22- Dams

Question 1

Hydroelectric dam

Answer 1

Large, man-made structure that holds water back to create a reservoir, which is the the used to generate electricity

Question 2

Hydroelectric dam pros and cons

Answer 2

Pro
- long life and low operating cost
-provides clean, domestic and renewable source of water
-storage in form of potential energy, ideal for meeting peak demands
Cons
-high capital cost, long construction periods that flood vast tracts of land
-may negatively affect fish and wildlife habitats
-pose a threat to communities downstream in event of catastrophic failure

Question 3

Cofferdam

Answer 3

Temporary barrier that redirects water into a diversion tunnel, preventing river from flowing through dam construction site

Question 4

Abutments

Answer 4

Sides of valley on which structure of dam rests

Question 5

Foundation

Answer 5

Floor of valley on which structure of dam sits

Question 6

Diversion tunnel

Answer 6

Tunnels constructed to allow water to be rerouted around the dam site so that construction can take place

Question 7

Concrete dams

Answer 7

Rely on weight of concrete and/or shape to hold back water
Pros
-rigid and offer Morse resistance to compressive forces
-offer significant resistance to water erosion, overtopping and piping failure
Cons
-relatively expensive
-more labour intensive
-concrete had low tensile strength
-construction requires appropriate sources of material, suitable access to the site and often necessitates the local construction of batching plants

Question 8

Gravity dams

Answer 8

Type of concrete dam
Use bulk weight and low center of gravity to hold back water
Can be built over long distances over relatively flat terrain
Very applicable in non-mountainous regions

Constructed where foundation material is competent and stable, and where there is a good source of aggregate

Question 9

Buttress dams

Answer 9

Type of concrete dam
Use multiple reinforced columns to support dam that has relatively thick structure
Often use half as much concrete as gravity, but this limits their max height

Can be constructed even if foundation materials are little weaker

Question 10

Arch dams

Answer 10

Type of concrete dam
Utilize strength of arch to displace the load of water behind it onto its abutments
Use less material than gravity dams, but are more expensive to construct due to extensive expertise required
Built across narrow, deep river gorges (have been considered for wider valleys)
Can only be built when walls of canyon are of unquestionable stability, must also be impervious to seepage around the dam, as this could cause failure

Question 11

Dam foundations

Answer 11

Must have adequate strength and low compressibility
Must also be impermeable enough to prevent excessive loss of water from reservoir

Question 12

Unfavourable geological conditions for dam foundation

Answer 12

1. Flat(low dipping) faults containing sheared material of low strength
2. Folded rocks containing thin, weak layers of shale
3. Weak soils or this susceptible to liquefaction
4. Highly permeable soils

Question 13

Earth dams def and pros and cons

Answer 13

Built with clay, sand and gravel
Relatively smaller in height and broad at the base

Pros
-constructed where foundation are weak
-safest of all dam types against risk of damage by earthquake
Cons
-require large volume of material
-susceptible to wave erosion, catastrophic failure and piping failure
-do not last as long
-extra care is required in designing internal structure

Question 14

Earth dam structure

Answer 14

Core of impermeable material (ex, rolled/compacted clay) supported by permeable shoulders of earth and rock fill
Impose lower stresses on the ground because of broad base

Question 15

Overtopping

Answer 15

When water flows over top of dam
Causes erosion of the dam
Can be prevented through use of a spillway

Question 16

Piping

Answer 16

Erosion of soil begins downstream and works towards the reservoir, developing a “pipe”
Piping can be recognized by an increased seepage flow rate, the discharge of muddy water, sinkholes or whirlpools
Piping failure can be attributed to poor construction methods (ex, inadequate compaction) or use of wrong construction materials