水利電力專業(yè)外文翻譯--溢洪道

1、<p>　　OVERFLOW SPILLWAY</p><p>　　An overflow spillway is a section of dam designed to permit water to pass over its crest. Overflow spillways are widely used on gravity, arch, and buttress dams. Some e

2、arth dams have a concrete gravity section designed to serve as a spillway. The design of the spillway for tow dams is not usually critical, and a variety of simple crest patterns are used. In the case of large dams it is

3、 important that the overflowing water be guided smoothly over the crest with a minimum of turbulence. If the ove</p><p>　　Cavities filled with vapor, air, and other gases will form in a liquid whenever the a

4、bsolute pressure of the liquid is close to the vapor pressure. This phenomenon, cavitations, is likely to occur where high velocities cause reduced pressure. Such conditions may arise if the walls of a passage are so sha

5、rply curved as to cause separation of flow from the boundary. The cavity, on moving downstream, may enter a region where the absolute is much higher. This causes the vapor in the cavity to conden</p><p>　　Th

6、e ideal spillway would take the form of the underside of the napped of a sharp-crested weir when the flow rate corresponds to the maximum design capacity of the spillway. More exact profiles may be found in more extensiv

7、e treatments of the subject. The reverse curve on the downstream face of the spillway should be smooth and gradual; A radius of about one-fourth of the spillway height has proved satisfactory. Structural design of an oge

8、e spillway is essentially the same as the design of a conc</p><p>　　A saving of concrete can be effected by providing a projecting corbel on the upstream face to control the flow in outlet conduits through t

9、he dam, a corbel will interfere with gate operation.</p><p>　　The discharge of an overflow spillway is given by the weir equation</p><p>　　Where Q=discharge, or </p><p>　　L=coeffici

10、ent</p><p>　　h=head on the spillway (vertical distance from the crest of the spillway to the reservoir level), m</p><p>　　The coefficient varies with the design and head. Experimental models are

11、 often used to determine spillway coefficient. End contractions on a spillway reduce the effective length below the actual length L. Square-cornered piers disturb the flow considerably and reduce the effective length by

12、the width of the piers plus about 0.2h for each pier.</p><p>　　Streamlining the piers or flaring the spillway entrance minimizes the flow disturbance. If the cross-sectional area of the reservoir just upstre

13、am from the spillway is less than five times the area of flow over the spillway, the approach velocity with increase the discharge a noticeable amount. The effect of approach velocity can be accounted for by the equation

14、</p><p>　　where is the approach velocity.</p><p>　　PROPERTIES OF CONCRETE</p><p>　　The characteristics of concrete should be considered in relation to the quality for any given cons

15、truction purpose. The closest practicable approach to perfection in every property of the concrete would result in poor economy under many conditions, and the most desirable structure is that in which the concrete has be

16、en designed with the correct emphasis on each of the various properties of the concrete, and not solely with a view to obtaining, say, the maximum possible strength. </p><p>　　Although the attainment of the

17、maximum strength should not be the sole criterion in design, the measurement of the crushing strength of concrete cubes or cylinders provides a means of maintaining a uniform standard of quality, and, in fact, is the usu

18、al way of doing so. Since the other properties of any particular mix of concrete are related to the crushing strength in some manner, it is possible that as a single control test it is still the most convenient and infor

19、mative.</p><p>　　The testing of the hardened concrete in prefabricated units presents no difficulty, since complete units can be selected and broken if necessary in the process of testing. Samples can be tak

20、en from some parts of a finished structure by cutting cores, but at consider one cost and with a possible weakening of the structure. It is customs, therefore, to estimate the properties of the concrete in the structure

21、on the oasis of the tests made on specimens mounded from the fresh concrete as it is place</p><p>　　Crushing strength</p><p>　　Concrete can be made having a strength in compression of up to abou

22、t 80N/,or even more depending mainly on the relative proportions of water and cement, that is, the water/cement ratio, and the degree of compaction. Crushing strengths of between 20 and 50 N/ at 28 days are normally obta

23、ined on the site with reasonably good supervision, for mixes roughly equivalent to 1:2:4 of cement: sand: coarse aggregate. In some types of precast concrete such as railway sleepers, strengths ranging from 40 to </p&

