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ce235-eh-lec-2

  1. 1. NUST Institute of Civil Engineering/Engr Sajjad Ahmad<br />1<br />
  2. 2. Engineering Hydrology(CE- 235)<br />CHAPTER - 2<br />2<br />PRECIPITATION<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>PRECIPITATION - OUTLINE<br /><ul><li>Forms of precipitation
  3. 3. Factors influencing precipitation formation
  4. 4. Measurement of precipitation
  5. 5. Computation of average rainfall over a basin</li></ul>3<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li> Falling of any form of moisture from atmosphere to ground<br />4<br />Engineering Hydrology(CE- 235)<br />PRECIPITATION<br />Transpiration<br />Evaporation<br />Water vapors<br />Atmospheric system<br />Precipitation<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>FORMS OF PRECIPITATION<br /><ul><li>Drizzle
  6. 6. Rain
  7. 7. Glaze
  8. 8. Sleet
  9. 9. Snow
  10. 10. Snowflake
  11. 11. Hail</li></ul>5<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>6<br />Engineering Hydrology(CE- 235)<br />DRIZZLE<br /><ul><li>Minute particles of water
  12. 12. Drop size < 0.5 mm
  13. 13. Intensity < 1.0 mm/hr
  14. 14. Do not produce runoff
  15. 15. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>7<br />Engineering Hydrology(CE- 235)<br />RAIN<br /><ul><li>Minimum drop size ≈ 0.5 mm
  16. 16. Maximum drop size ≈ 6.25 mm
  17. 17. Can produce runoff if
  18. 18. Rainfall intensity greater than the rate of infiltration
  19. 19. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>8<br />Engineering Hydrology(CE- 235)<br />GLAZE<br /><ul><li>Ice coating on drizzle or rain drops
  20. 20. Ice coating forms when
  21. 21. Drops comes in contact with cold surfaces on the ground
  22. 22. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>9<br />Engineering Hydrology(CE- 235)<br />SLEET<br /><ul><li>Drizzle or rain drops in frozen form
  23. 23. Drops freezes while falling through air at subfreezing temperature
  24. 24. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>10<br />Engineering Hydrology(CE- 235)<br />SNOW<br /><ul><li>Crystals of ice
  25. 25. Form due to sublimation
  26. 26. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>11<br />Engineering Hydrology(CE- 235)<br />SNOWFLAKE<br /><ul><li>Ice crystals fused together
  27. 27. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>12<br />Engineering Hydrology(CE- 235)<br />HAIL<br /><ul><li>Balls or lumps of ice
  28. 28. Diameter > 5 mm
  29. 29. Formed due to alternate freezing and drying
  30. 30. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>13<br />Engineering Hydrology(CE- 235)<br />FACTORS INFLUENCING PRECIPITATION FORMATION<br /><ul><li>Mechanism of cooling of air
  31. 31. Water vapor condensation and formation of droplets
  32. 32. Mechanism for growth of cloud droplets
  33. 33. Mechanism for accumulation of moisture
  34. 34. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>MECHANISM OF COOLING OF AIR<br /><ul><li>The warm air near the surface rises carrying moisture/water vapors with it
  35. 35. The air cools as it ascends due to lower temperature</li></ul>14<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad<br />15<br />
  36. 36. Engineering Hydrology(CE- 235)<br />CODENSATION OF WATER VAPORS<br /><ul><li>Water vapors condensed in presence of hygroscopic nuclei
  37. 37. These nuclei comes from sea salt or combustion products of sulfurous and nitrous acids
  38. 38. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></ul>16<br />
  39. 39. 17<br />Engineering Hydrology(CE- 235)<br />GROWTH OF CLOUD DROPLETS<br /><ul><li>Drops should be of sufficient size to over come the drag and uplift forces of air
  40. 40. Droplet size can grow by two means
  41. 41. Coalescence of droplets through collisions
  42. 42. Bergeron’s effect
  43. 43. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>18<br />Engineering Hydrology(CE- 235)<br />BERGERON’S EFFECT<br /><ul><li>If temp is b/w 10 to 20 F then mixture of water droplets and ice crystals exists in atmosphere
  44. 44. Due to higher saturation capacity around the droplets, some water evaporates
  45. 45. While due to low saturation capacity around the ice crystals vapors condense and ice crystal size increases
  46. 46. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>19<br />Engineering Hydrology(CE- 235)<br />ACCUMULATION OF MOISTURE<br /><ul><li>The quantity of water fall over basin is much more than the amount of moisture over the basin
