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Reservoir Dam Safety David Alexander University College London
The reservoir dam represents aparticular source of high vulnerabilitya point of conjunction between naturaland 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 concreteWorldwide:• 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 • irrigation and agricultural uses • recreation (tourism and sport).
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 2000catastrophic failureshave 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• conduct geological and geotechnical site survey• design dam to resist elastic and permanent displacements.
Case historiesSt Francis dam, California, 1928• 46.5 million m3 of water released at 11,00014,000 m3/secBaldwin Hills Reservoir, Calif., 1963• 5 dead and $15 million of damage• faulting and subsidence: leakage --> cracks --> collapse of earth dam.
Case historiesTeton Dam, Idaho (1976)• 302.8 million m3 of water released• 14 dead, $4001,000 million of damageStava Valley, Italy (1985)• collapse of two earth dams• 264 killed in mudflow 5 km long
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 damsSan Fernando Valley earthquake,California (1971)• M=6.1 earthquake nearly caused• failure of the Van Norman (earth) Dam• 80,000 people were at risk
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 Volume Year of Year of Magni- of dam of water const- biggest tude (m) (mn m3) ruction quakeKoyna, 103 2,708 1964 1967 6.5IndiaKremasta 165 4,750 1965 1966 6.3GreeceXingfengxang, 105 10,500 1959 1962 6.1ChinaOroville, 236 4,295 1968 1975 5.8CaliforniaKariba, 128 160,368 1959 1963 5.8ZimbabweHoover, 221 36,703 1936 1939 5.0Arizona
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.
Failure of sediment check-damsin a Mediterranean environment
Collapse of a damin 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.
Planning measures:• control of the techniques of design and construction• analysis of risks• monitoring of risks• evacuation plans for the downstream valley area. [X]