suspension or as dissolved.

​It primarily depends on-

• Nature of the soil in the catchment (as rocks generally don't dissolve, but small earthen particles get dissolved)
• Topography of the catchment as the more the slope more the velocity of the water.
• The intensity of rainfall as more rainfall leads to more runoff, thus increasing the sediment carrying capacity of any stream.

Explanation:

The maximum stress under empty reservoir condition occurs at heel of base because in empty reservoir condition resultant force shift towards heel of base and increases uplift at heel.

Uplift pressure or stress at the base of dam is given by: \(\sigma_{max} =\sum \frac{w}{b}\left ( 1+\frac{6e}{b} \right ) \) , Uplift pressure distribution below gravity dam is shown below:

Explanation:

r_c = 25 kN/m³

γ_w = 10 kW/m³

let length of Dam is pm

B is width of Dam

Hydrostatic force on dam, \({P_w} = \frac{1}{2}{\gamma_w}H \times H = \frac{1}{2} \times 10 \times 9 \times 9 = 405\;kN\) 

Force due to weight of Dam, W = γ_c × A × L = 25 × B × 10 × 1 = 250 B kN

‘P_w’ force try to overturn the dam about the and ‘w’ force try to script it.

To safety against overturning,

N_OT ≤ N_R

P_w × ≤ W × B/2

\(405 \times 3 \le 250\;B \times \frac{B}{2}\)

⇒ \(B \ge \sqrt {\frac{{405\; \times\; 2\; \times\; 3}}{{250}}} \)

⇒ \(B \ge \frac{9}{5}\sqrt 3 \;m\)

Concept:

Zones of storage in a Reservoir:

1. Full reservoir Level:- The full reservoir level is the highest water level to which the water surface will rise during normal operating conditions.

2. Maximum water level:- The maximum water level is the maximum level to which the water surface will rise when the design flood passes over the spillway.

3. Minimum pool level:- The minimum pool level is the lowest level up to which the water is withdrawn from the reservoir under ordinary conditions.

4. Dead Storage:- The volume of water held below the minimum pool level is called the dead storage. It is provided to cater for the sediment deposition by the impounding sediment laid in water. Normally it is equivalent to the volume of sediment expected to be deposited in the reservoir during the design life reservoir.

5. Live/Useful Storage:- The volume of water stored between the full reservoir level and minimum pool level is called useful storage. It assumes the supply of water for a specific period to meet the demand.

6. Bank Storage:- It is developed in voids of soil cover in the reservoir area and becomes available as seepage of water when water levels drops down. It increases the reservoir capacity over and above that given by elevation storage curves.

7. Valley storage:- The volume of water stored by the natural river channel in its valley up to the top of its banks before constructing of a reservoir is called the valley storage. The valley storage depends upon the cross-section of the river.

8. Flood/Surcharge storage:- It is storage contained between maximum reservoir level and full reservoir levels. It varies with the spillway capacity of the dam for a given design flood.

Explanation:

Ogee Fall:

• In this type of fall, an ogee curve (a combination of convex curve and concave curve) is provided for carrying the canal water from higher level to lower level.
• This fall is recommended when the natural ground surface suddenly changes to a steeper slope along the alignment of the canal.
• There is a heavy drawdown on the u/s side resulting in lower depth, higher velocities and consequent bed erosion.
• Kinetic energy is not well dissipated due to smooth transition.

Rapid Fall

• The rapid fall is suitable when the slope of the natural ground surface is even and long. It consists of a long sloping glacis with longitudinal slope which varies from 1 in 10 to 1 in 20. It is nowadays obsolete.

Stepped Fall

• Stepped fall consists of a series of vertical drops in the form of steps. This fall is suitable in places where the sloping ground is very long and requires long glacis to connect the higher bed level with lower bed level.
• This fall is practically a modification of the rapid fall. The sloping glacis is divided into a number of drops so that the flowing water may not cause any damage to the canal bed. Brick walls are provided at each of the drops.