DISTRIBUTION SYSTEM ELEMENTS AND ACCESSORIES

known as the water table. The depth of the water table-or the distance from the ground surface to the water level—varies considerably with the amount of rainfall. During droughts, the water table may be lowered, but during a rainy season it will probably rise. As you should understand from your studies of soil formation, the stratum over which groundwater accumulates is an irregular, rather than a continuously flat, plane. Therefore, unless the water is confined, it flows horizontally over the irregular stratum and is nearer the surface in some places than it is in other places. Where this underground water flows near the surface and the ground area is low, the water may flow out as a spring. Or, it may seep out and create a swampy area. The underground, flowing water also may become entrapped between impervious layers. In this case, enough water pressure may buildup to create an artesian well if the strata is penetrated by drilling or by a natural opening. In some regions of the world, there is not enough surface water or groundwater available to support the need for water. In these areas alternative sources are necessary. Rain, itself, can be an alternative source. In some locations, large catchment areas are constructed to collect rain and store it for future use. These catchment areas are usually constructed on the side of a mountain or a hill facing the prevailing direction of rainfall. In other areas, snow and ice may be used as alternative sources. Another source, although costly to develop for use, is seawater that has had the salt removed by desalination. Selection and Development of Water Sources When selecting a water source for development, the engineer must consider three primary factors: water quantity, water reliability, and water quality. The quantity factor considers the amount of water that is available at the source and the amount of water that will be required or demanded for use. The amount of water that maybe available at the source depends on variables, such as the amount of precipitation, the size of the drained area, geology, ground surface, evaporation, temperature, topography, and artificial controls. Water demands are estimated using per capita requirements and other controlling factors, such as water requirements for fire protection, industrial use, lawn sprinkling, construction, vehicles, and water delivered to other activities. The reliability of a water supply is one of the most important factors that the engineer considers when selecting a water source. A reliable water source is one that will supply the required amount of water for as long as needed. To determine the reliability of the water source, the engineer studies data, such as hydrological data, to determine the variations that maybe expected at the water source. Geological data should be studied since geological formations can limit the quantity and flow of water available. Also, legal advice may be necessary when selecting a water source since the laws regulating and controlling water rights may vary considerably from state to state and country to country. The third primary factor the engineer must consider when selecting a water source is the quality of the water. Practically all water supplies have been exposed to pollution of some kind. Therefore, to ensure that water is potable and palatable, it must be tested to determine the existence of any impurities that could cause disease, odor, foul taste, or bad color. In most cases, the water will require treatment for the removal of these impurities. In water treatment, the water is subjected to various filtration and sedimentation processes, and in nearly all cases is disinfected using chlorine or other disinfecting chemicals. Once the water source has been selected, develop- ment of the source can begin. Developing a water source includes all work that increases the quantity and improves the quality of the water or makes it more readily available for treatment and distribution. In developing a source, the engineer may use the construction of dams, digging or drilling of wells, and other improvements to increase the quantity and quality of the water. For a more detailed discussion of water source selection, development, and treatment, you should refer to chapter 9 of the UT1 TRAMAN. For NAVFAC guidance, you should refer to Water Supply System, MIL-HDBK-1005/7. Now that you are familiar with water sources, let us move onto water distribution. DISTRIBUTION SYSTEM ELEMENTS AND ACCESSORIES The elements of a water distribution system include distribution mains, arterial mains, storage reservoirs, and system accessories. These elements and accessories are described as follows: 1. DISTRIBUTION MAINS. Distribution mains are the pipelines that make up the distribution system. Their function is to carry water from the water source or treatment works to users. 2. ARTERIAL MAINS. Arterial mains are distribution mains of large size. They are interconnected with smaller distribution mains to form a complete gridiron system. 2-16