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HYDROLOGY PROJECTTechnical Assistance
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 CURVEOHS - 1
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
Stage-Discharge Relationship
Discharge (cumecs)2,0001,8001,6001,4001,2001,000800600400200
Wat
er L
evel
(m+m
.s.l)
599
598
597
596
595
594
OHS - 3
HYDROLOGY PROJECTTechnical Assistance
Analysis of stage-discharge data Station name : CHASKMAN Data from 1997 1 1 to 1997 12 30 Single channel Gauge Zero on 1997 7 29 = .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
594.00 595.19 -592.170 9.709 .7147E-03 595.19 595.95 -593.866 2.770 .1507E+02 595.95 600.00 -594.025 2.531 .2263E+02
Number W level Q meas Q comp DIFf Rel.dIFf Semr M M3/S M3/S M3/S 0/0 0/0
1 594.800 9.530 8.541 .989 11.58 3.75 2 595.370 36.480 46.661 -10.181 -21.82 2.12 3 596.060 127.820 136.679 -8.859 -6.48 2.90 4 596.510 231.400 226.659 4.741 2.09 2.06 5 598.080 738.850 783.019 -44.169 -5.64 3.63 6 597.700 583.340 610.359 -27.019 -4.43 3.03 Overall standard error = 5.904 Statistics per interval Interval Lower bound Upper bound Nr.of data Standard error 1 594.000 595.192 38 7.20 2 595.192 595.950 27 5.24 3 595.950 600.000 26 4.84
OHS - 4
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
CONTROL CONFIGURATION IN NATURAL CHANNEL
OHS - 7
HYDROLOGY PROJECTTechnical Assistance
SECTION CONTROL
OHS - 8
HYDROLOGY PROJECTTechnical Assistance
CHANNEL CONTROL (1)
OHS - 9
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
FITTING SINGLE CHANNEL SIMPLE RATING CURVE (2)
• EQUATIONS:– PARABOLIC TYPE:
Q = c2(h + a)2 + c1(h + a) +c0
– POWER TYPE:
Q = c(h + a)b
log Q = log c + b log(h + a),
Y = A + BX
OHS - 24
HYDROLOGY PROJECTTechnical Assistance
FITTING OF SINGLE CHANNEL SIMPLE RATING CURVE (3)
RELATION BETWEEN POWER TYPE RATINGCURVE AND MANNING EQUATION
MANNING:
Q = KmAR2/3S1/2
FOR RECTANGULAR X-SECTION:
A = B.H R H
MANNING: Q KmBS1/2.H5/3
POWER: Q = c(h + a)b
SO: c = KmBS 1/2 h + a = H and b = 5/3OHS - 25
HYDROLOGY PROJECTTechnical Assistance
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 <b<3 (TYPICALLY)
– COMPOUND: b > 5 ( ,, )
OHS - 26
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
COMPOUND CHANNEL RATING CURVE (1)
B
Br
hfhr
Qriver = (hrBr)(Kmrh2/3S1/2
and
Qfp = hf(B-Br)(Kmf hf 2/3S1/2
Qtotal = Qriver + Qfp
OHS - 45
HYDROLOGY PROJECTTechnical Assistance
COMPOUND CHANNEL RATING CURVE (2)
OHS - 46
HYDROLOGY PROJECTTechnical Assistance
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 = Qr
– ABOVE BANKFULL LEVEL:
OBSERVED FLOWS Qobs ARE CORRECTED FOR MAINCHANNEL FLOW Qr TO OBTAIN FLOOD PLAIN FLOW ONLY = Qf:
Qf = Qobs - Qr
OHS - 47
HYDROLOGY PROJECTTechnical Assistance
