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

Explanation:

A reservoir is an artificial or natural lake used to store water for various purposes. Two of the most important functions of reservoirs are:

• Controlling floods — Reservoirs regulate river flow by storing excess water during heavy rainfall or snowmelt, releasing it gradually to prevent downstream flooding.

• Generating hydropower — Reservoirs provide a constant supply of water to hydropower plants. The stored water is released through turbines to generate electricity, making use of the potential energy of the stored water.

 Additional Informatio

• Multi-purpose reservoirs serve various functions such as irrigation, drinking water supply, recreation, and navigation, in addition to flood control and power generation.

• Flood storage capacity of a reservoir is the volume available to store floodwaters temporarily.

• Hydropower generation depends on both the head (height of water) and flow rate — both managed by the reservoir.

• On the other hand, preventing percolation from fields and sewage disposal are not primary functions of a reservoir.

Explanation:

• The line that separates the saturated zone (where water pressure is positive) from the unsaturated zone in an earthen dam is known by all three names.
• Below this line, the soil is fully saturated, and positive hydrostatic (pore water) pressure exists.

Additional InformationSeepage Line:

• Represents the upper boundary of the flow zone through the dam body.

• Water seeps below this line and exerts positive pressure.

• Often identified through flow nets or finite element modeling in seepage analysis.

Phreatic Line:

• Another name for the seepage line, specifically in earth dams.

• Lies within the downstream face and is often parabolic in shape.

• Above this line, soil is unsaturated, and below it, fully saturated.​

Saturation Line:

• Term used interchangeably to indicate the limit of saturation within the soil mass.

• Denotes the start of hydrostatic pressure from capillary tension to positive pore pressure.

Explanation:

• The maximum elevation to which reservoir water surface will rise during normal operating conditions

• This defines the maximum water level that a reservoir is allowed to reach during standard or intended usage.

• It determines the design and operational limits of various hydraulic structures like spillways, gates, and power intakes.

• Beyond FRL, water is not normally stored, and any additional inflow is routed through spillways.

• The normal operating level in a reservoir

• FRL is considered the target water level during the reservoir’s routine operations.

• It ensures optimum storage for purposes such as irrigation, water supply, hydropower, and navigation.

• Operators aim to maintain this level to balance demand and storage without flooding or loss of utility.

• In seasonal reservoirs, water is typically stored up to FRL during monsoon and drawn down post-monsoon.

 Additional Information

Full Reservoir Level (FRL):

• The maximum elevation the reservoir water surface is allowed to reach during normal operations.

• Used for designing the capacity of the reservoir, penstocks, intake structures, etc.

Maximum Water Level (MWL):

• The highest level reached during design floods or emergency conditions, which may be above FRL.

• Determines the top of dam or freeboard design.

Dead Storage Level:

• The portion of the reservoir below the lowest outlet, not usable under gravity flow.

Live Storage:

• The volume of water stored between the dead storage level and FRL, usable for operational purposes.

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:

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.