  47. 47. Therefore continuous supply of moisture is required
  48. 48. This process is known as convergence
  49. 49. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>20<br />Engineering Hydrology(CE- 235)<br />CONVERGENCE<br /><ul><li>It is the process of inflow of moisture from other areas to the basin
  50. 50. It can be defined as
  51. 51. The net horizontal influx of air per unit area
  52. 52. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>21<br />Engineering Hydrology(CE- 235)<br />PRECIPITATION CLASSIFICATION BASED ON LIFTING MECHANISM<br /><ul><li>Convectional precipitation
  53. 53. Orographic precipitation
  54. 54. Cyclonic precipitation
  55. 55. Turbulent ascent precipitation
  56. 56. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>22<br />Engineering Hydrology(CE- 235)<br />CONVECTIONAL PRECIPITATION<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>23<br />Engineering Hydrology(CE- 235)<br />CONVECTIONAL PRECIPITATION<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>24<br />Engineering Hydrology(CE- 235)<br />CONVECTIONAL PRECIPITATION<br /><ul><li>Natural rising of warmer, lighter air in colder, denser surroundings
  57. 57. In the hot day, the ground surface gets heated, causing the warmer air to lift up
  58. 58. the colder air comes to take its place
  59. 59. The vertical air currents develop tremendous velocities the vapors condensed and Convective precipitation occurs
  60. 60. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>25<br />Engineering Hydrology(CE- 235)<br />OROGRAPHIC PRECIPITATION<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>26<br />Engineering Hydrology(CE- 235)<br />OROGRAPHIC PRECIPITATION<br /><ul><li>Orographic precipitation is caused by air masses which strike some natural topographic barriers like mountains, and cannot move forward
  61. 61. Hence rise up, causing condensation and precipitation
  62. 62. All the precipitation in Himalayan region is because of this nature
  63. 63. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>27<br />Engineering Hydrology(CE- 235)<br />CYCLONIC PRECIPITATION<br /><ul><li>Cyclonic precipitation is caused by lifting of an air mass due to the pressure difference
  64. 64. Cyclonic precipitation may be either frontal or non-frontal cyclonic precipitation
  65. 65. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>28<br />Engineering Hydrology(CE- 235)<br />FRONTAL PRECIPITATION<br /><ul><li>It results from the lifting of warm and moist air on one side of a frontal surface over colder, denser air on the other side
  66. 66. A front may be warm front or cold front depending upon whether there is active or passive accent of warm air mass over cold air mass
  67. 67. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>29<br />Engineering Hydrology(CE- 235)<br />COLD FRONT<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>30<br />Engineering Hydrology(CE- 235)<br />WARM FRONT<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>31<br />Engineering Hydrology(CE- 235)<br />STATIONARY FRONT<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>32<br />Engineering Hydrology(CE- 235)<br />MEASURMENT OF RAINFALL<br /><ul><li>Amount of precipitation
  68. 68. Vertical depth of water on a level surface
  69. 69. Expressed in inches, ft, cm etc
  70. 70. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>33<br />Engineering Hydrology(CE- 235)<br />MEASURMENT OF RAINFALL<br /><ul><li>Intensity or rate of precipitation
  71. 71. Precipitation amount per unit time
  72. 72. Expressed in inch pr hour or cm per hr etc
  73. 73. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>34<br />Engineering Hydrology(CE- 235)<br />MEASURMENT OF RAINFALL<br /><ul><li>Primary input used to estimate
  74. 74. Stream flow
  75. 75. Ground water infiltration
  76. 76. Accurate measurement is necessary
  77. 77. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>35<br />Engineering Hydrology(CE- 235)<br />MEASURMENT OF RAINFALL<br /><ul><li>Types of rain gauges used are
  78. 78. Non recording rain gauge
  79. 79. Recording rain gauge
  80. 80. Recording rain gauges also gives the intensity of rain fall
  81. 81. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>36<br />Engineering Hydrology(CE- 235)<br />NON RECORDING RAIN GAUGE<br /><ul><li>The most common is the non recording gauge called a Standard Rain Gauge. The SRG is 200mm diameter cylinder of 600mm height. It has a funnel on top and a plastic measuring tube (1/10th of diameter of cylinder) in the middle
