5 - Water Reservoirs

Question 1

As England is experiencing more heat waves, drouight and water security problems, what is there a greater need for?

Answer 1

Water reservoirs

Question 2

What is a drought defined as?

Answer 2

An extended period of weather when less than a third of the usual precipitation falls.

Question 3

In 2019, the UK passed the ‘net zero law’ what is this?

Answer 3

Green house emission must be net 0 by 2050.

Question 4

What is the main prupose of water resevoirs?

Answer 4

Irrigation (supplying water)

Question 5

Other than irrigation, what are some of the other pruposes of water reservoirs?

Answer 5

Hydropower, water supply, flood control, tourism, fish farming.

Question 6

What are some of disadvanatages of water reservoirs?

Answer 6

Flooding, resettlements, changes in river sediment balance, water stratification, hydrogen suphide, inteference with fish, small arthquakes during intial filling.

Question 7

How is hydrogen sulphide a disadvanatage of reserviors (how does it arise)?

Answer 7

Due to the removal of wood

Question 8

What does resettlements mean?

Answer 8

People having to move habitat

Question 9

What are some of the risks and challenges associated with water reservoirs?

Answer 9

Sedimentation, impluse waves, cavitation, dam break waves, aging.

Question 10

How often does a safety check of a resevoir need to occur?

Answer 10

Every 10 years.

Question 11

What is a gravity dam?

Answer 11

A concrete dam, with a sloped downstream face to resist the overturning force.

Question 12

How are gravity dams stable?

Answer 12

Due to their own weight

Question 13

What are gravity dams suitable for?

Answer 13

Narrow and wide valleys

Question 14

Why does a gravity dam’s foundation need to be impervious?

Answer 14

Otherwise there may be issues with uplift pressures with a high bearing stress.

Question 15

What is an embankment dam made of?

Answer 15

Compacted earth of rock

Question 16

How are embankement dams stable?

Answer 16

Due to their own weight

Question 17

What are embankment dams sutibale for?

Answer 17

Mostly wide valleys

Question 18

Why is the core of an embnakment dam impervious?

Answer 18

To avoid seepage erosion

Question 19

Why must an embankment dam not be overtopped beyond its splillway capacity?

Answer 19

To avoid surface erosion and dam failure

Question 20

What is an arch dam made of?

Answer 20

Concrete with an upstream face vertical or sloped

Question 21

How are arch dams stable?

Answer 21

Due to their opwn weight and the forces trasnferred to the abutments

Question 22

Do arch dams require less concrete than gravity dams?

Answer 22

Yes

Question 23

What are arch dams suitable for?

Answer 23

Narrow canyons with steep side walls composed of sound rock

Question 24

How is a buttress dam stable?

Answer 24

Due to the triangular shaped walls ( buttress)

Question 25

Do buttress dams use less materials than gravity dams?

Answer 25

Yes

Question 26

How are Buttress dams vulnerable?

Answer 26

Piers may be targeted during confilicts to cuase flooding.

Question 27

Why is a 'bottom outlet' required in a reservoir?

Answer 27

To lower the water levels, flush sediments.

Question 28

Why does the hieght of the bottom outlet need to be carefully selected?

Answer 28

To avoid blocking by sediment.

Question 29

What is an overflow structure?

Answer 29

A safety structure against overflow and dam overtopping, regulated with gates.

Question 30

How is the energy dissipated in overflow structures?

Answer 30

Through hydraulic jumps.

Question 31

What is the prupose of intake structures?

Answer 31

To withdraw water for power generation, irrigate or drinking water supply

Question 32

Why is the power house (turbines) often located several hundred meters below the reservoir?

Answer 32

So that the pipes / tunnels can be several kilometers long.

Question 33

What are scour holes created by?

Answer 33

Large energy heads

Question 34

As overflow structures are not typically used during flood events, what does this mean?

Answer 34

The water is not used for its main purpose ( irrigation + hydropower)

Question 35

What is the main option for dissipating the energy with a large energy head?

Answer 35

The use of a spillway chute

Question 36

What does a spillway chute do?

Answer 36

Conveys the flow from the upstram to the downstream portion

Question 37

What are the 2 options for the design of a spillway chute?

Answer 37

Make it smooth or stepped.

Question 38

What is the advantage of using a stepped spillway?

Answer 38

The stilling basin can be smaller (cheaper)

Question 39

Where is the remaining energy that is not dissipated on the spillway dissipated?

Answer 39

In the stilling nasin.

Question 40

What is the hydraulic jump equation use to compute?

Answer 40

The downstream water depth, H2

Question 41

If the acutal h2 is smaller than the computed one, what does this mean?

Answer 41

The hdyraulic jump moves downstream, therefore the stilling basin needs to be larger to prevent scour

Question 42

What additional energy dissipation mechanisms are often used?

Answer 42

Modify bottom gemetry, plan geometry, roughness and discharge addition

Question 43

What does the Froude Number equal?

Answer 43

Fr = v / root g*h

Question 44

In a tunnel/pipe system, what is the purpose of an entry trash rack?

Answer 44

To avoid the entrainment of stones and other solids damaging the turbines.

Question 45

What is the purpose of gates in a tunnel/pipe system?

Answer 45

To regulate the entry flow into the system

Question 46

What is the purpose of a tunnel in a tunnel/pipe system?

Answer 46

To accomodate high water pressures.

Question 47

What is the purpose of a surge shaft/tank in a tunnel/pipe system?

Answer 47

When the gates are closed, a pressure is released through the surge tank.

Question 48

What is the prupose of a pelton turbine in a tunnel/pipe system?

