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ESTABLISHING STAGE-DISCHARGE RELATION (1) WHY A STAGE-DISCHARGE RELATION? –FLOW IS THE VARIABLE OFTEN REQUIRED FOR HYDROLOGICAL ANALYSIS –CONTINUOUS MEASUREMENT.

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Presentation on theme: "ESTABLISHING STAGE-DISCHARGE RELATION (1) WHY A STAGE-DISCHARGE RELATION? –FLOW IS THE VARIABLE OFTEN REQUIRED FOR HYDROLOGICAL ANALYSIS –CONTINUOUS MEASUREMENT."— Presentation transcript:

1 ESTABLISHING STAGE-DISCHARGE RELATION (1) WHY A STAGE-DISCHARGE RELATION? –FLOW IS THE VARIABLE OFTEN REQUIRED FOR HYDROLOGICAL ANALYSIS –CONTINUOUS MEASUREMENT OF FLOW USUALLY IMPRACTICAL OR PROHIBITIVELY EXPENSIVE –STAGE OBSERVATIONS CONTINUOUSLY OR AT REGULAR SHORT TIME INTERVALS –STAGE OBSERVATION COMPARATIVELY EASY AND ECONOMICAL –RELATION BETWEEN STAGE AND DISCHARGE CAN BE ESTABLISHED: *THE DISCHARGE RATING CURVE OHS - 1

2 ESTABLISHING STAGE-DISCHARGE RELATION (2) GENERAL: –RATING CURVE ESTABLISHED BY CONCURRENT MEASUREMENTS OF STAGE h AND DISCHARGE Q COVERING EXPECTED RANGE OF RIVER STAGES AT SECTION OVER A PERIOD OF TIME –IF Q-h RATING CURVE NOT UNIQUE, THEN ADDITIONAL INFORMATION REQUIRED ON: *SLOPE OF WATER LEVEL (BACKWATER) *HYDROGRAPH h(t) (UNSTEADY FLOW) –Q-h EXTRAPOLATION MAY BE REQUIRED TO COVER FULL RANGE OF STAGES –RATING EQUATION IS USED TO TRANSFORM h(t) INTO Q(t) OHS - 2

3 OHS - 3

4 Analysis of stage-discharge data Station name : CHASKMAN Data from to Single channel Gauge Zero on =.000 m Number of data = 91 Power type of equation q=c*(h+a)**b is used Boundaries / coefficients lower bound upper bound a b c E E E+02 Number W level Q meas Q comp DIFf Rel.dIFf Semr M M3/S M3/S M3/S 0/0 0/ Overall standard error = Statistics per interval Interval Lower bound Upper bound Nr.of data Standard error OHS - 4

5 THE STATION CONTROL GENERAL: –THE SHAPE, RELIABILITY AND STABILITY OF THE Q-h RELATION ARE CONTROLLED BY A SECTION OR REACH OF CHANNEL AT AND/OR D/S OF GAUGING STATION = STATION CONTROL –ESTABLISHMENT OF Q-h RELATION REQUIRES UNDERSTANDING OF NATURE AND TYPE OF CONTROL AT A PARTICULAR STATION –ESTABLISHING A Q-h RELATION IS NOT SIMPLY CURVE FITTING OHS - 5

6 TYPES OF STATION CONTROLS CHARACTER OF RATING CURVE DEPENDS ON TYPE OF CONTROL, GOVERNED BY: –GEOMETRY OF THE CROSS-SECTION –PHYSICAL FEATURES OF THE RIVER D/S STATION CONTROLS CLASSIFIED IN MANY WAYS: –SECTION and CHANNEL CONTROLS –NATURAL and ARTIFICIAL CONTROLS –COMPLETE, COMPOUND and PARTIAL CONTROLS –PERMANENT and SHIFTING CONTROLS OHS - 6

