CE-235 EH Lec 3

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CE-235 EH Lec 3

  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 />(Contd…)<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. Precipitation classification based on lifting mechanism
  5. 5. Measurement of precipitation
  6. 6. 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>4<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.
  7. 7. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>EXAMPLE 2<br /><ul><li>Find out the missing storm precipitation of station ‘C’ given in the following table</li></ul>5<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>EXAMPLE 3<br /><ul><li>Precipitation station ‘X’ was in operative for part of a month during which a storm occurred. The storm totals at three surrounding stations A, B & C were respectively10.7, 8.9 & 12.2 cm. The normal annual precipitation amounts at station X, A, B & C are respectively 97.8, 112,93.5 & 119.9 cm
  8. 8. Estimate the storm precipitation for station ‘X’</li></ul>6<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>7<br />Engineering Hydrology(CE- 235)<br />Chances of error in rainfall assessment <br /><ul><li>Sir Alexander Binnie has shown that more errors are likely to be encountered in rainfall assessment if we use data of less than past 35 years
  9. 9. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>CONSISTANCY OF PRECIPITATION DATA BY DOUBLE MASS ANALYSIS<br /><ul><li>Double mass analysis is a commonly used data analysis approach for investigating the behavior of records made of hydrological or meteorological data at a number of locations. </li></ul>8<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>9<br />Engineering Hydrology(CE- 235)<br /> It is used to determine whether there is a need for corrections to the data to account for changes in data collection procedures or other local conditions. Such changes may result from a variety of things including changes in instrumentation, changes in observation procedures, or changes in gauge location or surrounding conditions.<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>10<br />Engineering Hydrology(CE- 235)<br /> Double mass analysis used for checking consistency of a hydrological or meteorological record and is considered to be an essential tool before taking it for analysis purpose.<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad<br />11<br />
  10. 10. 12<br />Engineering Hydrology(CE- 235)<br />DOUBLE MASS ANALYSIS<br /><ul><li>The double mass curve is obtained by plotting
  11. 11. X-axis ≈ Average accumulated precipitation of nearby stations
  12. 12. Y-axis ≈ Accumulated precipitation of the station under consideration
  13. 13. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>13<br />Engineering Hydrology(CE- 235)<br />DOUBLE MASS ANALYSIS<br /><ul><li>Arrange the data (recent to past)
  14. 14. Determine cumulative rain fall of the subjected station and of the nearby stations
  15. 15. Draw double mass curve
  16. 16. Part of the curve which lies in straight line requires no correction
  17. 17. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>14<br />Engineering Hydrology(CE- 235)<br />DOUBLE MASS ANALYSIS<br /><ul><li>All data lying after the deviation point from the straight line requires correction
  18. 18. To determine correction factor calculate the slope of the curve before and after the point of deviation
  19. 19. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>15<br />Engineering Hydrology(CE- 235)<br />DOUBLE MASS ANALYSIS<br /><ul><li>Pa =Adjusted precipitation
  20. 20. Po =Observed precipitation
  21. 21. Sa =Slope prior to the break in the curve
  22. 22. So =Slope after the break in the curve
  23. 23. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>Engineering Hydrology(CE- 235)<br />EXAMPLE 4<br /><ul><li>Check consistency of the data and correct if inconsistent
  24. 24. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></ul>16<br />
  25. 25. 17<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>18<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>19<br />Engineering Hydrology(CE- 235)<br />9415, 9024<br />Point of deviation<br />7665, 6923<br />Cumulative rainfall at stations X<br />YEAR 1950<br />4064, 3410<br />2045, 1656<br />Cumulative rainfall at nearby stations<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>20<br />Engineering Hydrology(CE- 235)<br />CALCULATION OF SLOPE<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>21<br />Engineering Hydrology(CE- 235)<br />CALCULATION OF SLOPE<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>22<br />Engineering Hydrology(CE- 235)<br />Correction factor<br />Applicable to the data before 1950<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>23<br />Engineering Hydrology(CE- 235)<br />Corrected precipitation<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad<br />24<br />Actual data curve<br />Cumulative rainfall at stations X<br />Corrected data curve<br />Cumulative rainfall at nearby stations<br />
