Dam Engineering: Types, Components, and Control Mechanisms
DAMS
Definition
Dams are water reservoirs designed to store water. Their primary functions include:
- Supplying water for consumption
- Controlling water flow to prevent flooding
- Harnessing water power and transforming it into other forms of energy (electrical, mechanical, etc.)
Classification of Dams
Types of Dams Based on Structure:
- Gravity Dams
- Lightweight Gravity Dams (Buttress Dams)
- Arch Dams
Types of Dams Based on Material:
- Concrete Dams
- Embankment Dams
- Composite Dams
Types of Dams Based on Application:
- Check Dams
- Storage Dams
- Flood Control Dams
- Diversion Dams
- Power Generation Dams
Types of Dams Based on Structure
Gravity Dams:
These dams feature an isosceles triangle profile, with a wider base to withstand greater water pressure. The weight of the dam’s material counteracts the reservoir’s pressure, transferring the thrust to the ground. Consequently, the soil must be capable of supporting the combined weight of the dam and the reservoir.
Forces Acting on Gravity Dams:
- Self-weight
- Hydrostatic pressure
- Uplift pressure
- Sediment or silt pressure
- Seismic force
- Weight of water on the upstream face
- Negative pressure correlation between the mantle and the downstream water face
- Friction of water discharge on the face
- Impact of waves and floating debris
- Ice pressure
- Earthquake effects
Advantages of Gravity Dams:
- Most common type of dam
- High durability
- Low maintenance requirements
Disadvantages of Gravity Dams:
- Require a large amount of construction material
- Limited wall height
Lightweight Gravity Dams (Buttress Dams):
These dams utilize ribs extending from the upstream face to buttresses, which transfer the water pressure to the foundation. The ribs support concrete slabs that hold back the water. Columns are incorporated to reduce the overall concrete volume. The water pressure is channeled into these structural elements.
Advantages of Lightweight Gravity Dams:
- Reduced material requirements
- Suitable for locations with limited construction material availability
Disadvantages of Lightweight Gravity Dams:
- High construction costs (require skilled labor)
- Necessitate a stable rock foundation
- Complex behavior in the crown liner and thermal expansion
Arch Dams:
Characterized by their curved and thin structure, arch dams often incorporate steel reinforcement bars or cables. The reinforcements distribute the load not only to the foundation but also to the abutments. These dams are typically constructed in narrow and deep gorges.
Types of Arch Dams:
- Constant-Radius Arch Dams: Feature a vertical upstream face with a consistent radius of curvature.
- Variable-Radius Arch Dams (Dome or Double-Curvature Dams): Have curves on both the upstream and downstream faces, with radii that decrease systematically with depth below the crest.
Advantages of Arch Dams:
- Minimize concrete requirements
- Ideal for tall, narrow gorges
Disadvantages of Arch Dams:
- Complex design and construction processes
- Require stirrup reinforcement and extremely hard, stable rock
- Challenging material transportation and placement
Types of Dams Based on Material
Concrete Dams:
Constructed using concrete, a compact, dense, and easily moldable material with high compressive strength, concrete dams are the most prevalent type today. However, they are more expensive and time-consuming to build, demanding specialized machinery and skilled labor. Concrete’s low tensile strength necessitates reinforcement to withstand bending stresses.
Types of Concrete Dam Structures:
- Gravity
- Arch
- Vault
- Buttress
- Ambursen (hollow gravity)
Embankment Dams:
These dams are built using natural materials that have not undergone chemical processing. The materials are compacted in layers to create the dam structure. Different types of permeable and impermeable materials are used in specific zones within the dam.
Types of Embankment Dams:
- Earth-fill Dams: Composed of over 50% earth or a mixture of earth, gravel, and sand.
- Rock-fill Dams: Primarily constructed using coarse rock.
- Earth-Rockfill Dams: Combine both earth and rock materials, with an impermeable core and permeable layers (shells) on either side.
Advantages of Embankment Dams:
- Cost-effective
- Relatively easy to construct due to material availability
- Do not require highly skilled labor
- More adaptable to less stable foundations
Disadvantages of Embankment Dams:
- Limited height potential
- Lower resistance compared to concrete dams
- Vulnerable to overtopping (water flowing over the crest)
Composite Dams:
These dams combine concrete and embankment materials, such as rubble, soil, and clay. They are typically gravity dams with large, low-rise walls. Composite dams offer rapid construction and relative cost-effectiveness. They are safe as long as the water level remains below the crest but can be extremely dangerous if overtopping occurs.
