Introduction to DAM

Lecture-04

DAM

A dam is a barrier built across a river, stream, or other waterway to manage the flow of water and create a reservoir, lake, or storage basin.

Its main functions include:

·        Water Storage: Holding water for agricultural irrigation, municipal water supply, and industrial use.

·        Flood Control: Regulating water flow to prevent flooding downstream.

·        Hydropower Generation: Utilizing the potential energy of water to generate electricity.

·        Recreation and Tourism: Facilitating activities such as boating, fishing, and tourism.

·        Navigation Improvement: Improving the navigability of waterways by maintaining stable water levels.

·        Sediment Control: Controlling the transport of sediment downstream.

Types of dams

Based on ancient origins

1.     Earth Dams

2.     Rock-fill Dams

3.     Solid Masonry Gravity Dams

Based on recent usage

1.     Hollow masonry gravity dams

2.     Timber dams

3.     Steel dams

4.     Arch dams

Based on material of construction

1.     Timber dams

2.     Steel dams

3.     Concrete dams

4.     Earth dams

5.     Rockfill dams

6.     Combined dams

Based on design criteria

·        Hydraulic design

1.     Non-overflow dams

2.     Overflow dams

3.     Composite dams

·        Stability consideration

1.     Gravity dams

2.     Non-gravity dams

Based on Use/purpose

1.     Storage dams

2.     Stage control/Diversion dams

3.     Barrier dams

Based on Height

1.     H ≤ 12m low dam

2.     12 ≤ H ≤ 30m medium

3.     H ≥ 30m high dam

Based on Rigid condition

1.     Solid masonry or concrete gravity dam.

2.     Arched masonry or concrete dam.

3.     Concrete butters dam.

4.     Steel dam.

5.     Timber dam.

Based on non-rigid condition

1.     Earth dam

2.     Rockfill dam

3.     Combined earth and rockfill dam.

Main types of dams

       Concrete dams

1.     Gravity dams

2.     Buttress dams

3.     Multiple ach dams

4.     Thick arch dams

5.     Thin arch dams

Embankment dams

1.     Rock fill dams

2.     Hydraulic fill dams

3.     Earthen embankments

4.     Composite dams

Therefore, no classification can be considered entirely distinct or absolute.

§  Characteristics of each kind of dam

§  Advantages and disadvantages of each kind of dam

Factors for selection of type of dams

Topography

The choice of dam type is primarily influenced by the site's topography.

• For low rolling plains area earth dam with a separate spillway: Earth Dam
• For a narrow V-shaped valley where the valley's top width is less than one-fourth its height: Arch Dam with a separate spillway.
• For a narrow stream in a U-shaped valley with high, rocky walls: Concrete Overflow Dam
• Intermediate Conditions: Factors like foundation conditions, spillway site location, and availability of construction materials are crucial for deciding the dam type.

Geology and foundation conditions

• Sound Rock Foundation: Suitable for any type of dam if no faults or fissures exist. Requires removal of disintegrated rock and sealing of seams/fractures by grouting.
• Dam Type: Gravity Dam, Rockfill Dam, Earth Dam, etc.
• Poor Rock or Gravel Foundation: Allows considerable under-seepage; needs effective water cutoffs or seals.
• Dam Type: Earth Dam, Rockfill Dam, Low Concrete Gravity Dam.
• Silt or Fine Sand Foundation: Prone to settlement, seepage, and toe erosion.
• Dam Type: Earth Dam, Low Concrete Gravity Dam (not suitable for Rockfill Dam).
• Clay Foundation: Prone to long-term consolidation causing cracks. Needs proper foundation treatment.
• Dam Type: Earth Dam (not suitable for Gravity Dam or Rockfill Dam).

Materials of Construction

• Availability of construction materials nearby
• Reduces transportation costs and overall construction expenses.
• Concrete Gravity Dam
• Suitable when sand, gravel, and crushed stone are available.
• Earth Dam
• Suitable when coarse and fine-grained soils are readily available.

Spill way size and location

• Large Spillway Needed:
• Spillway and dam are combined into one structure.
• Concrete dam is suitable.
• Small Spillway Needed:
• Earth dam or rock-fill dam is preferable.
• Spillway Located Away from Dam:
• Entire dam can be non-overflow type.
• Choice includes rigid or non-rigid dams.

 

Road way

• For supporting a roadway on top.
• Earth Dam or Gravity Dam.

Length & Height of Dam

• For Long length and low height: Earth Dam
• For Short length and greater height: Gravity Dam

Life of dam

• For Long life: Concrete or Masonry gravity dams
• For Intermediate life: Earth and Rock fill dams
• For Temporary storage: Timber dams

 

Factors governing selecting site for dam

Foundations

• Gravity Dams: Require sound rock as a foundation.
• Earth Dams: Can use any type of foundation with proper treatment.
• Ideal Foundation Properties: Should be free from seams, open pockets, or fault planes.
• Alternating Layers Concern: Hard and soft layer formations are undesirable due to water penetration weakening soft layers and causing movement.
• Sand, Stone, and Shale Layers: May cause slipping during trench excavation.
• Uniform Formation Preference: Best conditions occur with a single uniform foundation material.
• Multiple Rock Types Issue: Differing bearing strengths in foundations with varied rocks can cause differential settlement.

Topography

• The dam site should ideally have:
• A narrow gorge to minimize the dam's length.
• An upstream opening for a large reservoir basin.
• The majority of the dam should be positioned on higher ground than the river basin to:
• Lower construction costs.
• Simplify drainage of the dam section.

Site for spillway

• For earth or rockfill dams, the spillway should ideally have a suitable site nearby if constructed separately from the dam.
• If the spillway is integrated into the dam structure, no specific site is required for its placement.

Materials

• Materials for constructing a dam should be sourced locally.
• Minimizing transportation reduces construction costs significantly.

Reservoir and Catchment Area

• The site should offer adequate reservoir storage capacity at minimal cost.
• Minimize the cost of land and property submerged in the water spread area.
• Ensure minimal leakage through the reservoir’s sides and bed.
• Favor geological conditions that reduce percolation losses and maximize runoff.
• Prefer a deep reservoir for benefits like lower land cost per capacity, reduced evaporation losses, and less weed growth.

Communication

• Choose a site with road or rail connectivity.
• Ensure the site can be easily accessed for transportation.
• Consider transportation needs for cement, labor, machinery, food, and other equipment.

Locality

• The area around the site should be healthy.
• It should be free from mosquitoes and other pests.
• Labour and staff colonies will be built near the site.