22: Dams Flashcards
What is a hydroelectric dam? What are some pros and cons?
Hydroelectric dam:
- a large, man-made structure that holds water back to create a reservoir, which is then used to generate electricity
pros:
- long life and low operating cost
- clean, domestic, and renewable source of energy
- stored in potential energy, which is idea for meeting peak demands
cons:
- high capital cost
- long construction periods and flood vast tracts of land
- negatively affect fish and habitats
- pose a threat to downstream communities in the event of catastrophic failures!
What are some of the dam terminology?
Maximum water level?
Normal water level?
Spillway?
Sluiceway?
Gallery?
Heel?
Toe?
Maximum Water Level (MWL):
- the maximum fucking water level of the damn retard
Normal Water Level (NWL):
- bro u retarded or something?
Spillway:
- construction used to release the excess water of the reservoir, usually above NWL
Sluiceway:
- Opening in the dam near ground level used to clear silt accumulation from the reservoir
Gallery:
- small room-like structure left within the dam for checking operations
Heel:
- Contact point with the ground on the up-stream side
Toe:
- contact point with the ground on the downstream side
What are the 4 things you need to include when building a dam?
Cofferdam:
- a enclosure built around the dam area to allow the enclosed area to be pumped out
Diversion tunnel:
- Tunnels are constructed to allow water to be rerouted around the damn site so that construction can take place
Abutments:
- sides of the valley on which the structure of the dam rests on
Foundation:
- Floor of the valley on which the structure of the dam sits on
What are the 2 major categories for dams?
The type of dam constructed depends on the topography, geology of the site, and the source of earth material available for construction
2 major categories:
- concrete dams
- earth dams
- composite dams (combination of both)
What are some dam design issues?
dam stability:
- foundation / abutment failure
- overturning / sliding
spillway:
- dissipate energy (downstream erosion)
Dam leakage:
- erosion of dam material (earth dams) or foundation / abutment material
Reservoir slopes:
- landslides generated waves
What are the 4 types of concrete dams? HB the pros and cons of concrete dams?
Concrete dams:
- gravity dam
- arch dam
- buttress dam
- concrete-face (hybrid)
pros:
- rigid and offer more resistance to compressive forces
- better resistance to water erosion, overtopping, and piping failure
- can be shaped to support imposing loads
- reduce tensile stresses and minimize concrete usage (e.g. thin-arch dams)
cons:
- concrete is expensive
- more labor intensive than earth dams
- low tensile strength
- needs appropriate source of sand, gravel, aggregate
- suitable access of concrete usually requires construction of batching plants
What is a gravity dam? Good/bad places to build?
Gravity dam:
- uses their bulk weight and low center of gravity to hold back water in reservoir
- can be built across long distances over relatively flat terrain
- very applicable in non-mountainous regions
The gravity dam’s self weight acts vertically downward, transmitting the entire load onto the dams foundation!
MUST BE CONSTRUCTED WHERE THE FOUNDATION MATERIAL IS STABLE AND COMPETENT AND WHEN THERE IS A GOOD SOURCE OF CONCRETE AGGREGATE!
What is a buttress Dam? When / where should it be used?
Buttress Dam:
- uses multiple reinforced columns to support a dam that has a relatively thin structure.
- often use half as much concrete as gravity dams, but have a low heigh limit
this type of structure can be considered if the foundation materials are a little weaker / a smaller channel needs to be blocked!
What is an arch dam? When / where should it be used?
Arch dam:
- utilizes the strength of an arch to displace the load of the water behind it onto its abutments (sides / walls its attached too)
- use less material than gravity dams, but are more expensive due to expertise needed!
- build across narrow, deep river gorges, but can be considered in wider valleys (special)
- Can only be built where walls of a canyon are unquestionably stable/ They must also be impervious to seepage around the dam!
What are the principle attributes required of a dam foundation?
Principle attributes:
- adequate strength
- low compressibility
Design of foundation must also ensure that its impermeable enough to prevent excessive loss of water from the reservoir
What are some examples of unfavorable geological conditions?
Flat (low-dipping) faults containing sheared, gouge materials of very low strength
Weak soils or those susceptible to liquefaction
folded rocks containing thin, weak layers of shale present a potential for foundation failure
highly permeably soils
What are earth dams (structure, size). What are some pros and cons?
Earth dams:
- built with clay, sand, gravel and have a broad base.
- consist of a core of impermeable materials supported by shoulders of earth and rock fills to protect from erosion.
- relatively smaller in height and broader at the base (like a big pile lol)
- impose lower stresses on the ground than concrete dams of similar height!
Pros:
- are the safest of all dam types against the risk of damage by earthquakes
- can be constructed where the foundation / underlying materials are weak or where the suitable competent rocks are at a greater depth, meaning they can accommodate more differential settlements than concrete dams
cons:
- large size requires large volumes of suitable material which also needs to have varied grain size
- susceptible to wave erosion and catastrophic failure if overtopping and piping failures
- do not last as long as concrete dams
- compaction of the soil is required!
What is the slope stability of dam construction?
The slope stability of a dam refers to the ability of the dam’s slopes (both upstream and downstream) to resist failure or collapse under various loading conditions (water pressure, earthquake forces, and material self-weight).
What is overtopping when it comes to dam construction? What does this cause? How do you prevent this?
Overtopping:
- causes erosive action of water due to uncontrolled flow over the dam
- once erosion has begun it’s almost impossible to stop!
- It can be prevented through use of a spillway!
What is piping when it comes to dam construction? How can it be recognized? what is it usually attributed to? How can it be avoided?
Piping in an earth dam refers to the process of internal erosion caused by the movement of water through the dam’s body, its foundation, or its abutments. Can lead to dam failure!
Piping can be recognized by an increased seepage flow rage, the discharge of muddy water, sinkholes on or near the dam, or a whirlpool in the reservoir.
Usually attributed to poor construction methods (inadequate compaction of the clay fill) or use of wrong types of material
Can be avoided the lengthening the flow path of water under its foundation by installing a large concrete barrier the water must go under!