  82. 82. Due to smaller diameter 2.5mm rainfall fills 25mm of tube
  83. 83. Hence gives more accuracy
  84. 84. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>37<br />Engineering Hydrology(CE- 235)<br /><ul><li>Schematic diagram of symon’s rain gauge
  85. 85. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>38<br />Engineering Hydrology(CE- 235)<br />RECORDING TYPE RAIN GAUGE<br /><ul><li>Float type
  86. 86. Weighing type
  87. 87. Tipping bucket type
  88. 88. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>39<br />Engineering Hydrology(CE- 235)<br />FLOAT TYPE<br />Float<br />Rotating drum and graph paper<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>40<br />Engineering Hydrology(CE- 235)<br />FLOAT TYPE<br /><ul><li>Standard instrument used to quantify rainfall
  89. 89. Float within collecting bucket rises with level
  90. 90. Vertical movement marked by pen and shows rainfall depth on a paper chart
  91. 91. The chart is rotated by a spring-driven or electric clock at speeds of 1 revolution in 6, 9, 12, 24, or 192 hours
  92. 92. The rain gauge chart is a record of the accumulated of rainfall for the selected time interval
  93. 93. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>41<br />Engineering Hydrology(CE- 235)<br />FLOAT TYPE<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>42<br />Engineering Hydrology(CE- 235)<br />WEIGHING TYPE<br /><ul><li>It consists of a storage bin, which is weighed to record the mass
  94. 94. It weighs rain or snow which falls into a bucket, set on a platform with a spring or lever balance
  95. 95. The increasing weight of the bucket and its contents are recorded on a chart
  96. 96. The record shows accumulation of precipitation
  97. 97. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>43<br />Engineering Hydrology(CE- 235)<br />TIPPING BUCKET TYPE<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>44<br />Engineering Hydrology(CE- 235)<br />TIPPING BUCKET TYPE<br /><ul><li>Two containers on balance beam form a “tipping bucket”
  98. 98. Rain fills one container until its threshold weight reached
  99. 99. Bucket then tips over, emptying collected water into total container and continues to collect rainfall in other container
  100. 100. Magnet generates electric pulse which is recorded
  101. 101. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>45<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>46<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad<br />47<br />
  102. 102. 48<br />Engineering Hydrology(CE- 235)<br />SOURCES OF ERROR<br /><ul><li>Mistakes in reading the scale of gauge
  103. 103. Dents in collector rim may change its receiving area
  104. 104. Instrumental error in gauge or in their recording or measuring arrangements
  105. 105. Some rainwater may get lost due to splash from the collector
  106. 106. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad<br />49<br />
  107. 107. 50<br />Engineering Hydrology(CE- 235)<br />SOURCES OF ERROR<br /><ul><li>Some initial rainwater may get lost in moistening gauge funnel and inside surfaces
  108. 108. Blowing wind may tilt the rains from vertical which thus brings less rain catch in the gauge
  109. 109. Vertical upward air currents may impact upward acceleration to precipitation thus brings less rain catch in gauge
  110. 110. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>51<br />Engineering Hydrology(CE- 235)<br />SOURCES OF ERROR<br /><ul><li>Gauge inclined 10° from vertical will approx cater 1.5% less rainfall than it should
  111. 111. No rainfall recorded during tipping of bucket
  112. 112. Tipping of bucket may be affected due to rusting or accumulation of dust on pivot
  113. 113. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>52<br />Engineering Hydrology(CE- 235)<br />REMEDIAL MEASURES<br /><ul><li>Unavoidable Errors</li></ul>Equipment failure<br />Observer error <br /><ul><li>Avoidable Errors (related to site)
  114. 114. inclination - perpendicular to ground
  115. 115. Obstructions height
  116. 116. Wind
  117. 117. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>53<br />Engineering Hydrology(CE- 235)<br />Minimum distance from obstruction<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>54<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>55<br />Engineering Hydrology(CE- 235)<br />Example 1<br /><ul><li>A rain gauge recorded 125mm of precipitation. It was found later that the gauge was inclined at an angle of 20 degree to the vertical. Find the actual precipitation.
