CE-235 EH Lec 3
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  • 1. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
    1
  • 2. Engineering Hydrology(CE- 235)
    CHAPTER - 2
    2
    PRECIPITATION
    (Contd…)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • PRECIPITATION - OUTLINE
    • Forms of precipitation
    • 3. Factors influencing precipitation formation
    • 4. Precipitation classification based on lifting mechanism
    • 5. Measurement of precipitation
    • 6. Computation of average rainfall over a basin
    3
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 4
    Engineering Hydrology(CE- 235)
    Example 1
    • 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. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • EXAMPLE 2
    • Find out the missing storm precipitation of station ‘C’ given in the following table
    5
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • EXAMPLE 3
    • 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. Estimate the storm precipitation for station ‘X’
    6
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 7
    Engineering Hydrology(CE- 235)
    Chances of error in rainfall assessment
    • 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. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • CONSISTANCY OF PRECIPITATION DATA BY DOUBLE MASS ANALYSIS
    • 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.
    8
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 9
    Engineering Hydrology(CE- 235)
    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.
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 10
    Engineering Hydrology(CE- 235)
    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.
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
    11
  • 10. 12
    Engineering Hydrology(CE- 235)
    DOUBLE MASS ANALYSIS
    • The double mass curve is obtained by plotting
    • 11. X-axis ≈ Average accumulated precipitation of nearby stations
    • 12. Y-axis ≈ Accumulated precipitation of the station under consideration
    • 13. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 13
    Engineering Hydrology(CE- 235)
    DOUBLE MASS ANALYSIS
    • Arrange the data (recent to past)
    • 14. Determine cumulative rain fall of the subjected station and of the nearby stations
    • 15. Draw double mass curve
    • 16. Part of the curve which lies in straight line requires no correction
    • 17. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 14
    Engineering Hydrology(CE- 235)
    DOUBLE MASS ANALYSIS
    • All data lying after the deviation point from the straight line requires correction
    • 18. To determine correction factor calculate the slope of the curve before and after the point of deviation
    • 19. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 15
    Engineering Hydrology(CE- 235)
    DOUBLE MASS ANALYSIS
    • Pa =Adjusted precipitation
    • 20. Po =Observed precipitation
    • 21. Sa =Slope prior to the break in the curve
    • 22. So =Slope after the break in the curve
    • 23. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • Engineering Hydrology(CE- 235)
    EXAMPLE 4
    • Check consistency of the data and correct if inconsistent
    • 24. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
    16
  • 25. 17
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 18
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 19
    Engineering Hydrology(CE- 235)
    9415, 9024
    Point of deviation
    7665, 6923
    Cumulative rainfall at stations X
    YEAR 1950
    4064, 3410
    2045, 1656
    Cumulative rainfall at nearby stations
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 20
    Engineering Hydrology(CE- 235)
    CALCULATION OF SLOPE
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 21
    Engineering Hydrology(CE- 235)
    CALCULATION OF SLOPE
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 22
    Engineering Hydrology(CE- 235)
    Correction factor
    Applicable to the data before 1950
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 23
    Engineering Hydrology(CE- 235)
    Corrected precipitation
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
    24
    Actual data curve
    Cumulative rainfall at stations X
    Corrected data curve
    Cumulative rainfall at nearby stations
  • 26. Engineering Hydrology(CE- 235)
    EXAMPLE 5
    • The annual precipitation at station ‘A’ and the average precipitation at 15 surrounding stations are given in table 3.19 find
    • 27. Consistency of the record at station ‘A’
    • 28. Indicate the year in which there is a regime changes
    • 29. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
    25
  • 30. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
    26
  • 31. 27
    Engineering Hydrology(CE- 235)
    • Calculate cumulative rain fall of station A and near by stations
    • 32. Draw curve of cumulative rainfall
    • 33. Determine point of deviation
    • 34. Calculate slope before and after deviation
    • 35. Apply correction to the points lying after the deviation
    • 36. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 28
    Engineering Hydrology(CE- 235)
    Computation of Average Rainfall over a Basin
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 29
    Engineering Hydrology(CE- 235)
    Computation methods
    • Arithmetic Average Method
    • 37. Thiessen Polygon Method
    • 38. Distance weighting
    • 39. Isohyetal Method
    • 40. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 30
    Engineering Hydrology(CE- 235)
    Arithmetic mean method
    • If rainfall is uniformly distributed in areal pattern then this is the simplest method to estimate average rainfall over a catchment
    • 41. If P1, P2, P3, … Pn etc are the precipitation or rainfall values measured at ‘n’ gauge stations, then
    • 42. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 31
    Engineering Hydrology(CE- 235)
    Arithmetic mean method
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 32
    Engineering Hydrology(CE- 235)
    Example 5
    • 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. Find average precipitation
    • 44. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 33
    Engineering Hydrology(CE- 235)
    Thiessen polygon method
    • Rainfall recorded by each rain gauge weighted according to the area it is assumed to represent
    • 45. It is also called Weighted Mean Method
    • 46. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 34
    Engineering Hydrology(CE- 235)
    Thiessen polygon method
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 35
    Engineering Hydrology(CE- 235)
    Thiessen polygon method
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 36
    Engineering Hydrology(CE- 235)
    Steps for polygon
    • Draw area according to certain scale
    • 47. Connect all gauging stations
    • 48. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 37
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 38
    Engineering Hydrology(CE- 235)
    Steps for polygon
    • Draw perpendicular bisectors of all the lines joining the rain gauge network
    • 49. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 39
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 40
    Engineering Hydrology(CE- 235)
    Steps for polygon
    • Measure area of each polygon
    • 50. Calculate average precipitation
    • 51. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 41
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 42
    Engineering Hydrology(CE- 235)
    Distance weighting
    • This method is based on the distance between the centroid of basin and gauge
    • 52. The weight given to the precipitation is inversely proportional to the square of the distance between centroid of basin and gauge point
    Example 6
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 43
    Engineering Hydrology(CE- 235)
    Isohyetal method
    • An isohyet is a line on a rainfall map of the basin, joining places of equal rainfall readings
    • 53. An isohyetal map shows contours of equal rainfall on the ground
    • 54. Gives more accurate picture of rainfall distribution
    • 55. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 44
    Engineering Hydrology(CE- 235)
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 45
    Engineering Hydrology(CE- 235)
    Isohyetal method
    • Draw map of area
    • 56. Indicate points of rain gauges
    • 57. Write rainfall value at gauge points
    • 58. Draw isohyets
    • 59. Measure area enclosed or b/w every two isohyets
    • 60. NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • 46
    Engineering Hydrology(CE- 235)
    Isohyetal method
    • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
  • NUST Institute of Civil Engineering/Engr Sajjad Ahmad
    47