CE-235 EH Lec 3

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

3. PRECIPITATION - OUTLINE Forms of precipitation Factors influencing precipitation formation Precipitation classification based on lifting mechanism Measurement of precipitation Computation of average rainfall over a basin 3 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad

4. 4 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad 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. Example 1

5. 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 Station A B C D E Storm precipitation (cm) 9.7 8.3 ? 11.7 8.0 Normal annual precipitation (cm) 100.3 109.5 93.5 125.7 117.5

6. 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 Estimate the storm precipitation for station ‘X’ 6 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad

7. 7 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad 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 Chances of error in rainfall assessment

8. 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. 9 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad 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.

10. 10 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad 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.

11. NUST Institute of Civil Engineering/Engr Sajjad Ahmad 11

12. 12 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad The double mass curve is obtained by plotting X-axis ≈ Average accumulated precipitation of nearby stations Y-axis ≈ Accumulated precipitation of the station under consideration DOUBLE MASS ANALYSIS

13. 13 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Arrange the data (recent to past) Determine cumulative rain fall of the subjected station and of the nearby stations Draw double mass curve Part of the curve which lies in straight line requires no correction DOUBLE MASS ANALYSIS

14. 14 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad All data lying after the deviation point from the straight line requires correction To determine correction factor calculate the slope of the curve before and after the point of deviation DOUBLE MASS ANALYSIS

15. 15 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad P a =Adjusted precipitation P o =Observed precipitation S a =Slope prior to the break in the curve S o =Slope after the break in the curve DOUBLE MASS ANALYSIS

16. EXAMPLE 4 Check consistency of the data and correct if inconsistent NUST Institute of Civil Engineering/ Engr Sajjad Ahmad 16 Engineering Hydrology (CE- 235)

17. 17 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad year annual rain at station X (mm) Cumulative rainfall at station X avg annual rainfall at nearby station (mm) cumulative avg annual rainfall at nearby stations 1972 188 188 264 264 1971 185 373 228 492 1970 310 683 386 878 1969 295 978 297 1175 1968 208 1186 284 1459 1967 287 1473 350 1809 1966 183 1656 236 2045 1965 304 1960 371 2416 1964 228 2188 234 2650 1963 216 2404 290 2940 1962 224 2628 282 3222 1961 203 2831 246 3468 1960 284 3115 264 3732 1959 295 3410 332 4064 1958 206 3616 231 4295 1957 269 3885 234 4529 1956 241 4126 231 4760 1955 284 4410 312 5072 1954 223 4633 360 5432

18. 18 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad year annual rain at station X (mm) Cumulative rainfall at station X avg annual rainfall at nearby station (mm) cumulative avg annual rainfall at nearby stations 1953 173 4806 234 5666 1952 282 5088 333 5999 1951 218 5306 236 6235 1950 246 5552 251 6486 1949 284 5836 284 6770 1948 493 6329 361 7131 1947 320 6649 282 7413 1946 274 6923 252 7665 1945 322 7245 274 7939 1944 437 7682 302 8241 1943 389 8071 350 8591 1942 305 8376 228 8819 1941 320 8696 312 9131 1940 328 9024 284 9415 1939 308 9332 315 9730 1938 302 9634 280 10010 1937 414 10048 343 10353

19. 19 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Point of deviation Cumulative rainfall at stations X Cumulative rainfall at nearby stations YEAR 1950 7665, 6923 2045, 1656 4064, 3410 9415, 9024

20. 20 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad CALCULATION OF SLOPE

21. 21 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad CALCULATION OF SLOPE

22. Correction factor 22 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Applicable to the data before 1950

23. 23 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Corrected precipitation 1949 198.8 1948 252.7 1947 197.4 1946 176.4 1945 191.8 1944 211.4 1943 245 1942 159.6 1941 218.4 1940 198.8 1939 220.5 1938 196 1937 240.1

24. NUST Institute of Civil Engineering/Engr Sajjad Ahmad 24 Actual data curve Corrected data curve Cumulative rainfall at stations X Cumulative rainfall at nearby stations

25. EXAMPLE 5 The annual precipitation at station ‘A’ and the average precipitation at 15 surrounding stations are given in table 3.19 find Consistency of the record at station ‘A’ Indicate the year in which there is a regime changes NUST Institute of Civil Engineering/ Engr Sajjad Ahmad 25 Engineering Hydrology (CE- 235)

26. NUST Institute of Civil Engineering/Engr Sajjad Ahmad 26 year annual rain at station X (mm) avg annual rainfall at nearby station (mm) 1990 36 60.5 1989 42 44 1988 36 24 1987 12 49.5 1986 48 47.5 1985 54 38.5 1984 30 38.5 1983 18 55 1982 42 60.5 year annual rain at station X (mm) avg annual rainfall at nearby station (mm) 1981 36 27.5 1980 42 19.5 1979 42 36.5 1978 36 57 1977 69.5 55 1976 62.5 22 1975 50.5 60 1974 21.5 25 1973 16 27.5 1972 90 57 1971 50.5 71.5

27. 27 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Calculate cumulative rain fall of station A and near by stations Draw curve of cumulative rainfall Determine point of deviation Calculate slope before and after deviation Apply correction to the points lying after the deviation

28. 28 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Computation of Average Rainfall over a Basin

29. 29 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Arithmetic Average Method Thiessen Polygon Method Distance weighting Isohyetal Method Computation methods

30. 30 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad If rainfall is uniformly distributed in areal pattern then this is the simplest method to estimate average rainfall over a catchment If P 1 , P 2 , P 3 , … P n etc are the precipitation or rainfall values measured at ‘n’ gauge stations, then Arithmetic mean method

31. 31 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Arithmetic mean method

32. 32 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad 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 Find average precipitation

33. 33 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Thiessen polygon method Rainfall recorded by each rain gauge weighted according to the area it is assumed to represent It is also called Weighted Mean Method

34. 34 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Thiessen polygon method Gauge no / name Precipitation (cm) Area (Km^2) AxP 1 25 64 1600 2 31 100 3100 ... … …

35. 35 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Thiessen polygon method

36. 36 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Draw area according to certain scale Connect all gauging stations Steps for polygon

37. 37 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad

38. 38 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Draw perpendicular bisectors of all the lines joining the rain gauge network Steps for polygon

39. 39 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad

40. 40 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Measure area of each polygon Calculate average precipitation Steps for polygon

41. 41 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad

42. 42 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad This method is based on the distance between the centroid of basin and gauge The weight given to the precipitation is inversely proportional to the square of the distance between centroid of basin and gauge point Distance weighting Example 6

43. 43 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad An isohyet is a line on a rainfall map of the basin, joining places of equal rainfall readings An isohyetal map shows contours of equal rainfall on the ground Gives more accurate picture of rainfall distribution Isohyetal method

44. 44 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad

45. 45 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Draw map of area Indicate points of rain gauges Write rainfall value at gauge points Draw isohyets Measure area enclosed or b/w every two isohyets Isohyetal method

46. 46 Engineering Hydrology (CE- 235) NUST Institute of Civil Engineering/ Engr Sajjad Ahmad Isohyet range Mean isohyetal value (cm) Area b/w isohyets (Km^2) A x P < 6 5.5 (assume) 10 55 6-7 6.5 50 325 7-8 7.5 70 525 8-9 8.5 60 510 9-10 9.5 50 475 >10 10.5 (assumed) 30 315 Isohyetal method

47. NUST Institute of Civil Engineering/Engr Sajjad Ahmad 47