  118. 118. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>56<br />Engineering Hydrology(CE- 235)<br />ADVANTAGES OF RECORDING RAIN GAUGES<br /><ul><li>Rainfall is recorded automatically, therefore no need of attendant
  119. 119. Recording rain gauge also gives intensity of rainfall at any time, while non-recording gauge gives only total rainfall for time interval
  120. 120. Recording rain gauges can be installed far off places
  121. 121. Possibility of human error is obviated
  122. 122. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>57<br />Engineering Hydrology(CE- 235)<br />DISADVANTAGES OF RECORDING RAIN GAUGES<br /><ul><li>Costly in comparison to non-recording gauges
  123. 123. Error in recording rainfall due to fault in electrical or mechanical mechanism
  124. 124. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>58<br />Engineering Hydrology(CE- 235)<br />Analysis of precipitation data<br /><ul><li>Point data analysis
  125. 125. Estimation of missing precipitation record
  126. 126. Simple arithmetic mean method
  127. 127. Normal ratio method
  128. 128. Consistency of precipitation data or double mass curve analysis
  129. 129. Estimation of average precipitation over a basin
  130. 130. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>59<br />Engineering Hydrology(CE- 235)<br />Arithmetic Mean Method<br /><ul><li>Interpolation station ≈ gauging station whose data is missing
  131. 131. Index station ≈ surrounding stations around interpolation station whose data is used to calculate the missing rainfall data
  132. 132. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>60<br />Engineering Hydrology(CE- 235)<br />Arithmetic Mean Method<br /><ul><li>If Px is the missing precipitation then
  133. 133. Here n ≈ number of nearby stations
  134. 134. And Pi is the precipitation at ith station
  135. 135. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>61<br />Engineering Hydrology(CE- 235)<br />Arithmetic Mean Method<br /><ul><li>Lets suppose there are three index stations then the precipitation at interpolation station will be given as
  136. 136. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>62<br />Engineering Hydrology(CE- 235)<br />Normal Ratio Method<br /><ul><li>If Px is the missing precipitation then
  137. 137. Here n ≈ number of nearby stations
  138. 138. And Pi is the precipitation at ith station
  139. 139. Nx and Ni are the normal annual precipitation value for station x and ith respectively
  140. 140. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>63<br />Engineering Hydrology(CE- 235)<br />Applicability of above methods<br /><ul><li>Arithmetic average method can be used iff following satisfies
  141. 141. Nx and N1 , N2 and N3 are the normal annual precipitation value for station x and 1,2 and 3rd station respectively
  142. 142. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>64<br />Engineering Hydrology(CE- 235)<br />Applicability of above methods<br /><ul><li>If the condition not satisfies then only normal ratio method can be used
  143. 143. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>65<br />Engineering Hydrology(CE- 235)<br />THANKS<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li>

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