24、gt;<p>　　The crushing strength of concrete is influenced by a number of factors in addition to the water/cement ratio and the degree of compaction. The more important factors are </p><p>　　Type of cem

25、ent and its quality. Both the rate of strength gain and the ultimate strength may be affected.</p><p>　　Type and surface texture of aggregate. There is considerable evidence to suggest that some aggregates p

26、roduce concrete of greater compressive and tensile strengths than obtained with smooth river gravels.</p><p>　　Efficiency of curing. A loss in strength of up to about 40 per cent may result from premature dr

27、ying out. Curing is therefore of considerable, importance both in the field and in the making of tests. The method of curing concrete test cubes given in BS 1881 should, for this reason, be strictly adhered to.</p>

28、<p>　　Temperature In general, the rate of hardening of concrete is increased by an increase temperature. At freezing temperatures the crushing strength may remain low for some time.</p><p>　　Age Und

29、er normal conditions increase in strength with age, the rate of increase depending on the type of cement with age. For instance, high alumina cement produces concrete with a crushing strength at 21 hours equal to that of

30、 normal Portland cement concrete at 28 days. Hardening continues but at a much slower rate for a number of years.</p><p>　　The above refers to the static ultimate load. When subjected to repeated loads concr

31、ete fails at a load smaller than the ultimate static load, a fatigue effect. A number of investigators have established that after several million cycles of loading, the fatigue strength in compression is 50-60 per cent

32、of the ultimate static strength.</p><p>　　Tensile and flexural strength</p><p>　　The tensile strength of concrete varies from one-eighth of the compressive strength at early ages to about one- t

33、wentieth later, and is not usually taken into account in the design of reinforced concrete structures. The tensile strength is, however, of considerable importance in resisting cracking due to changes in moisture content

34、 or temperature. Tensile strength tests are used for concrete roads and airfields.</p><p>　　The measurement of the strength of concrete in direct tension is difficult and is rarely attempted. Two more practi

35、cal methods of assessing tensile strength are available. One gives a measure of the tensile strength in bending, usually termed the flexural strength. BS 1881:1970 gives details concerning the making and curing of flexur

36、e test specimens, and of the method test. The standard size of specimen is 150 ×150×750 long for aggregate of maximum size 40. If the largest nominal size of the agg</p><p>　　A load is applied thro

37、ugh two rollers at the third points of the span until the specimen breaks. The extreme fiber stresses, that is, compressive at the top and tensile at the bottom, can then be computed by the usual beam formulae. The beam

38、will obviously fail in tension since the tensile strength is much lower than the compressive strength. Formulae for the calculation of the modulus of rupture are given in BS 1881:1970.</p><p>　　Test specimen

39、s is the form of beams are sometimes used to measure the modulus of rupture or flexural strength quickly on the site. The two halves of the specimen may then be crushed so that besides the flexural strength the compressi

40、ve strength can be approximately determined on the same sample. The test is described in BS 1881:1970.</p><p>　　Values of the modulus of rupture are utilized in some methods of design of unreinforced concret

41、e roads and runways, in which reliance is placed on the flexural strength of the concrete to distribute concentrated loads over a wide area.</p><p>　　More recently introduced is a test made by splitting cyli

42、nders by compression across the diameter, to give what is termed the splitting tensile strength; Details of the method are given in BS 1881:1970.</p><p>　　Values of the modulus of rupture are utilized in som

43、e methods of design of unreinforced concrete roads and runways, in which reliance is place on the flexural strength of the concrete to distribute concentrated loads over a wide area.</p><p>　　More recently i

44、ntroduced is a test made by splitting cylinders by compression across the diameter, to give what is termed the splitting tensile strength; Details of the method are given in BS 1881:1970. the testing machine is fitted wi

45、th an extra bearing bar to distribute the load along the full length of the cylinder Plywood strips, 12mm wide and 3mm thick are inserted between the cylinder and the testing machine bearing surfaces top and bottom.</

46、p><p>　　From the maximum applied load at failure the tensile splitting strength is calculated as follows:</p><p>　　Where splitting tensile strength, N/</p><p>　　P=maximum applied load

