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# CE-235 EH Lec 3

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### CE-235 EH Lec 3

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
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
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
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