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EXTRAPOLATION OF RATING CURVE

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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EXTRAPOLATION OF RATING CURVE

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  1. 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

  2. HIGH FLOW EXTRAPOLATION • METHODS: • DOUBLE LOG PLOT METHOD • STAGE-AREA/STAGE-VELOCITY METHOD • THE MANNING’S EQUATION APPROACH • THE CONVEYANCE SLOPE METHOD OHS - 2

  3. 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 OHS - 3

  4. X-SECTION KHAMGAON, 1997 OHS - 4

  5. OHS - 5

  6. 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 OHS - 6

  7. ILLUSTRATION OF STAGE-AREA/STAGE-VELOCITY METHOD OHS - 7

  8. 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 MANNING’S EQUATION METHOD OHS - 8

  9. MANNING’S EQUATION METHOD OHS - 9

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  13. 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 CONVEYANCE-SLOPE METHOD OHS - 13

  14. OHS - 14

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  16. INTERPRETATION OF WETTED PERIMETER FLOOD PLAIN RIVER OHS - 16

  17. 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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