Classification of Dams.
Dams may be classified according to their purpose as diverting dams or weirs and as storage dams. The former may be located upon any portion of a stream where the conditions are favorable, and the water used for manifold purposes, being conveyed by means of canals, flumes, tunnels and pipe-lines to places of intended use. These dams are generally low and may be either of a temporary or permanent character, depending upon the uses to which the water is put. Temporary dams are made of brush, logs, sand bags, gravel and loose rock. The more permanent structures are built of stone and concrete masonry.
Storage dams may be classified according to the kind of material entering into their structure, as follows: (1) Earth; (2) Earth and Timber; (3) Earth and Rock-fill; (4) Rock-fill; (5) Masonry; (6) Composite Structures.
Low dams forming service reservoirs for domestic water supplies and for irrigation comprise by far the most numerous class. They are not designed to impound a large volume of water and therefore may be built across a small ravine or depression, or even upon the summit of a hill, by excavating the reservoir basin and using the material excavated to form the embankment. These reservoirs may be used in connection with surface or gravity systems, artesian wells, or underground supplies obtained by pumping. The dams forming these reservoirs being of moderate size and height may vary greatly in shape and dimensions. The form may be made to suit the configuration of the dam site. When the earthwork requires it, they may be lined with various materials to secure water-tightness. Often such dams are made composite in character, partly of earth and partly of masonry or some other material. They are also frequently accompanied by numerous accessories, such as settling-basins, aerating devices and covers, which present a diversity in form and appearance. A presentation of the different types of dams thus employed, with a discussion of the questions pertaining to utility in design and economy in construction, would be exceedingly valuable and of general interest. Service reservoirs will receive only a passing notice, with the hope expressed that some competent authority will discuss them in the future.
CHAPTER II.
Preliminary Studies and
Investigations.
The preliminary studies and investigations which should be made prior to the construction of any dam for the storage of water have to do with (1) the Catchment Area, (2) the Reservoir basin, and (3) the Dam site.