Extension of rating curves

It is always extremely difficult to obtain velocity measurements and hence estimated discharges at high stages. The range of the stage-discharge relationship derived from measurements is nearly always exceeded by flood flows. Hydrologists responsible for river gauging should make determined attempts to measure flood peaks, particularly at stations where the rating at high flows is in doubt. However, there are several techniques that can be adopted to assess the discharge at stages beyond the measured limit of the rating curve, but all extensions are strictly only valid for the same shape of cross-section and same boundary roughness.

Shift correction

Qate Recorded Measured Stage Corrected Estimated stage discharge correction stage discharge

May 1

"i

0,

A,

Mi +A,

Ol

2

h2

a'2

H2 +Aj

Q'2

3

m3

q3

a3

h3 + a3

q3

4

H4

a;

/y4 +A;

Q;

5

Hs

-A*

Ms +A'

01

6

m6

M6-

7

H-,

Q7

-A,

H-, -A7

q7

Fig. 7.24 Stout's method.

Fig. 7.24 Stout's method.

7.7.5.1 Logarithmic extrapolation

If the rating curve plots satisfactorily as a straight line on log-log paper, it may be extrapolated easily to the higher stages. However, in using this method, especially if the extrapolation exceeds 20 per cent of the largest gauged discharge, other methods should be applied to check the result. Alternatively, the straight-line equation fitted to the logarithmic rating curve could be used to calculate higher discharges, with similar reservations to check by another method.

7.7.5.2 Velocity-area method

In addition to the rating curve, plots of A versus H and V versus H can be drawn. The overall mean velocity across the channel V calculated from,

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