  26. 26. Engineering Hydrology(CE- 235)<br />EXAMPLE 5<br /><ul><li>The annual precipitation at station ‘A’ and the average precipitation at 15 surrounding stations are given in table 3.19 find
  27. 27. Consistency of the record at station ‘A’
  28. 28. Indicate the year in which there is a regime changes
  29. 29. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></ul>25<br />
  30. 30. NUST Institute of Civil Engineering/Engr Sajjad Ahmad<br />26<br />
  31. 31. 27<br />Engineering Hydrology(CE- 235)<br /><ul><li>Calculate cumulative rain fall of station A and near by stations
  32. 32. Draw curve of cumulative rainfall
  33. 33. Determine point of deviation
  34. 34. Calculate slope before and after deviation
  35. 35. Apply correction to the points lying after the deviation
  36. 36. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>28<br />Engineering Hydrology(CE- 235)<br />Computation of Average Rainfall over a Basin<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>29<br />Engineering Hydrology(CE- 235)<br />Computation methods<br /><ul><li>Arithmetic Average Method
  37. 37. Thiessen Polygon Method
  38. 38. Distance weighting
  39. 39. Isohyetal Method
  40. 40. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>30<br />Engineering Hydrology(CE- 235)<br />Arithmetic mean method<br /><ul><li>If rainfall is uniformly distributed in areal pattern then this is the simplest method to estimate average rainfall over a catchment
  41. 41. If P1, P2, P3, … Pn etc are the precipitation or rainfall values measured at ‘n’ gauge stations, then
  42. 42. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>31<br />Engineering Hydrology(CE- 235)<br />Arithmetic mean method<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>32<br />Engineering Hydrology(CE- 235)<br />Example 5<br /><ul><li>Six rain gauges were installed in a relatively flat area and storm precipitation from these gauges were recorded as 3.7, 4.9, 6.8, 11.4, 7.6 and 12.7 cm respectively from gauges 1, 2 ….6
  43. 43. Find average precipitation
  44. 44. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>33<br />Engineering Hydrology(CE- 235)<br />Thiessen polygon method<br /><ul><li>Rainfall recorded by each rain gauge weighted according to the area it is assumed to represent
  45. 45. It is also called Weighted Mean Method
  46. 46. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>34<br />Engineering Hydrology(CE- 235)<br />Thiessen polygon method<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>35<br />Engineering Hydrology(CE- 235)<br />Thiessen polygon method<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>36<br />Engineering Hydrology(CE- 235)<br />Steps for polygon<br /><ul><li>Draw area according to certain scale
  47. 47. Connect all gauging stations
  48. 48. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>37<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>38<br />Engineering Hydrology(CE- 235)<br />Steps for polygon<br /><ul><li>Draw perpendicular bisectors of all the lines joining the rain gauge network
  49. 49. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>39<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>40<br />Engineering Hydrology(CE- 235)<br />Steps for polygon<br /><ul><li>Measure area of each polygon
  50. 50. Calculate average precipitation
  51. 51. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>41<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>42<br />Engineering Hydrology(CE- 235)<br />Distance weighting<br /><ul><li>This method is based on the distance between the centroid of basin and gauge
  52. 52. The weight given to the precipitation is inversely proportional to the square of the distance between centroid of basin and gauge point </li></ul>Example 6<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>43<br />Engineering Hydrology(CE- 235)<br />Isohyetal method<br /><ul><li>An isohyet is a line on a rainfall map of the basin, joining places of equal rainfall readings
  53. 53. An isohyetal map shows contours of equal rainfall on the ground
  54. 54. Gives more accurate picture of rainfall distribution
  55. 55. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>44<br />Engineering Hydrology(CE- 235)<br /><ul><li>NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>45<br />Engineering Hydrology(CE- 235)<br />Isohyetal method<br /><ul><li>Draw map of area
  56. 56. Indicate points of rain gauges
  57. 57. Write rainfall value at gauge points
  58. 58. Draw isohyets
  59. 59. Measure area enclosed or b/w every two isohyets
  60. 60. NUST Institute of Civil Engineering/Engr Sajjad Ahmad</li></li></ul><li>46<br />Engineering Hydrology(CE- 235)<br />Isohyetal method<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 />

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