H Y D R O L O G Y P R O JE C T Technical Assistance EXTRAPOLATION OF RATING CURVE • WHY: – GAUGING DOES NOT COVER FULL RANGE OF FLOW – CALIBRATION AT HIGH FLOW IS DIFFICULT: * OCCURS INFREQUENTLY * SHORT DURATION – AT HIGH STAGES A DIFFERENT CONTROL MAY EXIST (FLOODPLAIN, DIFFERENT ROUGHNESS) • WHERE: – EXTRAPOLATION AT STATE DATA PROCESSING CENTRE OHS - 1
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EXTRAPOLATION OF RATING CURVE WHY: –GAUGING DOES NOT COVER FULL RANGE OF FLOW –CALIBRATION AT HIGH FLOW IS DIFFICULT: *OCCURS INFREQUENTLY *SHORT DURATION.
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HYDROLOGY PROJECTTechnical Assistance
EXTRAPOLATION OF RATING CURVE
• WHY:– GAUGING DOES NOT COVER FULL RANGE OF
FLOW
– CALIBRATION AT HIGH FLOW IS DIFFICULT:* OCCURS INFREQUENTLY
* SHORT DURATION
– AT HIGH STAGES A DIFFERENT CONTROL MAY EXIST (FLOODPLAIN, DIFFERENT ROUGHNESS)
• WHERE:– EXTRAPOLATION AT STATE DATA
PROCESSING CENTRE
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HYDROLOGY PROJECTTechnical Assistance
HIGH FLOW EXTRAPOLATION
• METHODS:
– DOUBLE LOG PLOT METHOD
– STAGE-AREA/STAGE-VELOCITY METHOD
– THE MANNING’S EQUATION APPROACH
– THE CONVEYANCE SLOPE METHOD
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HYDROLOGY PROJECTTechnical Assistance
DOUBLE LOG PLOT METHOD
• CONDITIONS:– HYDRAULIC AND GEOMETRIC
CHARACTERISTICS DO NOT CHANGE BEYOND MEASURED RANGE
• IMPLICATION:– POWER TYPE RATING CURVE FITTED TO THE
LAST (HIGHEST) SEGMENT WILL BE CONSIDERED APPLICABLE TO EXTRAPOLATED RANGE AS WELL
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HYDROLOGY PROJECTTechnical Assistance
X-SECTION KHAMGAON, 1997
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HYDROLOGY PROJECTTechnical Assistance
STAGE-AREA/STAGE-VELOCITY METHOD
• USE:– TO EXTRAPOLATE RATING CURVE BEYOND
MEASURED RANGE BASED ON:* MEASURED STAGE-X-SECTIONAL AREA RELATION
* EXTRAPOLATED STAGE-VELOCITY PLOT
• NOTE:– ASSUMPTION IS THAT STAGE-VELOCITY PLOT
CAN EASILY BE EXTRAPOLATED SINCE RATE OF INCREASE OF VELOCITY AT HIGHER STAGES DIMINISHES RAPIDLY
– FROM A HYDRAULIC POINT OF VIEW ASSUMPTION DOUBTFULL SINCE U::R^2/3
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ILLUSTRATION OF STAGE-AREA/STAGE-VELOCITY
METHOD
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MANNING’S EQUATION METHOD
METHOD– K* = f(h) PLOTTED
FROM DISCHARGE MEASUREMENTS
– K* BECOMES APPROX. CONSTANT FOR h>>
– K* WITH Ar AND Rr FROM MEASUREMENTS EXTRAPOLATED
– FLOOD PLAIN TREATED BY MANNING EQUATION
– S CAN BE ESTIMATED FROM TOPOSHEETS
2/13/2 SRKu m
3/2*
3/2*
2/1*
R
uK
RKu
SKK m
fpriver
fpfpfpmfp
rrriver
QQQ
SRAKQ
RAKQ
2/1
,
3/2*
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MANNING’S EQUATION METHOD
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HYDROLOGY PROJECTTechnical Assistance
CONVEYANCE-SLOPE METHOD
• METHOD:– CONVEYANCE AND
ENERGY SLOPE ARE EXTRAPOLATED SEPARATELY
– A AND R FROM FIELD SURVEY
– Km ESTIMATED IN FIELD
– K EXTRAPOLATED
– S FOLLOWS FROM (Q/K)2 VERSUS h
– EXISTING S-TENDENCY EXTRAPOLATED
3/2
2/1
2/13/2
ARKK
with
KSQ
or
SARKQ
m
m
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INTERPRETATION OF WETTED PERIMETER
FLOOD PLAINRIVER
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EXTRAPOLATION WITH HYMOS
• FIRST:– COMPUATATION OF GEOMETRIC AND
HYDRAULIC QUANTITIES IN MEASRED RANGE
• NEXT:– PLOT OF FOLLOWING PARAMETERS AS f(h)
* SURFACE WIDTH
* WETTED PERIMETER
* X-SECTIONAL AREA
* HYDRAULIC RADIUS
* AR2/3, Q, u, conveyance K, AND S
• FINALLY:– K EXTRAPOLATED AND S ASYMPTOTICALLY
EQUAL TO BEDSLOPE: Q = KS1/2 OHS - 17
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HYDROLOGY PROJECTTechnical Assistance
Computation of cross-sectional parameters
Station : KHAMGAON Date : 1997 1 1
Section 1
Left bound -160.00 m, right bound .00 m from initial point Water boundaries included
K-Manning: 30.0, Sqrt(S).K-Manning: .4135
Stage Width Wetted Area Hydraulic A*R**2/3 Q Perimeter radius m m m sq-m m m**(8/3) m3/s