Brian Clark, age 11, of Jamestown, N.Y., for his question:
HOW IS A DAM BUILT
A dam is a barrier placed across a river to stop the flow of water. Many kinds of dams are built. The final selection of material used depends on the character of the damsite.
Rock filled or stone masonry dams may be most economical where rock is abundant. Timber dams are often built where lumber is plentiful.
Concrete is a common construction material for dams, but cement and gravel are not always available without heavy transportation costs.
Earth dams prove to be most economical in many locations.
In narrow canyons, thin arch dams may prove most suitable. But in wide river valleys, where the length of the dam would be very great, multiple arch dams, flat slab dams and dams built of earth, steel or timber may be less costly.
A builder must first gather and study much information. The site where the dam is to be erected must be examined for its formation, quality of foundation and the availability of suitable construction materials.
A careful analysis must be made of the stream flow characteristics. The area to be covered by the reservoir that the dam creates must be outlined when determining the height of the dam at any given site. This requires detailed topographic mapping and geological studies.
All property in the reservoir area must be bought or relocated. Sometimes this requires the relocation of entire towns, highways, railroads and utilities.
Engineers also must determine the amount of mud, silt and debris the dam will stop. This will determine the useful life of the reservoir, because when the reservoir becomes filled with this material, it can no longer hold water.
If the dam is to be used for generating power, outlets must be provided that will connect to generating equipment. If the water is to be used for irrigation, outlets must be constructed.
Means must then be found to bypass the flow of the water so the foundation can be constructed.
To divert the flow of the river from a damsite, frequently half of the riverbed is excavated at one time. The other half is used for the flow of the river.
In some cases, it is more economical to bore a tunnel through an adjacent canyon wall, permitting the entire flow of the river to pass around the damsite. To accomplish this, cofferdams, or small dams placed temporarily across a stream, are built upstream to divert the river into the tunnel. After the dam has been built high enough, the diversion tunnel is closed with gates and permanently plugged.
In designing the dam, provision must be made to bypass water when the reservoir is full, without overtopping the dam. For this purpose, a spillway is built.
In order to release water from behind the dam when the reservoir is not full, dams often are equipped with reservoir outlets. These outlets consist of specially designed valves that can be opened and closed under high water pressure.
Among the many kinds of valves used for this purpose are needle valves, gate valves, slide valves and cylinder gates.