Types of Dams Based on Application
Check Dams:
These small dams are built in the upper and middle reaches of rivers to trap sediment and debris, preventing siltation in downstream reservoirs. They allow water to pass through while retaining solids ranging from fine particles to large rocks. Check dams are often used for erosion control and sediment management.
Flood Control Dams:
Designed to regulate the flow of water during flood events, these dams help mitigate downstream flooding by storing excess water and releasing it gradually. They may also serve secondary purposes, such as power generation or irrigation.
Diversion Dams:
These dams raise the water level to facilitate its diversion for various purposes, such as irrigation or hydroelectric power generation. They control sedimentation in the channel, ensuring a consistent water supply to intake structures. Diversion dams are typically low-rise structures.
Power Generation Dams:
These dams are specifically engineered to generate hydroelectric power. They create a height difference (head) and store a sufficient volume of water to drive turbines. The type of turbine used depends on the head and flow characteristics:
- High Head, Low Flow: Pelton turbines
- Low Head, High Flow: Francis or Kaplan turbines
Components of a Dam and Reservoir
Reservoir:
The volume of water impounded by the dam.
Lake/Impoundment:
The area of the valley inundated by the reservoir.
Dam Site:
The specific location where the dam is constructed.
Dam Structure:
Faces:
The two vertical or near-vertical surfaces of the dam: the upstream face (in contact with the reservoir) and the downstream face.
Crest:
The top surface of the dam, often incorporating a walkway or road.
Abutments:
The sides of the valley that support the ends of the dam. Abutments are crucial structural elements in arch dams.
Foundation:
The base or foundation upon which the dam wall rests.
Spillway:
A channel designed to safely discharge excess water from the reservoir, either in a controlled or uncontrolled manner.
Types of Spillways:
- Uncontrolled Spillways: Allow water to flow freely over the crest.
- Controlled Spillways: Regulate water discharge using gates.
Types of Spillway Gates:
- Vertical Axis Gates: Include cylinder gates, sluice gates, and roller gates.
- Horizontal Axis Gates: Include taintor gates, sector gates, and drum gates.
Intake Structures:
Structures that regulate water withdrawal from the reservoir for various purposes, such as irrigation or water supply. They can be located on the upstream face or within the dam body.
Bottom Outlet:
A conduit or channel at the base of the dam for releasing water from the reservoir for sediment flushing or reservoir drawdown.
Sluiceway:
A gated passageway through the dam used to control water flow and sediment release.
Control Tower:
A structure housing equipment and personnel for monitoring and controlling dam operations.
Fish Ladder:
A structure that allows fish to migrate upstream past the dam, typically consisting of a series of pools connected by small waterfalls or ramps.
Internal Components:
Gallery:
A network of tunnels within the dam wall used for inspection, maintenance, and instrumentation. Galleries can be dry or flooded and provide access to various levels within the dam.
Drainage System:
A system of drains and filters designed to collect and remove seepage water from within the dam and its foundation, reducing uplift pressure and preventing internal erosion.
Instrumentation:
Sensors and monitoring equipment embedded within the dam to measure various parameters, such as water levels, pressures, strains, and displacements. This data is essential for assessing dam safety and performance.
Dam Control and Monitoring Elements
Dams are equipped with various systems to monitor their structural integrity and ensure safe operation. These systems include:
Pendulums:
Direct and inverted pendulums are used to measure horizontal movements within the dam structure. Their accuracy and long-term stability make them valuable tools for monitoring dam deformations.
Deformmeters:
These instruments measure relative displacements between points within the dam, typically at joints or cracks. They help monitor the expansion and contraction of concrete, as well as any potential movement along fault lines.
Piezometers:
Piezometers measure water pressure at specific points within the dam and its foundation. They provide information about uplift pressure, seepage flow, and the effectiveness of the drainage system.
Geodetic-Topographic Monitoring:
This involves using surveying techniques to monitor the dam’s position and any potential movements. Fixed base stations and mobile surveying equipment are used to track changes in elevation, tilt, and horizontal displacement.
Seepage Weirs:
These weirs are installed to measure the flow rate of seepage water through the dam or its foundation. Continuous monitoring of seepage flow helps detect any anomalies or potential problems.
Other Instrumentation:
Dams may also be equipped with a range of other instruments, including:
- Temperature sensors
- Strain gauges
- Tiltmeters
- Seismometers
These instruments provide comprehensive data on the dam’s behavior, enabling engineers to assess its safety, identify potential issues, and implement necessary maintenance or repairs.