Answer 48

To apply hydropower when there is very high pressure heads.

Question 49

What is Bernoulli's equation between 2 cross sections 1 and 2?

Answer 49

p1 / pg + v1^2 / 2g + z1 - change in H = p2/pg + v2^2 / pg + z2

Question 50

What is the change in H in Bernoulli equation?

Answer 50

The ttoal head loss of continuous and local losses.

Question 51

Due to the conservation of mass, what does the flow rate in tunnel 1 equal?

Answer 51

The flow rate in tunnel 2: V1A1 = V2A2

Question 52

What is the equation of continuous losses?

Answer 52

Friction factor * (L/D) *u^2 ./2g

Question 53

What is reynolds number when calculating the continuous losses?

Answer 53

Re = vD / viscosity

Question 54

What is kl?

Answer 54

The local loss coefficient

Question 55

What is the equation of the local losses of a system?

Answer 55

hl = kl * u^2 / 2g

Question 56

When is an axial flow/ Kaplan turbine used?

Answer 56

For low heads 2-70m

Question 57

In an axial flow/Kaplan turbine, why is the propeller blade angle adjustable?

Answer 57

To optimise the performance for different dischagres and power outputs.

Question 58

When is a Francis turbine used?

Answer 58

For medium heads, between 10 and 300

Question 59

How does a Francis turbine work?

Answer 59

Water enters the outer part of the casing and flows inward through the array of adjustable guide vanes to the runner, which drives the generator.

Question 60

When is a pelton turbine/wheel used?

Answer 60

For large heads, 80 -1600m.

Question 61

What is the difference between Pelton turbines and reaction/Francis turbines?

Answer 61

The chasing is not full of water. The water is deflected by the buckets, and the change in momentum applies a force to the buckets to drive the shaft.

Question 62

Waters hammers or pressure surges can be significant and have to be taken into account for the design of pipe systems. Why is this?

Answer 62

To avoid pipe damage, leaks or bursts

Question 63

What happens to the pressure as the time of valve closure decreases?

Answer 63

Pressure increases.

Question 64

When a valve/gate is closed instantaneuously, what is the fluid upstream of the valve still flowing at?

Answer 64

It orgininal velocity

Question 65

When a valve/gate is closed, what is generated?

Answer 65

A shock wave

Question 66

Where does a shock wave travel?

Answer 66

Back along the pipe and the fluid between the valve/gate and wave in now compressed. The pressure is raised to p + change in pressure.

Question 67

Once the shockwave has travelled through the pipe and to the reservoir, what is the pipe therefore full of?

Answer 67

Compressed and pressurised fluid.

Question 68

Due to a shockwave making the whole pipe compressed and pressurised, what then happens?

Answer 68

The reservoir head is unchanged resulting in an in-equilibrium and the fluid begins to discharge in reverse into the reservoir. The compression wave reaches the valve (dead end) resulting in a negative pressure at the valve, producing a negtive shock wave traveling back towards the reservoir This is all hydraulic losses.

Question 69

What is Ew?

Answer 69

The bulk modulus of elasticity of water, a measure of its compressibility

Question 70

Under what assumptions is the Joukovsky equation valid?

Answer 70

Straight, uniform pipe Uniform pipe friction, minimal frictin pressure drop in piping 1D fluid flow, linear elastic piping

Question 71

What are the limitations for the Joukovsky equation?

Answer 71

If the assumptions are violated, the maximum pressure prediction may be too conservative

Question 72

How does a surge tank help reduce pressure?

Answer 72

Relieves valves upstream in proximity to the valve

Question 73

Is it expensive to design pipe systems to resist water hammer?

Answer 73

Yes

Question 74

What is the maximum surge, Smax used for?

Answer 74

To desing the tank height and cross sectional area such that no-overtopping and air entrainment into the pipe system is occured

Question 75

What are the 3 things that can happen to power?

Answer 75

It is either consumed, stored or lost

Question 76

What is PSH?

Answer 76

An operational flexible renewable energy source.

Question 77

Why are solar and wind not opeerational flexible?

Answer 77

They are generated at particular times, rather than being generated straight away.

Question 78

What time range is PSH good for?

Answer 78

Long term storage, hours to weeks.

Question 79

How is water stored when pumped from a lower to elevated water reservoir?

Answer 79

The form of gravitational potential energy

Question 80

What are the benefits of pumped hydropower storage?

Answer 80

Supports net 0, low cost of energy storage, flexible and good life span.

Question 81

What are the 4 main components of pumped-storage hydroelectricity?

Answer 81

2 Water reservoirs, water piping, powerhouse and a grid connection

Question 82

What are the benefits of integrating PSH into an existing hydropower system down a river?

Answer 82

Large water volumes

Question 83

What are the negatives of integrating PSH into an existing hydropower system down a river?

Answer 83

Expensive, limited number of site, environmental impacts.

Question 84

What are the benefits of intergrating PSH schemes into off river-systems?

Answer 84

Cheaper than on river, faster construction, re-use of old mining sites as reservoirs, same water is cycled repeatedly.

Question 85

What are the negatives of integrating PSH schemes into off-river systems?

Answer 85

Less water than on river systems

Question 86

Name four design options to decrease the stilling basin length downstream of a spillway at a dam without changing the approach flow water depth and flow velocity.

Answer 86

* Change the bottom geometry by adding a positive or negative step. * Change the plan geometry by increasing the stilling basin width. * Change the boundary roughness by adding blocks. * Redirect part of the discharge in the vertical or opposite the flow direction.