7 CONTROL CONFIGURATION IN NATURAL CHANNEL OHS - 7

8 SECTION CONTROL OHS - 8

9 CHANNEL CONTROL (1) OHS - 9

10 BACKWATER EFFECT S LxLx h0h0 hxhx OHS - 10

11 CHANNEL CONTROL (2) EXTENT OF CHANNEL CONTROL: –FIRST ORDER APPROXIMATION OF BACKWATER EFFECT (rectangular channel): at x = 0: h 0 = h e +  h 0 at x = L x : h x = h e +  h x Backwater:  h x =  h 0.exp[(-3.S.L x )/(h e (1-Fr 2 )] Froude: Fr 2 = u 2 /(gh) often << 1 Manning: Q = K m Bh e 5/3 S 1/2 So with q = Q/B: h e = {q/(K m S 1/2 )} 3/5 ln(  h x /  h 0 ) = -3.S.L x /h e at:  h x /  h 0 = 0.05: L x = h e /S OHS - 11

12 ARTIFICIAL CONTROL OHS - 12

13 SHIFTING CONTROLS SHIFTING CONTROLS RESULT FROM: –SCOUR AND FILL IN AN UNSTABLE CHANNEL –GROWTH AND DECAY OF AQUATIC WEEDS –OVERSPILLING AND PONDING IN AREAS ADJOINING THE RIVER REQUIRES: –LARGE GAUGING EFFORT AND MAINTENANCE COST TO OBTAIN RECORD OF ADEQUATE QUALITY OHS - 13

14 FITTING RATING CURVES (1) SIMPLE RATING CURVE: DISCHARGE DEPENDS ON: * STAGE ONLY COMPLEX RATING CURVE: DISCHARGE DEPENDS ON: *STAGE, AND *SLOPE OF ENERGY LINE OR *RATE OF CHANGE OF STAGE WITH TIME OHS - 14

15 FITTING OF RATING CURVES (2) POSSIBLE CAUSE(S) OF SCATTER IN STAGE- DISCHARGE PLOT: –STATION AFFECTED BY VARIABLE BACKWATER –UNSTEADY FLOW EFFECTS –SCOURING/SEDIMENTATION OF BED –CHANGES IN VEGETATION CHARACTERISTICS –OBSERVATIONAL ERRORS OHS - 15

16 PERMANENT CONTROL OHS - 16

17 VARIABLE BACKWATER (1) OHS - 17

18 VARIABLE BACKWATER (2) OHS - 18

19 UNSTEADY FLOW OHS - 19

20 RIVER BED CHANGES OHS - 20

21 EFFECT OF VEGETATION OHS - 21

22 FITTING RATING CURVES (3) MAIN CASES: –SIMPLE RATING CURVE *SINGLE CHANNEL *COMPOUND CHANNEL –RATING CURVE WITH BACKWATER CORRECTION: *NORMAL FALL *CONSTANT FALL –RATING CURVE WITH UNSTEADY FLOW CORRECTION –RATING CURVE WITH SHIFT ADJUSTMENT OHS - 22

23 FITTING SINGLE CHANNEL SIMPLE RATING CURVE (1) TO BE CONSIDERED: –EQUATIONS USED –PHYSICAL BASIS EQUATION PARAMETERS –DETERMINATION OF DATUM CORRECTION –NUMBER AND RANGE OF RATING SEGMENTS –DETERMINATION OF RATING CURVE COEFFICIENTS –ESTIMATION OF UNCERTAINTY IN RATING CURVE OHS - 23

24 FITTING SINGLE CHANNEL SIMPLE RATING CURVE (2) EQUATIONS: –PARABOLIC TYPE: Q = c 2 (h + a) 2 + c 1 (h + a) +c 0 –POWER TYPE: Q = c(h + a) b log Q = log c + b log(h + a), Y = A + BX OHS - 24

25 FITTING OF SINGLE CHANNEL SIMPLE RATING CURVE (3) RELATION BETWEEN POWER TYPE RATING CURVE AND MANNING EQUATION MANNING: Q = K m AR 2/3 S 1/2 FOR RECTANGULAR X-SECTION: A = B.H R  H MANNING: Q  K m BS 1/2.H 5/3 POWER: Q = c(h + a) b SO: c = K m BS 1/2 h + a = H and b = 5/3 OHS - 25

