喜歡就充值下載吧。。資源目錄里展示的文件全都有，，請(qǐng)放心下載，，有疑問(wèn)咨詢(xún)QQ:1064457796或者1304139763 ==================== 喜歡就充值下載吧。。資源目錄里展示的文件全都有，，請(qǐng)放心下載，，有疑問(wèn)咨詢(xún)QQ:1064457796或者1304139763 ==================== Reservoirs and water When a barrier is constructed across some river in the form of a dam, water gets stored up on the upstream side of the barrier, forming a pool of water, generally called a reservoir. Broadly speaking, any water collected in a pool or a lake may be termed as a reservoir. The water stored in reservoir may be used for various purposes. Depending upon the purposes served, the reservoir may be classified as follows: ①storage or conservation reservoir ; ② flood control reservoirs ; ③distribution reservoirs ; ④multipurpose reservoir. 1． storage or conservation reservoirs A city water supply, irrigation water supply or a hydroelectric project drawing water directly from a river or a stream may fail to satisfy the consumers (consumers’ ) demands during extremely low flows, while during high lows, it may become difficult to carry out their operation due to devastating floods. A storage or a conservation reservoir can retain such excess supplies during periods of peak flows and can release them gradually during low flows as and when the need arises Incidentally, in addition to conserving water for later use, the storage of floodwaters may also reduce flood damage below the reservoir. Hence, a reservoir can be used for controlling floods either solely or in addition to other purposes. In the former case, it is known as “flood control reservoir” or “single purpose flood control reservoir”, and in the later case, it is called a “multipurpose reservoir 2． flood control reservoir A flood control reservoir or generally called flood-mitigation reservoir, stores a portion of the flood flows in such a way as to minimize the flood peaks at the areas to be protected downstream. To accomplish this, the entire inflow entering the reservoir is discharged till the outflow reaches the safe capacity of the channel downstream. The inflow in excess of this rate is stored in the reservoir, which is then gradually released so as to recover the storage capacity for next flood. A reservoir with gates and valves installation at the spillway and at the sluice outlets is known as a storage-reservoir, while on the other hand, a reservoir with fixed ungated outlets is known as a retarding basin. Functioning and disadvantage of a retarding basin: A retarding basin is usually provided with an uncontrolled spillway and an uncontrolled orifice type sluiceway. The automatic regulation of outflow depending upon the availability of water, takes place from such a reservoir. The maximum discharging capacity of such a reservoir should be equal to maximum safe carrying capacity of the channel downstream. As flood occurs, the reservoir gets filled and discharges through sluiceways. As the reservoir elevation increases, outflow discharge increases. The water level goes on rising until the flood has subsided and the inflow becomes equal to or less than the outflow. After this, water gets automatically withdrawn from the reservoir until the stored water is completely discharged. The advantages of a retarding basin over a gate controlled detention basin are: ①cost of gate installations is saved. ② there are no gates and hence, the possibility of human error and negligence in their operation is eliminated. ③since such a reservoir is not always filled, much of land below the maximum reservoir level will be submerged only temporarily and occasionally and can be used for agriculture, although no permanent habitation can be allowed on this land. Fig. 8.1 the principle of flood control reservoir Functioning and advantages of a storage reservoir: A storage reservoir with gated spillway and gated sluiceway, provides more flexibility of operation, and thus gives us better control and increased usefulness of the reservoir. Storage reservoir are, therefore, preferred on large rivers which require better control, while retarding basins are preferred on small rivers. In storage reservoirs, the flood crest downstream can be better controlled and regulated properly so as not to cause their coincidence. This is the biggest advantage of such a reservoir and outweighs its disadvantages of being costly and involving risk of human error in installation and operation of gates. 3． distribution reservoir A distribution reservoir is a small storage reservoir constructed within a city water supply system. Such a reservoir can be filled by pumping water at a certain rate and can be used to supply water even at rates higher than the inflow rate during periods of maximum demands (called critical periods of demand). Such reservoirs are, therefore, helpful in permitting the pumps or water treatment plants to work at a uniform rate, and they store water during the hours of no demand or less demand and supply water from their “storage” during the critical periods of maximum demand. 4． Multipurpose reservoirs A reservoir planned and constructed to serve not only one purpose but various purposes together is called a multipurpose reservoir. Reservoir, designed for one purpose, incidentally serving other purposes, shall not be called a multipurpose reservoir, but will be called so, only if designed to serve those purposed also in addition to its main purpose. Hence, a reservoir designed to protect the downstream areas from floods and also to conserve water for water supply, irrigation, industrial needs, hydroelectric purposes, etc. shall be called a multipurpose reservoir. Water is best known and most abundant of all chemical compounds occurring in relatively pure form on the earth’s surface. Oxygen, the most abundant chemical element, is present in combination with hydrogen to the extent of 89 percent in water. Water covers about three fourths of the earth's surface and permeates cracks of much solid land. The Polar Regions (原文polar regions) are overlaid with vast quantities of ice, and the atmosphere of the earth carries water vapor in quantities from 0.1 percent to 2 percent by weight. It has been estimated that the amount of water in the atmosphere above a square mile of land on a mild summer day is of the order of 50,000 tons. In addition to the direct use of water in our homes and on the farm, there are many indirect ways in which water affects our lives. In manufacturing, generation of electric power, transportation, recreation, and in many other ways, water plays a very important role. Our use of water is increasing rapidly with our growing population. Already there are acute shortages of both surface and underground waters in many locations. Careless pollution and contamination of our streams, lakes, and underground sources has greatly impaired the quality of the water which we do have available. It is therefore of utmost importance for our future that good conservation and sanitary measures be practiced by everyone.