Sediment Control Measures in Reservoirs


seminar report

  Seminar Report On
“SEDIMENT CONTROL MEASURE IN RESERVOIR”

By

PRATIK SOLANKI

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Sediment Control Measures in Reservoirs

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25 Tallest Dams In The World


Throughout history man has been building dams for various reasons, whether it was to prevent floods, generate electricity, or create a water supply. Starting thousands of years ago in the middle east as small walls, today dams are immensely huge power generation facilities that fulfill a number of tasks and take years to build. So, whether you recognize the impact these architectural wonders have had on your life or not, these are the 25 tallest dams in the world. (courtesy: http://list25.com)

25  Toktogul Dam

A hydroelectric and irrigation dam on the Naryn River in the Jalal-Abad Province of Kyrgyzstan, this dam is the shortest on our list at 215 meters high.

Toktogul Dam

24  Longtan Dam

Longtan DamLongtan Dam is a large roller-compacted concrete gravity dam on the Hongshui River in China. It stands at 216.5 meters.

23  Glen Canyon Dam

Glen Canyon DamNamed for Glen Canyon, a colorful series of gorges most of which now lies under the reservoir, the dam created the second largest artificial lake in the United States.

22 Dworshak Dam

Dworshak DamAt 219 meters, Dworshak is the third tallest dam in the United States and the tallest straight-axis concrete dam in the Western Hemisphere.

21 Contra Dam, Switzerland

Contra DamCommonly known as the Verzasca Dam and the Locarno Dam, the Contra is an arch dam on the Verzasca River in Switzerland.

20  Hoover Dam, Arizona

Hoover DamOnce known as Boulder Dam, the Hoover Dam is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the US states of Arizona and Nevada.

19  Luzzone Dam, Switzerland

Luzzone DamFeaturing the world’s highest artificial climbing wall on one of its sides, this dam stands at 225 meters.

18  Bhakra Dam, India 

Bhakra DamThis is a concrete gravity dam across the Sutlej River near the border between Punjab and Himachal Pradesh in northern India.

17  Karun-4 Dam, Iran

Karun-4 DamAn arch dam on the Karun River in Iran its primary objectives are electric power supply and flood control.

16  Shuibuya Dam, China

Shuibuya DamBuilt not just for power but also to promote flood control, navigation, tourism and fishery, the Shuibuya Dam is 233 meters tall.

15  Chirkey Dam, Russia

Chirkey DamThis arch dam on the Sulak River is the tallest arch dam in Russia at 232.5 meters.

14  El Cajón Dam, Honduras

El Cajón DamOfficially known as Central Hidroeléctrica Francisco Morazán, this dam is a hydroelectric power plant located in Western Honduras.

13  Oroville Dam, California

Oroville DamAt 230 meters high this earthfill embankment dam on the Feather River in California is the tallest in the United States.

12  Ertan Dam, China

Ertan DamThis 240 meter high arch dam can be found on the Yalong River, a tributary of the Yangtze River in Sichuan Province, southwest China

11  Sayano–Shushenskaya Dam, Russia

Sayano–Shushenskaya DamLocated on the Yenisei River, near Sayanogorsk in Russia, this dam is the largest power plant in the country and the sixth-largest hydroelectric plant in the world.

10  Mica Dam, Canada

Mica DamThis 243 meter high dam spans the Colombia River 135 kilometres north of Revelstoke, Canada.

9  Deriner Dam, Turkey

Deriner DamNamed after İbrahim Deriner, who died while serving as the Chief Engineer of its research team, the dam is located on the Çoruh River 3 miles east of Artvin, Turkey.

8  Laxiwa Dam, China

Laxiwa DamThe Laxiwa Dam is a 250 meter high arch dam on the Yellow River in Qinghai Province, northwest China.

7  Mauvoisin Dam, Switzerland

Mauvoisin DamWith Mont Blanc de Cheilon in the background the Mauvoisin Dam creates the Lac de Mauvoisin in the Swiss alps.

6  Tehri Dam, India

Tehri DamThis multi-purpose rock and earth-fill embankment dam on the Bhagirathi River near Tehri in Uttarakhand, India is 261 meters high.

5  Vajont Dam, Italy

Vajont DamA disused dam north of Venice, Italy, in 1963 a landslide caused the overtopping of the dam and around 2,000 deaths.

4  Inguri Dam, Georgia

Inguri DamThis hydroelectric dam on the Inguri River in Georgia is the second highest concrete arch dam in the world.

3  Grande Dixence Dam, Switzerland

Grande Dixence DamA concrete gravity dam on the Dixence River in Switzerland, at 285 meters it is the tallest of its kind in the world.

2  Xiaowan Dam, China

Xiaowan DamAn arch dam on the Lancang (Mekong) River in China, its primary purpose is to provide hydroelectric power.

1  Nurek Dam, Tajikistan

Nurek DamThis earth fill embankment dam on the Vakhsh River in Tajikistan is currently the tallest dam in the world at 300 meters.

Phreatic Line and Horizontal Drain In Earth fill Dams


Earth dams are generally built of locally available materials in their natural state with a minimum of processing. Homogeneous earth dams are built whenever only one type of material is economically available.

The material must be sufficiently impervious to provide an adequate water barrier and slopes must be relatively flat to make it safe against piping and sloughing.

The general design procedure is to make a first estimate on the basis of experience with similar dams and then to modify the estimate as required after conducting a stability analysis except where there is a surplus of material.

The United States Department of the Interior Bureau of Reclamation (USBR) and other agencies suggested limits for the upstream and the downstream slopes for different types of materials and dams.

Phreatic Line and Horizontal Drain In Earth fill Dams

The upstream slopes of most of the earth dams in actual practice usually vary from 2.0 (horizontal):1 (vertical) to 4:1 and the downstream slopes are generally between 2:1 and 3:1 (USBR 2003). Free board depends on the height and action of waves. USBR (2003) recommends normal free-board about 1.5 to 3 m depending on the fetch. The width of the dam crest is determined by considering the nature of embankment materials, height and importance of structure, possible roadways requirements, and practicability of construction. A majority of dams have the crest widths varying between 5 and 12 m.

About 30% of dams had failed due to the seepage failure, viz piping and sloughing. Recent comprehensive reviews by Foster et al. (2000a,b) and Fell et al. (2003) show that internal erosion and piping are the main causes of failure and accidents affecting embankment dams; and the proportion of their failures by piping increased from 43% before 1950 to 54% after 1950. The sloughing of the downstream face of a homogeneous earth dam occurs under the steady-state seepage condition due to the softening and weakening of the soil mass when the top flow line or phreatic line intersects it. Regardless of flatness of the downstream slope and impermeability of soil, the phreatic line intersects the downstream face to a height of roughly one-third the depth of water . It is usual practice to use a modified homogeneous section in which an internal drainage system in the form of a horizontal blanket drain or a rock toe or a combination of the two is provided. The drainage system keeps the phreatic line well within the body of the dam. Horizontal filtered drainage blankets are widely used for dams of moderate height.USBR constructed the 50 m high Vega dam, which is one of the highest with a homogenous section and a horizontal downstream drain.

The minimum length of the horizontal blanket drain required to keep the phreatic line within the body of the dam by a specified depth and also equations for maximum downstream slope cover and minimum and maximum effective lengths of the downstream filtered drainage system.

The position of the phreatic line influences the stability of the earth dam because of potential piping due to excessive exit gradient and sloughing due to the softening and weakening of the soil mass as if it touches the downstream slope or intersects it. When the dam embankment is homogeneous or when the downstream zone is of questionable permeability, a horizontal drainage blanket is provided to keep the phreatic line well within the dam body, to allow adequate embankment and foundation drainage, and to eliminate piping from the foundation and the embankment.

As the dams are made of fine-grained soil, saturation may occur due to the capillary rise above the phreatic surface so it is necessary to account for capillary rise while calculating the minimum length of the downstream filtered drainage. Though the suction head in the soil matrix above the phreatic surface within the dam body due to capillary rise generally improves the stability of the downstream slope, once the capillary fringe intersects the downstream slope the pressure changes from negative (suction) to atmospheric and the downstream face may become a seepage face leading to its failure. Hence the phreatic line should not intersect the downstream slope and it should be a distance greater than capillary rise below the sloping face so that the chances of the sloughing or piping may be nullified.