sensors at the other side. By installing transmitters and receivers on
each bank along a line joining them at an angle across the stream,
the difference in travel times of the two pulses in the two opposite
directions is measured; this is related to the mean velocity of the
stream. This method again requires sophisticated electronic equip-
ment (Herschy, 1999) to process the data from the transmitters and
receivers, and can only be used in weed-free, clear water and stable
bed streams. It is expensive to install and operate, although a high
accuracy of flow measurement is possible.- Integrating floats.This technique is based on the principle of moving
floats, the floats being air bubbles released from nozzles located at
regular intervals from a compressed air line laid across the bed of the
stream. The areal spread (Sargent, 1981) of the rising bubbles at
the water surface (photographed with special equipment) gives the
measure of the mean velocity of the stream.
(e) Natural control
The natural control of a river reach is a particular section (gauging station)
where discharges and their corresponding stages are measured and a
unique relationship (called the rating curve) between them is established.
This relationship can then be used to estimate discharges from observed
stages.
The rating curve, once established, has to be checked periodically for
its validity and, if necessary, adjustments have to be made. Considerable
deviations may occur as a result of morphological changes of the river bed
(scouring, deposition, vegetal growth, etc.), the presence of flood waves
and any changes introduced along the reach of the river in the vicinity of
the control section, both upstream and downstream of it.
The rating curve generally assumes the form
Qa(H z)b (8.36)whereHis the stage, and a,b, and z(stage at zero discharge) are the para-
meters relating to the control section.
If three values of discharges Q 1 ,Q 2 , and Q 3 are selected from the
established rating curve such that Q 2 (Q 1 Q 3 )1/2, the stage at zero dis-
chargezcan be obtained from (WMO, 1980)
z(H 1 H 3 H^22 )/(H 1 H 3 2 H 2 ) (8.37)whereH 1 ,H 2 , and H 3 are the stages corresponding, respectively, to the
three discharges.