47、in N</p><p>　　l=length of cylinder in mm</p><p>　　d=diameter in mm</p><p>　　As in the case of the compressive strength, repeated loading reduces the ultimate strength so that the fa

48、tigue strength in flexure is 50-60 per cent of the static strength.</p><p>　　Shear strength</p><p>　　In practice, shearing of concrete is always accompany compression and tension caused by ben

49、ding, and even in testing is impossible to staminate an element of bending. </p><p>　　RESERVOIRS</p><p>　　When a barrier is constructed across some river in the form of a dam, water gets stored

50、up on the upstream side of the barrier, forming a pool of water, generally called a reservoir.</p><p>　　Broadly speaking, any water collected in a pool or a lake may be termed as a reservoir. The water store

51、d in reservoir may be used for various purposes. Depending upon the purposes served, the reservoirs may be classified as follows:</p><p>　　Storage or Conservation Reservoirs.</p><p>　　Flood Cont

52、rol Reservoirs.</p><p>　　Distribution Reservoirs.</p><p>　　Multipurpose reservoirs.</p><p>　　(1) Storage or Conservation Reservoirs. A city water supply, irrigation water suppl

53、y or a hydroelectric project drawing water directly from a river or a stream may fail to satisfy the consumers’ demands during extremely low flows, while during high flows; it may become difficult to carry out their oper

54、ation 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 </p><p>　　Incid

55、entally, in addition to conserving water for later use, the storage of flood water may also reduce flood damage below the reservoir. Hence, a reservoir can be used for controlling floods either solely or in addition to o

56、ther 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’.</p><p>　　(2) Flood Co

57、ntrol Reservoirs 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 a

58、ccomplish this, the entire inflow entering the reservoir is discharge till the outflow reaches the safe capacity of the channel downstream. The inflow in excess of this rate is stored in stored in the reservoir, which is

59、 then gradually released so as to recover the s</p><p>　　The flood peaks at the points just downstream of the reservoir are thus reduced by an amount AB. A flood control reservoir differs from a conservation

60、 reservoir only in its need for a large sluice-way capacity to permit rapid drawdown before or after a flood.</p><p>　　Types of flood control reservoirs. There are tow basic types of flood-mitigation reservo

61、ir.</p><p>　　Storage Reservoir or Detention basins.</p><p>　　Retarding basins or retarding reservoirs.</p><p>　　A reservoir with gates and valves installation at the spillway and at

62、 the sluice outlets is known as a storage-reservoir, while on the other hand, a reservoir with ungated outlet is known as a retarding basin.</p><p>　　Functioning and advantages of a retarding basin:</p>

63、;<p>　　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

64、such a reservoir. The maximum discharging capacity of such a reservoir should be equal to the maximum safe carrying capacity of the channel downstream. As flood occurs, the reservoir gets filled and discharges through sl

65、uiceways. As the reservoir elevation increases, outflow discharge increase</p><p>　?、?Cost of gate installations is save.</p><p>　?、?There are no fates and hence, the possibility of human error

66、and negligence in their operation is eliminated.</p><p>　　Since such a reservoir is not always filled, much of land below the maximum reservoir level will be submerged only temporarily and occasionally and c

67、an be successfully used for agriculture, although no permanent habitation can be allowed on this land.</p><p>　　Functioning and advantages of a storage reservoir:</p><p>　　A storage reservoir wi

68、th gated spillway and gated sluiceway, provides more flexibility of operation, and thus gives us better control and increased usefulness of the reservoir. Storage reservoirs are, therefore, preferred on large rivers whic

69、h require batter 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

70、 in installation and operation of gate</p><p>　　(3) Distribution Reservoirs A distribution reservoir is a small storage reservoir constructed within a city water supply system. Such a reservoir can be fil

71、led 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, help

72、ful in permitting the pumps or water treatment plants to work at a uniform rate, and they store water during the hours of</p><p>　　(4) Multipurpose Reservoirs A reservoir planned and constructed to serve

73、not only one purpose but various purposes together is called a multipurpose reservoir. Reservoir, designed for one purpose, incidentally serving other purpose, shall not be called a multipurpose reservoir, but will be ca