26 FITTING OF SINGLE CHANNEL SIMPLE RATING CURVE (4) POWER b IN POWER TYPE RATING CURVE VARIES WITH SHAPE OF CROSS-SECTION: –RECTANGULAR: b = 1.7 –TRIANGULAR: b = 2.5 –PARABOLIC: b = 2.0 –IRREGULAR: 1.2 5 (,, ) OHS - 26

27 FITTING OF SINGLE CHANNEL SIMPLE RATING CURVE (5) DATUM CORRECTION a: Q = c(h + a) b so: Q = 0 for a = - h METHODS TO DETERMINE a: –TRIAL AND ERROR –ARITHMETIC PROCEDURE –COMPUTER-BASED OPTIMISATION OHS - 27

28 TRIAL AND ERROR PROCEDURE FOR a OHS - 28

29 ARITHMETIC PROCEDURE TO DETERMINE a JOHNSON PROCEDURE: –SELECT AT LOWER AND UPPER END OF ESTIMATED RATING CURVE Q 1 AND Q 3 WITH CORRESPONDING h 1 AND h 3 –DETERMINE: (Q 2 ) 2 = Q 1.Q 3 and h 2 = f(Q 2 ) SO: Q 1 /Q 2 = Q 2 /Q3 AND: (h 1 +a)/(h 2 +a) = (h 2 +a)/(h 3 +a) –YIELDING: a = (h 2 2 -h 1 h 3 )/(h 1 +h 3 -2h 2 ) OHS - 29

30 RATING CURVE SEGMENTS NUMBER AND RANGES : *DETERMINED BY DISTINCT CHANGES IN THE CROSS- SECTION AND HENCE CAN BE IDENTIFIED FROM GEOMETRY OF CROSS-SECTION OF CONTROL SECTION *CAN ALSO BE IDENTIFIED FROM DOUBLE LOGARITHMIC PLOT OF STAGE VERSUS DISCHARGE, SHOWN AS A DISTINCT BREAK (plot h-a 1 vs Q) *APPLY SOME OVERLAP WHEN FITTING PARAMETERS FOR EACH SEGMENT *SPLIT UP A SEGMENT IF CURVATURE IS CONSIDERABLE TO AVOID ODD b-VALUES OHS - 30

31 RATING CURVE SEGMENTS (2) CONTROL SECTION PROFILE BRIDGE SECTION PROFILE BREAK IN RATING CURVE OHS - 31

32 FITTING OF SINGLE CHANNEL SIMPLE RATING CURVE (8) h1h1 OHS - 32

33 DETERMINATION OF RATING CURVE COEFFICIENTS (1) PER SEGMENT ( FOR POWER TYPE Q = c(h+a) b ): –FIRST AN ESTIMATE FOR a IS MADE BY: * COMPUTERISED JOHNSON METHOD *OR FORCED BY USER –NEXT THE POWER b AND COEFFIENT c ARE ESTIMATED BY LEAST SQUARES METHOD ON THE LOGARITHMS OF Q AND (h+a) –PREVIOUS STEPS ARE REPEATED (IF a IS DETERMINED BY JOHNSON METHOD) TO OPTIMISE THE VALUES FOR a, b AND c, LEADING TO A MINIMUM LEAST SQUARES FOR VALUES OF a WITHIN 1 m OF FIRST ESTIMATE OHS - 33

34 DETERMINATION OF RATING CURVE COEFFICIENTS (2) OHS - 34

35 DETERMINATION OF RATING CURVE COEFFICIENTS (3) OHS - 35

36 DETERMINATION OF RATING CURVE COEFFICIENTS (4) OHS - 36

37 DETERMINATION OF RATING CURVE COEFFICIENTS (5) OHS - 37

38 DETERMINATION OF RATING CURVE COEFFICIENTS (6) FINALLY THE VALUES FOR b AND c FOLLOW FROM  AND  : b =  AND c = 10  OHS - 38

39 BREAK SEGMENT -1 SEGMENT-2 OHS - 39

40 Analysis of stage-discharge data Station name : KHED Data from to Single channel Given boundaries for computation of rating curve(s) interval lower bound upper bound nr. of data Power type of equation q=c*(h+a)**b is used Boundaries / coefficients lower bound upper bound a b c E E+02 Number W level Q meas Q comp DIFf Rel.dIFf Semr M M3/S M3/S M3/S 0/0 0/ Overall standard error = Statistics per interval Interval Lower bound Upper bound Nr.of data Standard error OHS - 40

41 STANDARD ERROR OF ESTIMATE IN STAGE-DISCHARGE RELATION OHS - 41

42 UNCERTAINTY IN RATING CURVE FIT STAGE-DISCHARGE EQUATION IS A LINE OF BEST FIT TO THE MEASUREMENTS THE CURVE PROVIDES A BETTER ESTIMATE THAN ANY OF THE INDIVIDUAL MEASUREMENTS POSITION OF THE LINE IS ALSO SUBJECT TO UNCERTAINTY: –STANDARD ERROR OF THE MEAN RELATIONSHIP Smr OHS - 42

43 CONFIDENCE LIMITS OF RATING CURVE Where: t = Student t-value at 95% probability P i = ln(h i + a) S 2 P = variance of P If n = 25 the: S mr  20% S e indicating the advantage of using the curve over the individual measurements OHS - 43

44 FITTING OF RATING CURVES IN HYMOS FOLLOWING STEPS ARE REQUIRED: *SELECT THE REQUIRED PERIOD AND STATION *CHECK THE MAXIMUM RANGE OF WATER LEVELS IN THE TIME PERIOD *INSPECT THE AVAILABLE STAGE DISCHARGE DATA TOGETHER WITH A REPRESENTATIVE CROSS-SECTION OF THE CONTROL *IDENTIFY THE BREAKS IN THE SCATTER PLOT *ELIMINATE OUTLIERS IF UNRELIABLE (MIND OTHER REASONS FOR SCATTER!!!) *SELECT EQUATION TYPE AND ‘a’ FORCED OR FREE *SELECT THE INTERVALS WITH OVERLAPS TO FORCE INTERSECTIONS *INSPECT THE PLOT AND THE TABULAR OUTPUT *REPEAT IF RESULT IS UNSATISFACTORY *SAVE THE CURVE PARAMETERS IF ACCEPTABLE OHS - 44

45 COMPOUND CHANNEL RATING CURVE (1) B BrBr hfhf hrhr Q river = (h r B r )(K mr h 2/3 S 1/2 and Q fp = h f (B-B r )(K mf h f 2/3 S 1/2 Q total = Q river + Q fp OHS - 45

46 COMPOUND CHANNEL RATING CURVE (2) OHS - 46

47 COMPOUND CHANNEL RATING CURVE (3) COMPUTATIONAL PROCEDURE (1): –FIRST THE RATING CURVE IS FITTED FOR THE MAIN CHANNEL UP TO BANKFULL LEVEL –THIS CURVE IS EXTENDED TO RIVER STAGES ABOVE BANKFULL LEVEL = Q r –ABOVE BANKFULL LEVEL: OBSERVED FLOWS Q obs ARE CORRECTED FOR MAINCHANNEL FLOW Q r TO OBTAIN FLOOD PLAIN FLOW ONLY = Q f : Q f = Q obs - Q r OHS - 47

48 COMPOUND CHANNEL RATING CURVE (4) COMPUTATIONAL PROCEDURE (2): –LAST WATER LEVEL RANGE IS USED TO FIT THE CURVE FOR THE FLOOD PLAIN FLOW Q f ALONE HENCE: –h < BANKFULL: Q = c 1 (h + a 1 ) b1 –h  BANKFULL Q = c 1 (h + a 1 ) b1 + c 2 (h + a 2 ) b2 OHS - 48

49 OHS - 49

50 RATING CURVE WITH BACKWATER CORRECTION NO UNIQUE STAGE-DISCHARGE CURVE WHEN STATION CONTROL IS AFFECTED BY OTHER CONTROLS DOWNSTREAM CAUSES: *FLOW REGULATION D/S *LEVEL IN MAIN RIVER OR TRIBUTARY AT CONFLUENCE *WATER LEVEL IN RESERVOIR D/S *VARIABLE TIDAL EFFECT *D/S CONSTRICTION WITH VARIABLE CAPACITY DUE TO WEED GROWTH *RIVERS WITH RETURN OF OVERBANK FLOW OHS - 50

51 BACKWATER EFFECT S LxLx h0h0 hxhx OHS - 51

52 CHANNEL CONTROL EXTENT OF CHANNEL CONTROL: –FIRST ORDER APPROXIMATION OF BACKWATER EFFECT (rectangular channel): at x = 0: h 0 = h e +  h 0 at x = L x : h x = h e +  h x Backwater:  h x =  h 0.exp[(-3.S.L x )/(h e (1-Fr 2 )] Froude: Fr 2 = u 2 /(gh) often << 1 Manning: Q = K m Bh e 5/3 S 1/2 So with q = Q/B: h e = {q/(K m S 1/2 )} 3/5 ln(  h x /  h 0 ) = -3.S.L x /h e at:  h x /  h 0 = 0.05: L x = h e /S OHS - 52

53 BACKWATER VARIABLE BACKWATER: CAUSES VARIABLE ENERGY SLOPE FOR THE SAME STAGE HENCE: DISCHARGE IS A FUNCTION OF BOTH STAGE AND OF SLOPE: SLOPE-STAGE-DISCHARGE RELATION GENERALLY: ENERGY SLOPE APPROXIMATED BY WATER LEVEL SLOPE OHS - 53

54 BACKWATER CORRECTION (1) FALL BETWEEN MAIN AND AUXILIARY STATION TAKEN AS MEASURE FOR SURFACE SLOPE m = MEASURED r = REFERENCE S = SLOPE F = FALL VALUE OF POWER P THEORETICALLY 0.5 OHS - 54

55 BACKWATER CORRECTION (2) TWO PROCEDURES FOR BACKWATER CORRECTION: –CONSTANT FALL METHOD *STAGE-DISCHARGE RELATION IS AFFECTED BY BACKWATER AT ALL TIMES –NORMAL (OR LIMITING) FALL METHOD *STAGE-DISCHARGE AFFECTED ONLY WHEN THE FALL REDUCES BELOW A GIVEN VALUE OHS - 55

56 CONSTANT FALL METHOD MANUAL PROCEDURE –SELECT AN AVERAGE FALL, CALLED THE REFERENCE FALL F r –CREATE A RATING CURVE h-Q r WHERE: Q r = Q/  (F m /F r ) –CREATE A SECOND RELATION FOR Q m /Q r = f(F m /F r ) –USE SECOND RELATION TO UPDATE Q r AND THE STAGE-DISCHARGE RELATION h-Q r, etc. USE: Q = Q r (F m /F r ) p with F m from observations F r from procedure Q r from rating curve OHS - 56

57 CONSTANT FALL METHOD OHS - 57

58 CONSTANT FALL RATING OHS - 58

59 CONSTANT FALL COMPUTATIONAL PROCEDURE FITTING: –FIRST A REFERENCE FALL IS SELECTED –A RATING CURVE IS FITTED BETWEEN h AND Qr –VALUE OF p IS OPTIMISED USE: –FOR GIVEN h AND FALL Fm, Qr AND Fr FROM THE STORED INFORMATION –DISCHARGE FROM SECOND RELATION OHS - 59

60 CONSTANT FALL METHOD WITH HYMOS (1) OHS - 60

61 CONSTANT FALL METHOD WITH HYMOS OHS - 61

62 NORMAL FALL METHOD FOR BACKWATER CORRECTION (1) MANUAL PROCEDURE: –PLOT STAGE AGAINST DISCHARGE AND MARK THE BACKWATER FREE MEASUREMENTS –FIT A RATING CURVE FOR THE BACKWATER FREE MEASUREMENTS: Qr-h RELATION –PLOT FALL VERSUS STAGE AND DRAW A LINE FOR THE NORMAL OR LIMITING FALL Fr –COMPUTE Qm/Qr AND Fm/Fr FOR EACH OBSERVATION AND DRAW AVERAGE CURVE –ADJUST THE CURVES BY HOLDING TWO CONSTANT AND PLOTTING THE THIRD, ETC. OHS - 62

63 NORMAL FALL METHOD FOR BACKWATER CORRECTION (2) OHS - 63

64 NORMAL FALL METHOD FOR BACKWATER CORRECTION (3) OHS - 64

65 NORMAL FALL METHOD FOR BACKWATER CORRECTION (4) OHS - 65

66 NORMAL FALL METHOD FOR BACKWATER CORRECTION (5) USE OF THE PROCEDURE WITH h AND F m GIVEN: –READ Fr FROM F r - h CURVE –CALCULATE F m /F r –READ Q/Q r FROM Q m /Q r - F m /F r RELATION –READ Q r FROM Q r - h RELATIONSHIP –MULTIPLY Q/Q r WITH Q r TO COMPUTE Q OHS - 66

67 NORMAL FALL METHOD FOR BACKWATER CORRECTION COMPUTATIONAL PROCEDURE: –COMPUTE BACKWATER FREE RATING CURVE –DERIVE Fr FROM Fm, Qm AND Qr –FIT PARABOLA TO Fr - h DATA –OPTIMISE PAR. p USE: –WITH ABOVE REATIONS FOR Qr-h AND Fr-h APPLY LAST EQUATION OHS - 67

68 RATING CURVE WITH UNSTEADY FLOW CORRECTION (1) NOTE: –WATER SURFACE SLOPE ON FRONT SIDE OF FLOOD WAVE STEEPER THAN ON BACK SIDE –DISCHARGE PROPORTIONAL WITH ROOT OF SLOPE HENCE: – FOR THE SAME STAGE, THE DISCHARGE IS LARGER FOR RISING STAGES THAN FOR FALLING STAGES –RATING CURVE HAS TO BE ADJUSTED TO ACCOMMODATE FOR THESE EFFECTS OHS - 68

69 RATING CURVE WITH UNSTEADY FLOW CORRECTION (2) Qm = measured discharge Qr = steady state discharge c = flood wave celerity S0 = bed slope (energy slope for steady flow) dh/dt = change of h per unit of time Procedure: –trial Qr - h relation is established from measurements where dh/dt = 0 –compute 1/cS0 and fit a relation for 1/cS0 = f(h) OHS - 69

70 RATING CURVE WITH UNSTEADY FLOW CORRECTION (3) CORRECTION REQUIRED IF FACTOR (1+1/cS 0.  h/  t) 1/ CORRECTION FACTOR HIGH WHEN: –BED SLOPE IS SMALL –CELERITY IS SMALL –  h/  t IS LARGE USE: –OBTAIN Q r VIA Q r -h FROM OBSERVED h –OBTAIN 1/cS 0 VIA 1/cS 0 -h FROM OBSERVED h –OBTAIN  h/  t FROM HYDROGRPAH –APPLY JONES FORMULA TO COMPUTE ACTUAL (UNSTEADY) FLOW OHS - 70

71 EXAMPLE UNSTEADY FLOW CORRECTION(1) OHS - 71

72 EXAMPLE UNSTEADY FLOW CORRECTION(2) OHS - 72

73 EXAMPLE UNSTEADY FLOW CORRECTION(3) OHS - 73

74 EXAMPLE UNSTEADY FLOW CORRECTION(4) OHS - 74

75 EXAMPLE UNSTEADY FLOW CORRECTION(5) OHS - 75

76 UNSTEADY FLOW WITH HYMOS (BEFORE CORRECTION) OHS - 76

77 UNSTEADY FLOW WITH HYMOS (WITH CORRECTION) OHS - 77

78 SHIFTING CONTROL (1) CONSIDERATION: –A STABLE CONTROL IS A DESIRABLE PROPERTY OF A GAUGING STATION –ALLUVIAL STREAM-BEDS ARE NOT STABLE DUE TO SILTATION AND SCOUR (MOVING DUNES AND BARS) –AS A CONSEQUENCE THE STAGE-DISCHARGE RELATION WILL VARY –EXTENT AND FREQUENCY OF VARIATION DEPENDS ON: *TYPICAL BED MATERIAL SIZE * FLOW VELOCITIES OHS - 78

79 OHS - 79

80 SHIFTING CONTROL (3) INDETERMINATE Q-h OHS - 80

81 SHIFTING CONTROL (4) ALTERNATIVE: u-R PLOT OHS - 81

82 SHIFTING CONTROL (5) APPROACHES FOUR POSSIBLE APPROACHES: –FITTING A SIMPLE RATING CURVE BETWEEN SCOUR EVENTS –VARYING THE SHIFT PARAMETER –APPLICATION OF STOUT’S SHIFT METHOD –FLOW DETERMINED FROM DAILY GAUGING OHS - 82

83 SHIFTING CONTROL (6) SIMPLE RATING BETWEEN EVENTS USE: –WHERE RATING SHOWS LONG PERIOD OF STABILITY –WHERE SUFFICIENT GAUGINGS PER PERIOD ARE AVAILABLE –WHERE SHIFTS IN RATING ARE EASILY IDENTIFIABLE: *PLOT DATA WITH DATE *FLOOD EVENTS CAUSE CHANGE *NOTES IN THE FIELD RECORD BOOK ON REASONS FOR SHIFT OHS - 83

84 SHIFTING CONTROL(7) VARYING SHIFT PARAMETER USE: –WHERE RATING SHOWS PERIODS OF STABILITY BUT INSUFFICIENT DATA ARE AVAILABLE FOR NEW RATING –THEN PARAMETER “a” IS ADJUSTED AS SHOWN LEFT: h r = rated h for Q m h m = observed stage for Q m CHECK APPLICABILITY OF  a FOR FULL OR PARTIAL RANGE OF h Q = c 1 (h+a 1 +  a) b1 Q=c 1 (h+a 1 ) b1 OHS - 84

85 SHIFTING CONTROL (8) STOUT’s METHOD (1) PROCEDURE: –FIT A MEAN RELATION FOR ALL POINTS IN PERIOD –DETERMINE hr FROM Qm –DETERMINE  h FOR INDIVIDUAL MEAS. –DETERMINE  h t BY LINEAR INTERPOLATION BETWEEN  h’s –  h t ARE USED TO CORRECT RATING h r =(Q m /c) 1/b - a  h = h r - h m Q t = c 1 (h t +  h t +a 1 ) b1  h t = f(  h i,  h j ) OHS - 85

86 SHIFTING CONTROL(9) STOUT’s METHOD (2) OHS - 86

87 SHIFTING CONTROL (10) STOUT’s METHOD (3) WHEN: –GAUGING IS FREQUENT –MEAN RATING IS REVISED PERIODICALLY –IF PREVIOUS METHODS DO NOT APPLY ASSUMPTION: –SHIFTS GRADUAL CHANGES IN RATING DRAWBACK: –ERRORS IN MEASUREMENT ARE MIXED DEVIATIONS DUE TO SHIFTS IN CONTROL –INDIVIDUAL MEASUREMENT ERRORS HAVE SEVERE CONSEQUENCES DIFFERENT FROM ORDINARY RATING CURVE OHS - 87

88 OHS - 88

89 OHS - 89

90 OHS - 90

91 SHIFTING CONTROL (11) DAILY GAUGING WHEN: –IF BROAD SCATTER IS AVAILABLE NEITHER FROM BACKWATER NOR FROM SCOUR –CALCULATED SHIFT IS ERRATIC –HENCE WHEN NON OF OTHER PROCEDURES APPLY NOTE: –IMPORTANT PARTS OF THE HYDROGRAPH MAY BE MISSED –BETTER TO RELOCATE THE STATION UNLESS URGENT NEED OHS - 91


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