L22 - Dams

what are hydroelectric dams?

dam that creates a reservoir of water and uses high → low elevation difference to drive turbines

picture of hydroelectric dam

goes from reservoir of high elevation, through turbine, and river of low elevation

what are the pros of hydroelectric dams? (4)

• long lifespan

• low operating costs

• domestic and renewable source of energy

• stored as potential energy, which is ideal for meeting peak demands

what are the cons of hydroelectric dams? (5)

• high capital cost

• long construction periods

• flood vast tracts of land

• hurts fish/other wildlife habitats

• risk to downstream communities from a dam in the event of a dam catastrophe

dam terminology picture

upstream:

• gallery - room in left of dam for dam operations

downstream:

• spillway: construction used to release excess reservoir water

• sluiceway: dam opening near ground used to clear silt accumulation from reservoir

how are dams built? what four tools are used?

1. cofferdam: temporary barrier redirecting water into a diversion tunnel, preventing river flow through dam construction

2. abutments: sides of the valley where the dam structure rests

3. foundation: floor of valley where the dam sits

4. diversion tunnel: tunnels made to divert water before construction of the dam, keeping the river bed dry

what are the types of dams?

• concrete dams

• earth dams

• composite dams (both concrete/earth)

what factors are considered when designing a dam?

1. dam stability

2. spillway

3. dam leakage

4. reservoir slopes

what are the types of concrete dams?

• gravity

• arch

• buttress

• concrete-face (hybrid)

what are the pros of concrete dams?

• rigid and offers more resistance to compression

• resistance to water erosion

• resistance to overtopping and piping failure

• can be shaped to support imposed loads

what are the cons of concrete?

• concrete is expensive

• lots of labour required

• low tensile strength, susceptible to different settlements

how do gravity dams work? when are they best used?

use their bulk weight and a low centre of gravity to hold back water

• best used for long distances over flat terrain (non-mountainous regions)

gravity vs buttress dams question

A: gravity dams can support tall loads, and till is uncertain and is not the best choice of aggregate.

what are buttress dams? what is their limitation?

• dams that use multiple reinforced columns to support a dam with a thin structure

• because it uses less concrete, this limits their height

what are arch dams? what is unique about them? contrast with gravity dams.

• dams that utilize strength of an arc shape

  • uses less materials than gravity dams, but are more expensive due to the higher expertise needed to build them

where are arch dams built?

• narrow, deep river gorges

• can only be built where the walls of a canyon are definitely stable and will not leak

what are bad geological conditions for a dam’s foundation?

1. flat faults with sheared materials of low strength

2. folded rocks with thin, weak layers of shale, which has the potential for foundational failure

3. weak soils susceptible to liquefaction

4. highly permeable soils

what are earth dams. what is their shape like?

dams built with clay, sand, and gravel ( “earth fill”)

• smaller in height, broad base, trapezoidal shape

what are the pros of earth dams?

• can accommodate more differential settlement compared to concrete dams

• safest dam type against risk of earthquake damage

what are the cons of earth dams?

• large size of earth dams requires large volumes of suitable materials with varied grain sizes

• susceptible to wave erosion

• susceptible to catastrophic failure if overtopped/piping fail

• not long-lasting

• requires extra care with internal design

picture of earth dam components, list earth dam components

• filter

• shoulder

• core

• transition zone

• toe drain

how does water pass through an earth dam?

through the filter, shoulder, then toe drain

how do earth dams impose lower stresses on ground compared to concrete dams?

broad base

what kind of materials are needed for earth dam construction?

large amounts of clay, sand, and gravel

how do glacial deposits provide earth dam construction material (clay, sand, gravel)?

clay: deposited in still water in front of retreating glaciers or in glacier lakes

sand/gravel: eskers and outwash plans are easy/accessible sources for sand/gravel

how does overtopping work

overtopping causes erosive action of water

• continuous water flow erodes through dam

what is the risk of pipe failure in dams?

• if seepage is uncontrolled, then it can erode soil from foundation or dam, resulting in rapid failure of dam

what geomechanical and hydrogeological factors are there to consider when designing/maintaining an earth dam?

1. overtopping

2. piping - seepage due to water percolating through the dam/dam foundation, seepage must be controlled

3. seepage control

what are the causes of piping failures?

• poor construction methods

• using the wrong type of construction materials

during construction of an earth dam, great effort is taken to compact the clay core. Why?

E - both B (increase strength of the clay) and C (decrease permeability of the clay)