COMPOUND CHANNEL RATING CURVE (4)
COMPUTATIONAL PROCEDURE (2):– LAST WATER LEVEL RANGE IS USED TO FIT
THE CURVE FOR THE FLOOD PLAIN FLOW Qf
ALONE
HENCE:– h < BANKFULL:
Q = c1(h + a1)b1
– h BANKFULL
Q = c1(h + a1)b1 + c2(h + a2)b2
OHS - 48
HYDROLOGY PROJECTTechnical Assistance
Rating curve (compound channel)
Rating Curve Measurements
Discharge (m3/s)600550500450400350300250200150100500
Wat
er L
evel
(m+M
SL
)53
52
51
50
49
48
47
46
45
OHS - 49
HYDROLOGY PROJECTTechnical Assistance
RATING CURVE WITH BACKWATER CORRECTION
NO UNIQUE STAGE-DISCHARGE CURVEWHEN
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
HYDROLOGY PROJECTTechnical Assistance
BACKWATER EFFECT
S
Lx
h0
hx
OHS - 51
HYDROLOGY PROJECTTechnical Assistance
CHANNEL CONTROL
EXTENT OF CHANNEL CONTROL:– FIRST ORDER APPROXIMATION OF
BACKWATER EFFECT (rectangular channel):
at x = 0: h0 = he + h0
at x = Lx: hx = he +hx
Backwater: hx = h0.exp[(-3.S.Lx)/(he(1-Fr2)]
Froude: Fr2 = u2/(gh) often << 1
Manning: Q = KmBhe5/3S1/2
So with q = Q/B: he = {q/(KmS1/2)}3/5
ln(hx/h0) = -3.S.Lx/he
at: hx/h0 = 0.05: Lx = he/S OHS - 52
HYDROLOGY PROJECTTechnical Assistance
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 SLOPEOHS - 53
HYDROLOGY PROJECTTechnical Assistance
BACKWATER CORRECTION (1)
ASRKQ m2/13/2
• 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
p
r
m
p
r
m
r
m
F
F
S
S
Q
Q
OHS - 54
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
CONSTANT FALL METHOD• MANUAL PROCEDURE
– SELECT AN AVERAGE FALL, CALLED THE REFERENCE FALL Fr
– CREATE A RATING CURVE h-Qr WHERE:
Qr = Q/(Fm/Fr)
– CREATE A SECOND RELATION FOR
Qm/Qr = f(Fm/Fr)
– USE SECOND RELATION TO UPDATE Qr AND THE STAGE-DISCHARGE RELATION h-Qr, etc.
• USE: Q = Qr(Fm/Fr)p with Fm from observations
Fr from procedure
Qr from rating curveOHS - 56
HYDROLOGY PROJECTTechnical Assistance
CONSTANT FALL METHOD
OHS - 57
HYDROLOGY PROJECTTechnical Assistance
CONSTANT FALL RATING
OHS - 58
HYDROLOGY PROJECTTechnical Assistance
CONSTANT FALL COMPUTATIONAL PROCEDURE
p
m
rmr F
FQQ
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
p
r
mr F
FQQ
OHS - 59
HYDROLOGY PROJECTTechnical Assistance
CONSTANT FALL METHOD WITH HYMOS (1)
OHS - 60
HYDROLOGY PROJECTTechnical Assistance
CONSTANT FALL METHOD WITH HYMOS
OHS - 61
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
NORMAL FALL METHOD FOR BACKWATER CORRECTION (2)
OHS - 63
HYDROLOGY PROJECTTechnical Assistance
NORMAL FALL METHOD FOR BACKWATER CORRECTION (3)
OHS - 64
HYDROLOGY PROJECTTechnical Assistance
NORMAL FALL METHOD FOR BACKWATER CORRECTION (4)
OHS - 65
HYDROLOGY PROJECTTechnical Assistance
NORMAL FALL METHOD FOR BACKWATER CORRECTION (5)
USE OF THE PROCEDURE WITH h AND Fm
GIVEN:
– READ Fr FROM Fr - h CURVE
– CALCULATE Fm/Fr
