Reservoir Dam Safety

David Alexander University College London

The reservoir dam represents a particular source of high vulnerability a point of conjunction between natural and anthropogenic disaster potentials.

Types of reservoir dam:

• earth dam and rock-fill dam (embankment dam): triangular section, built of compacted sediments and boulders (most common type)

• gravity dam: weight of concrete or dressed stone supports structure • concrete arch dam: deflects pressure of impounded water onto adjacent foundations and abutments

• buttress dam: has supports on its downstream side.

Fewer than 10% of dams are

made of concrete

Worldwide: • more than 50,000 dams are higher than 15 m • more than 40 are higher than 180 m.

Dams and reservoirs are used for:-

• water supply for domestic and industrial use

• electrical power generation

• reduction of flood risk

• acquifer recharge

• recreation (tourism and sport).

• irrigation and agricultural uses

Problems:

• pollution of stored water (with sediment, etc.)

• loss of storage capacity (siltation, leakage or breaching of dam)

• seiching in violent windstorms or earthquakes (seiche: a oscillatory motion of an enclosed body of water)

• surges and water waves caused by landslides.

Problems (contd.)

• failure of the dam leading to floods

• instability beneath the foundations

• overtopping if sluices or spillway fail

• loss of free board (caused by subsidence or uplift)

• creation of earthquakes.

Brief classification of emergencies associated with reservoir dams:

• filtration: slow loss of water, brief and without serious consequences

• rapid loss of water through piping, cracks or in highly permeable terrain

• partial or total collapse of the dam.

More than 2000 catastrophic failures have been recorded

• 10% in the 20th century

• more than 9000 dead in the 20th century.

According to a study of 308 dam disasters:

• 40% were caused by foundation failures

• 23% resulted from spillway failures

• 37% were due to errors of design or construction, site inadequacies, subsidence or earthquakes.

Disasters have resulted from:

• poor design, construction and maintenance (foundation failures, inadequate spillway capacity)

• site inadequacies

• extensive rains and swollen rivers, leading to flood discharges

• earthquakes

• landslides.

To design safe dams:

• locate dam away from active faults

• overdesign to allow for displacements

• use wide sections that resist cracking

• use self-healing materials

• design dam to resist elastic and permanent displacements.

• conduct geological and geotechnical site survey

St Francis dam, California, 1928 • 46.5 million m3 of water released at 11,00014,000 m3/sec

Case histories

Baldwin Hills Reservoir, Calif., 1963 • 5 dead and $15 million of damage • faulting and subsidence: leakage --> cracks --> collapse of earth dam.

Baldwin Hills, California, 1963

Baldwin Hills, California, 1963

Case histories

Teton Dam, Idaho (1976) • 302.8 million m3 of water released • 14 dead, $4001,000 million of damage

Stava Valley, Italy (1985) • collapse of two earth dams • 264 killed in mudflow 5 km long

Teton Dam, Idaho, 1976

Val di Stava, 1985

<--Main valley

^ Mudflow

Case histories

Vajont Dam, Italy (1963) • 1,925 dead, 3 villages flattened • 240 million m3 of rock slid at 100 km/hr into reservoir • wall of water 100 m high

Shear plane

Body of landslide

Mount Toc

Longarone reconstructed

Seismicity caused by tectonic forces can cause:-

• liquefaction: - of the foundations of the dam - of earth dams - of the reservoir sideslopes

• elastic or permanent displacements and overtopping of the dam

• direct damage to the dam and collapse.

Earthquakes and reservoir dams

San Fernando Valley earthquake, California (1971) • M=6.1 earthquake nearly caused • failure of the Van Norman (earth) Dam • 80,000 people were at risk

Van Norman Dam, California, 1971

Induced seismicity:-

• occurs where hydraulic conductivity in underlying rocks is high

• high pore water pressures cause rapid fault movements to occur

• the weight of water, if its depth exceeds 100 m

• induced seismicity does not only occur in areas of natural seismicity

• earthquake can occur some years after the dam is built.

Dam Height of dam

(m)

Volume of water

(mn m3)

Year of const-

ruction

Year of biggest quake

Magni-tude

Koyna, India

103 2,708 1964 1967 6.5

KremastaGreece

165 4,750 1965 1966 6.3

Xingfeng

xang, China

105 10,500 1959 1962 6.1

Oroville, California

236 4,295 1968 1975 5.8

Kariba, Zimbabwe

128 160,368 1959 1963 5.8

Hoover, Arizona

221 36,703 1936 1939 5.0

Earthquakes and reservoir dams

Koyna Reservoir, India (1967) • capacity 2,800 million m3 • filling of reservoir caused M=6.2 earthquake • 177 killed and 2200 injured

Natural dams

• most natural dams fail within two weeks of formation

• landslide debris or glacier ice blocks river and causes impoundment of discharge

• high pore water pressures cause failure of dam

• failure usually leads to an outburst flood.

A landslide dam that did not fail

Val Pola sturzstrom landslide-

induced natural dam

Bormio (Alps) 1987

Val Pola landslide dam stabilization works

Failure of sediment check-dams in a Mediterranean environment

Collapse of a dam in the Chi-Chi, Taiwan,

1999 earthquake

Planning measures:

• surveillance of small dams, not only large ones

• careful study of various risks (earthquake, landslide, subsidence, flood, etc.)

• careful study of the environment of the dam and the reservoir

• geotechnical testing.

[X]

• control of the techniques of design and construction

• analysis of risks

• monitoring of risks

• evacuation plans for the downstream valley area.

Planning measures: