Dam Sites

The type and size of dam constructed depends on the need for and the amount of water available, the topography and geology of the site, and the construction materials that are readily obtainable. Dams can be divided into two major categories according to the type of material with which they are constructed, namely, concrete dams and earth dams. The former category can be subdivided into gravity, arch and buttress dams, whereas rolled fill and rockfill embankments comprise the other. As far as dam construction is concerned, safety must be the primary concern, this coming before cost. Safety requires that the foundations and abutments be adequate for the type of dam selected. 

Hoover Dam, Colorado, completed In the 1930s but still one of the largest and most Impressive arch dams In the world. 508 

Buttress dams provide an alternative to other concrete dams in locations where the foundation rocks are competent. A buttress dam consists principally of a slab of reinforced concrete that slopes upstream and is supported by a number of buttresses whose axes are normal to the slab (Fig. 9.20). The buttresses support the slab and transmit the water load to the foundation. They are rather narrow and act as heavily loaded walls, thus exerting substantial unit pressures on the foundations. 

Earth dams are embankments of earth with an impermeable core to control seepage (Fig. 9.21). This usually consists of clayey material. If sufficient quantities are not available, then concrete or asphaltic concrete membranes are used. The core normally is extended as a cut-off or grout curtain below ground level when seepage beneath the dam has to be controlled. 

Marddap Dam, near Mariental, Namibia, and exampie of an embankment dam. 

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Approximately 10% of U.S. electrical energy is produced by hydroelectric dams.6 Although there are few economic and environmentally acceptable dam sites remaining, in some places it is possible to use wind power, or perhaps even the ocean tides, to generate electricity. Here the opportunity for chemists and chemical engineers is the invention and production of modern materials that can make such approaches possible. 

A 5-acre field, belonging to the Arkansas Southeastern Branch Experiment Station at Kelso, Ark., was chosen as the experimental site. The relationship of the site to its environs is shown in Figure 2. Irrigation water was introduced from a head ditch on the north side of the plot and flowed through the %-mile furrows to a drainage ditch, which was sampled as it left the field. The ditch ran % mile until the water was held for reuse by a check dam. It was estimated that the water was diluted 20-fold at the dam site. The dam held the drainage of 800 acres of cultivated land. 

Fig. 12-26. Mercury and Zn contents of soils, fluvioglacial cover and bedrock at the Bastyan dam-site road cutting (from Carr ct al., 1986).

The dam is on the Columbia River, which at the dam site follows the steep, western wall of a canyon. Between the dam and the eastern canyon wall is an 0.5-mile wide terrace made up of pervious strata (ranging from silts to gravels) deposited by the river in its earlier courses. These ancient riverbeds, some of them directly exposed on the present riverbanks, could readily drain the reservoir. A grout curtain, 1800 ft long, was designed and constructed to prevent such drainage. 

The Hoover Dam site on the Colorado River between Arizona and Nevada and the Aswan Dam site on the Nile River in Egypt are examples of good locations for the harvesting of hydropower. Today, most of the good sites for hydropower are already developed. In most places, there is little hope for further significant expansion of this energy source. The only large hydropower facility currently under... 

Precedence for rock mass consolidation in relation to tunnel drainage has been reported by Lombardi (1992) in relation to the driving of an investigation adit through a confined, fractured, marly-limestone aquifer near the Zeuzier dam in Switzerland. Recorded settlements at the dam site of -13 cm were measured even though the adit was... 

According to the statistic data from the weather station at the Three-Gorges Dam site, the mean... 

Bock, C.A., Moneymaker, B.C. (1934). Stratigraphy and structural geology of the Hiwassee River Basin in vicinity of Coleman dam site. TVA Knoxville TN. 

Reservoir volume can be estimated, firstly, by planimetering areas upstream of the dam site for successive contours up to proposed top water level. Secondly, the area between two successive contours is multiplied by the contour interval to give the interval volume, the summation of the interval volumes providing the total volume of the reservoir site. 

Leakage from a reservoir takes the form of sudden increases in stream flow downstream of the dam site with boils in the river and the appearance of springs on the valley sides. It may be associated with major defects in the geological structure, such as solution channels, fault zones or buried channels through which large and essentially localized flows take place. Seepage is a more discreet flow, spread out over a larger area but may be no less in total amount. 

Fresh metamorphosed rocks such as quartzite and hornfels are very strong and afford excellent dam sites. Marble has the same advantages and disadvantages as other carbonate rocks. Generally, gneiss has proved a good foundation rock for dams. 

Well cemented shales, under structurally sound conditions, present few problems at dam sites, though their strength limitations and elastic properties may be factors of importance in the design of concrete dams of appreciable height. They, however, have lower moduli of elasticity and lower shear strength values than concrete and, therefore, are unsatisfactory foundation materials for arch dams. Moreover, if the lamination is horizontal and well developed, then the foundations may offer little shear resistance to the horizontal forces exerted by a dam. A structure keying the dam into such a foundation is then required. 

Talus or scree may clothe the lower slopes in mountainous areas and, because of its high permeability, must be avoided in the location of a dam site, unless It is sufficiently shallow to be economically removed from under the footprint of the dam. 

Wherever possibie, construction materiais for an earth dam should be obtained from within the future reservoir basin. Accordingiy, the investigation for the dam site and the surrounding area shouid determine the avaiiabiiity of impervious and pervious materials for the embankment, sand and gravel for drains and filter blankets, and stone for riprap. 

When the downstream dam site is considered, the retained reservoir level will be 2475 a.s.l. The specified phreatic surface in this model was selected to be the same level of water damming. The colluvium and heavily weathered bedrock were considered to be permeable while the bedrock was assumed to be impermeable. The shear strength parameters of the saturated deposit are selected as 0.7 times of the dry condition referring to the research of Liu (2009). The hydrostatic pressure on the slope was also considered. Figure 9 shows the pore pressure of the colluvium and the phreatic line can be obtained from the result. Figure 10 illustrates the displacement contour and vectors after 1000 iteration steps. Figure 11 shows the cross sectional map of the displacement in Y direction after partial saturation. It can be concluded that, unlike the seismic condition, the... 

The project is located within the Central Himalaya (Fig. 2). The proposed dam site and portion of the head race tunnel is represented by quartzo feldspathic gneiss, biotite gneiss with bands of mica schist, quartzite with minor metabasics and interbedded limestone of Thimpu (Sure) Formation, whereas towards the proposed powerhouse site, rocks of Chekha Formation comprising mainly schist and quartzite intruded by leucogran-ite are exposed (Bhargava 1995). 

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