– READ Q/Qr FROM Qm/Qr - Fm/Fr RELATION
– READ Qr FROM Qr - h RELATIONSHIP
– MULTIPLY Q/Qr WITH Qr TO COMPUTE Q
OHS - 66
HYDROLOGY PROJECTTechnical Assistance
NORMAL FALL METHOD FOR BACKWATER CORRECTION
p
m
rmr Q
QFF
/1
• 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
2hchbaFr
p
r
mr F
FQQ
OHS - 67
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
RATING CURVE WITH UNSTEADY FLOW CORRECTION (2)
dt
dh
ScQQ rm
0
11
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)
dtdh
Scrm
/
1)/(1 2
0
2
0
1chhba
Sc
OHS - 69
HYDROLOGY PROJECTTechnical Assistance
RATING CURVE WITH UNSTEADY FLOW CORRECTION (3)
• CORRECTION REQUIRED IF FACTOR (1+1/cS0.h/t)1/2 < 0.95 OR >1.05
• CORRECTION FACTOR HIGH WHEN:– BED SLOPE IS SMALL
– CELERITY IS SMALL h/t IS LARGE
• USE:– OBTAIN Qr VIA Qr-h FROM OBSERVED h
– OBTAIN 1/cS0 VIA 1/cS0-h FROM OBSERVED h
– OBTAIN h/t FROM HYDROGRPAH
– APPLY JONES FORMULA TO COMPUTE ACTUAL (UNSTEADY) FLOW OHS - 70
HYDROLOGY PROJECTTechnical Assistance
EXAMPLE UNSTEADY FLOW CORRECTION(1)
OHS - 71
HYDROLOGY PROJECTTechnical Assistance
EXAMPLE UNSTEADY FLOW CORRECTION(2)
OHS - 72
HYDROLOGY PROJECTTechnical Assistance
EXAMPLE UNSTEADY FLOW CORRECTION(3)
OHS - 73
HYDROLOGY PROJECTTechnical Assistance
EXAMPLE UNSTEADY FLOW CORRECTION(4)
OHS - 74
HYDROLOGY PROJECTTechnical Assistance
EXAMPLE UNSTEADY FLOW CORRECTION(5)
OHS - 75
HYDROLOGY PROJECTTechnical Assistance
UNSTEADY FLOW WITH HYMOS(BEFORE CORRECTION)
OHS - 76
HYDROLOGY PROJECTTechnical Assistance
UNSTEADY FLOW WITH HYMOS(WITH CORRECTION)
OHS - 77
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical AssistanceOHS - 79
HYDROLOGY PROJECTTechnical Assistance
SHIFTING CONTROL (3)INDETERMINATE Q-h
OHS - 80
HYDROLOGY PROJECTTechnical Assistance
SHIFTING CONTROL (4)ALTERNATIVE: u-R PLOT
OHS - 81
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical Assistance
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:
hr = rated h for Qm
hm = observed stage for
Qm
CHECK APPLICABILITY
OF a FOR FULL OR
PARTIAL RANGE OF h
nhha m
n
ir /)(
1
Q = c1(h+a1+a)b1
Q=c1(h+a1)b1
OHS - 84
HYDROLOGY PROJECTTechnical Assistance
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 ht BY LINEAR INTERPOLATION BETWEEN h’s
ht ARE USED TO CORRECT RATING
hr =(Qm/c)1/b - a
h = hr - hm
Qt = c1(ht+ht+a1)b1
ht = f(hi, hj)
OHS - 85
HYDROLOGY PROJECTTechnical Assistance
SHIFTING CONTROL(9)STOUT’s METHOD (2)
OHS - 86
HYDROLOGY PROJECTTechnical Assistance
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
HYDROLOGY PROJECTTechnical AssistanceOHS - 88
HYDROLOGY PROJECTTechnical AssistanceOHS - 89
HYDROLOGY PROJECTTechnical AssistanceOHS - 90
HYDROLOGY PROJECTTechnical Assistance
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