vessel. The sounder works on a principle based on the propagation speed
of sound in water, and measurement of the corresponding time lapse.
Since the distance recorded is the shortest distance between the transducer
and river bed, care must be taken in using the sounders in an irregular
river bed. In such cases high-frequency sounders (small angle of spread
from transducer) are preferable. Optical instruments such as the sextant
and the range finder, and electronic instruments such as a radiolog system,
are commonly used to establish the location of sounded points. Appropri-
ate data processing (hand or computerized) finally produces the bed
profile.
8.4.4 Discharges(a) Area–velocity methods
- Floats.River discharges are usually computed from velocity mea-
surements, the simplest method being timing the movement of a float
over a known distance. Several types of float are in use, providing
data from which mean flow velocities can be derived (BSI, 1969a, b,
1986; Herschy, 1999). The float technique is more often used for
reconnaissance estimates of flow. Surface floats are very useful to
identify flow patterns, e.g. in estuarial surveys. - Current meters.The flow velocities are measured using current
meters at systematically distributed points over the cross-section.
The area of the cross-section is determined from soundings and the
discharge obtained as:
Q∑AiVi. (8.33)In the case of shallow streams, a miniature type current meter
attached to a graduated staff may be operated by the gauger wading
across the section, measuring both the stages and velocities simultan-
eously. Wide rivers may be gauged from a boat held in position along
a fixed alignment across the section. Sometimes, permanent installa-
tions such as a cable car (gauger travels with car and operates current
meter) or a cableway (gauger operates current meter from the bank
with mechanical winches) are used to carry out velocity and water
level measurements.(b) Dilution methods
Dilution gauging is an alternative solution in streams with steep gradients,
and in shallow torrents where conventional current metering techniques
cannot be used.