05/03/2023 1

•RESERVOIRS

RESERVOIR • Why store?

– Raise head, smooth flow, conflicts and trade offs• Determination of reservoir volume/height

– Mass curve approach, simulation approach• Dam design

– Forces on a dam, Types of dam, Seepage, Spillways• Technical problems with dams

– Silting, failure• Social impact of dams

Reservoir

On the web (not necessarily on dams.org)– Find an example of a “bad” dam project– Find an example of a “good” dam project

We will be discussing– Who benefits and who loses when a dam is built?– Who makes the decisions?– What makes a dam good or bad?

Reservoir: Why store ?• Raise head

– Hydro power– Allow greater flow to irrigation

• Smooth flow– Reliable Hydro power– Off season irrigation– Flood control– Domestic Water supply

• Other reasons– Fishing– Leisure

Reservoirs; Why store: Conflicts• Maximising head vs. Maximising storage

– If the storage is used, the head is reduced

• Water use vs. flood control

–Water use prefers a full reservoir (for use later)–Flood control the reservoir should be empty (so floodwater

can fill it)

THE ANATOMY OF A RESERVOIR

RESERVOIRS: HOW MUCH TO STORE

3 2( ) ( )h f V f A

RESERVOIRS: HOW MUCH TO STORE

n in outQ Q Q

in stream precipitationQ Q Q

......out used spill evaporation leakage seapageQ Q Q Q Q Q

precipitationQ AP

Reservoirs: how much to store: mass curve

RESERVOIRS: DAM DESIGN: FORCES ON A DAM

2

2hF

2hF W t

W

is the specific weight of water

Reservoirs: Dam design: Types of DamType Materials Typical cross section Plan view

Gravity Concrete, rubble masonry

Arch Concrete

Buttress Concrete, ferrocement, timber, steel

Embankment Earth, rock

Reservoirs Dam design: Seepage: Flow nets

Lines of constant potential, and constant flow– Perpendicular– Should form a series of squares

reservoirs: seepage: typical flow nets

Reservoirs: Typical flow nets: Earth dams

reservoirs: zoned earth dams: piping

Toe drain

Blanket drain

Chimney and blanket drain

Impermeable core and blanket

Reservoirs: Seepage: Earth dams

Reservoirs: Seepage: Zoned earth dams

Seepage: Zoned earth dams

Ramganga project

Reservoirs: Zoned earth dams

Kishan rockfill dam

Reservoirs: Spillways: TypesSide channel

Chute

Shaft

Syphon

Ogee

Reservoirs: Spillways: Side channel

Reservoirs: Spillways: Chute spillways

Reservoirs: Spillways: Shaft spillways

Reservoirs: Spillways: Ogee spillways

Reservoirs: Spillways: Ogee spillways

Reservoirs: Spillways: Hydraulic jump

Reservoirs: Spillways: Hydraulic jump

V = Stream velocity (m s-1)R = Hydraulic radiusS = Slopen = Manning

roughness

Spillways: Hydraulic jump Manning's equation

2 3 1 2R SVn

p = pressure (Pa)r = Density (kg m-3)g = Gravity (m s-

2)v = velocity (m s-

1)z = elevation (m)

Spillways: Hydraulic jump Bernoulli's equation

2

constant2

p v zg gr

Spillways: Hydraulic jump: Sums: Critical depth

E

h

2

2vE hg

crith

minE

Tranquil

Rapid

E = Specific Energy (m)

h = head (m)g = Gravity (m s-

2)v = velocity (m s-

1)

Spillways: Hydraulic jump: Sums: Head loss

22

2 2L dvh H h hg

Hd

h1

h2

22

2vg

21

2vg

h

hL

Hd

h1

h2

22

2vg

21

2vg

h

hL

Spillways: Hydraulic jump: Height of jump

1 22 21 1 1 1

22

2 4h h v hh

g

Spillways: Hydraulic jump: Location of the hydraulic jump

Spillways: Encouraging a jump

Technical problems with dams: Silting

Technical problems with dams: Silting

Technical problems with dams: Failure modes

Overtopping – inadequate spillways (1/3) Foundation failure (1/3)

– Excess seepage through foundation– Piping – insufficient drainage/grading

Wave action Erosion of downstream face Slump Bank stability Seismic activity Degradation – lack of maintenance Material supplies sub specification

Reservoirs: Technical problems with dams: Failure:

                                             

   

Technical problems with dams: Failure:

                                                                               

   

TECHNICAL PROBLEMS WITH DAMS: FAILURE: PIPING DAM

Dec 22 3:00 pm Dec 23 7:00 am

Dec 23 9:30 am Dec 23 3:30 pm

TECHNICAL PROBLEMS WITH DAMS: FAILURE: DAM SLIDE

TECHNICAL PROBLEMS WITH DAMS: FAILURE: DAM EARTHQUAKE DAMAGE

Technical problems with dams: Failure: dam bank slip

Reservoirs: Social impact of dams Climate change

Fertility of downstream banks

Displacement

Changes in local economy

Deforestation Possibility of financial collapse

Possibility of failure

Reservoirs: Summary• Storage needs may conflict

• Mass balance and the mass curve are useful ways to size reservoirs

• Seepage is an issue with earth dams but can be dealt with by zoning

• There are several varieties of spillway, including side channel, chute, shaft, syphon, and ogee

• Hydraulic jump is a useful technique to reduce spillwater levels

• Dams can fail by silting, overtopping, foundation failure wave action, erosion and lack of maintenance

• Dams can have social and environmental impacts