A solution of a known strength of tracer (e.g. NaCl, rodamine dye,
etc.) is injected into the flow at a constant rate (or by sudden injection)
over the cross-section and samples are taken at the downstream end at
regular time intervals. As soon as the strength of the samples has obtained
a constant value (plateau level) the flow rate, Q, in the channel can be cal-
culated using the equationQqN (8.34)whereqis the rate of injection of the tracer and NC 1 /(C 2 C 0 ),C 1 being
the concentration of the injected solution, C 2 the plateau concentration of
the samples and C 0 the background concentration already present in the
stream.
The gulp (or sudden) injection or integration method results in the
following equation:QVC (^1)
t
(Ct C 0 )dt (8.35)
whereVis the volume of solution injected, and Ctis the concentration of
tracer at the sampling point during a period of time dt.
(c) Gauging structures
Discharges (modular flows) can also be measured using the standard
gauging structures (artificial control) such as the broad-crested weir, the
Venturi flume and the Crump weir, etc., with one single measurement of
the stage upstream of the structure with respect to its sill level. Section 8.5
provides further details.
(d) Modern techniques of river gauging
The existing methods of river gauging are not always suitable, e.g. for
navigable rivers with locks, and non-obtrusive techniques of gauging are
preferred in such cases (Cole, 1982).
- Electromagnetic flow gauge.A large coil buried across and beneath
the river bed is magnetized by an external source and the water
moving through the coil induces an electromotive force (e.m.f.)
which is proportional to the average velocity through the cross-
section. The probe system (Herschy, 1999) to sense the induced
e.m.f. is very sophisticated and needs highly skilled personnel. The
installation is expensive both to set up and to operate. - Ultrasonic flow gauge.Acoustic sound pulses transmitted through the
water from transmitters from one side of the river are received by