74、lled so, only if designed to serve those purposes 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,</p><

75、p>　　THE ELECTRIC POWER SYSTEM </p><p>　　A great amount of effort is necessary to maintain an electric power supply within the requirement of the various types of customers served. Large investments are ne

76、cessary, and continuing advancements in methods must be made as loads steadily increase from year to year. Some of the requirements for electric power supply are recognized by most consumers, such as proper voltage, avai

77、lability of power on demand, reliability, and reasonable cost. Other characteristics, such as frequency, wave shape,</p><p>　　The voltage of the power supply at the customer’s service entrance must be held s

78、ubstantially constant. Variations in supply voltage are, from the customer’s view, detrimental in various respects. For example, below-normal voltage substantially reduces the light output from incandescent lamps. Above-

79、normal voltage increase the light output but substantially reduces the life of the lamp. Motor operate at below-normal voltage draw abnormally high current and may overheat, even when carrying no mor</p><p>

80、　　Power must be available to the consumer in any amount that be may require from minute to minute. For example, motors may be turned on or off, without advance warning to the electric power company. As electrical energy

81、cannot be stored (except to a limited extent in storage batteries), the changing loads impose severe demands on the control equipment of any electrical power system. The operating staff must continually study load patter

82、ns to predict in advance those major load changes that follow </p><p>　　The demands for reliability of service increase daily as our industrial and social environment becomes more complex. Modern industry is

83、 almost locally dependent on electric power for its operation. Homes and office buildings are lighted, heated, and ventilated by electric power. In some instances loss of electric power may even pose a threat a life itse

84、lf. Electric power, like everything else that is man-made can never be absolutely reliable. Occasional interruptions to service in limited areas </p><p>　　The cost of electric power is a prince consideration

85、 in the design and operation of electric power is a prime consideration in the design and operation of electric power system. Although the cost of almost all commodities has risen steadily over the past many years, the c

86、ost per kilowatt-hour of electrical energy has actually declined. This decrease in cost has been possible because of improved efficiencies of the generating stations and distribution systems. Although franchises often gr

87、ant the </p><p>　　The power supply requirements just discussed are all well known to most electric power users. There are, however, other specifications to the electric power supply which are so effectively

88、handled by the power companies that consumers are seldom aware that such requirements are of importance.</p><p>　　The frequency of electric power supply in the United States is almost entirely 60 hertz (form

89、erly cycles per second). The frequency of a system is dependent entirely upon the speed at which the supply generator is rotated by its prime mover. Hence frequency control is basically a matter of speed control of the m

90、achines in the generating stations. Modern speed-control systems are very effective and hold frequency almost constant. Deviations are seldom greater than 0.02 hertz. </p><p>　　In an ac system the voltage co

91、ntinually varies with time, at one instant being positive and a short time later being negative, going through 60 complete cycles of change in each second. Ideally a plot of the time change should be a sine wave.</p&g

92、t;<p>　　In poorly designed generating equipment, harmonics may be present and the wave shape may be somewhat. The presence of harmonics produces unnecessary losses in the customer’s equipment and sometime produces

93、 hum in nearby telephone lines. The voltage wave shape is basically determined by the construction of the generation equipment. The power companies put specification limitations on the harmonic content of generator volta

94、ges and so require equipment manufactures to design and build their machine</p><p>　　ENVIRONMENT POLLUTION</p><p>　　The existence of pollution in the environment, as a national and a world probl

95、em, was not generally recognized until the 1960s.Today many people regard pollution as a problem that will not go away, but one that could get worse in the future. It is increasingly being appreciated that the general ef

96、fects of pollution produce a deterioration of the quality of the environment. This usually means that pollution is responsible for dirty streams, rivers and sea shorts, atmospheric contamination, the di</p><p&

97、gt;　　The present increasing emphasis upon pollution may create the impression that there has been a relatively sudden deterioration of the environment, that was not apparent twenty or thirty years ago. This is not the ca

98、se. Pollution must have started at the time when man began to use the natural resources of the environment for his own benefit. At he began to develop a settled life in small communities, the activities of clearing trees