Contents preface (VII) introduction 1—37

14.7. CHOICE OF DAMS
Most of the dams can be grouped into one of the following two categories: (i) Embankment dams, and

(ii) Concrete dams.

Embankment dams include earth-fill dams and rock-fill dams. Concrete dams include gravity dams, arch dams and buttress dams. Preliminary designs and estimates will usually be required for different types of dams before one can decide the suitability or otherwise of one type of dam in comparison to other types. The cost of construction is the most important factor to be considered while making the final selection of the type of dam. Besides, the characteristics of each type of dam, as related to the physical features of the site and its adaptation to the purposes of the dam, as well as safety, and other relevant limitations are also to be considered for selecting the best type of dam for a particular site. The following are the important factors which affect the choice of the type of dam:
(i) Topography,
(ii) Geology and foundation conditions, (iii) Material available, and
(iv) Size and location of spillway.

PLANNING OF WATER RESOURCE PROJECTS

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Topography of the site dictates the first choice of the type of dam. A concrete dam would be the obvious choice for narrow stream flowing between high and rocky abutments (i.e., deep gorges). Broad valleys in plains would suggest an embankment dam with a separate spillway.

Geological and hydrogeological characteristics of the strata which are to carry the weight of the dam determine the foundation conditions. Any type of dam can be constructed on solid rock foundations. Well-compacted gravel foundations are suitable for concrete gravity dams of small height, earth-fill, and rock-fill dams. However, effective cutoffs are required to check the foundation seepage. Silt or fine sand foundations can support concrete dams of small height and earth-fill dams. Problems of settlement, piping, and the foundation seepage are associated with this type of foundation. Non-uniform foundations containing different types of strata will usually require special treatment before any type of dam is constructed on such foundations.
If the construction materials to be used in large quantity for the construction of the dam are available in sufficient quantity within a reasonable distance from the site, the cost of the dam will be considerably reduced due to saving on transportation. If suitable soils for the construction of an earth-fill dam are locally available in nearby borrowpits, choice of an earth-fill dam would be the most economical. The availability of sand and gravel (for concrete) near the dam site would reduce the cost of a concrete dam.
Spillway is a major part of any dam and its size, type, and the natural restrictions in its location will affect the selection of the type of dam. Spillway requirements are decided by the runoff and streamflow characteristics. As such, spillway on dams across streams of large flood potential can become the dominant part of the dam and put the selection of the type of dam to a secondary position. For large spillways, it may be desirable to combine the spillway and dam into one structure. This is possible only in concrete dams. Embankment dams are based on more conservative design assumptions and, hence, spillway is generally not constructed as part of the embankment. On the other hand, excavated material from a separate spillway can be advantageously used for the construction of an embankment dam.