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Groundwater investigation techniques 165

Fig. 5.20 Standard sectional method for calculating stream discharge using results from current metering. The cross-section is divided into
20 sections (or about 5% of the total width and containing less than 10% of the total discharge) and the current meter set in the middle of
each section (mid-section method) or at either side and the velocities averaged (mean section method).

Fig. 5.21 The steady-state or constant
rate injection dilution gauging method of
stream discharge measurement. In this
example, tracer of known concentration,
c, is dispensed from a Mariotte bottle
at a constant rate, q. The sample
concentration, C, is for a water sample
taken where complete mixing of the tracer
has occurred over the mixing length of the
stream flowing with steady discharge, Q.

cq = CQ eq. 5.15 that does not vary along the length of the reach and
also no losses of tracer, for example in dead zones
where c is the initial tracer concentration, q is the rate along the banks of the river.
of injection, C is the concentration in the stream at In the slug injection method, a known volume
some downstream point and Q is the unknown dis- of tracer solution, v, of known concentration, c, is
charge. Thus, the discharge can be determined from injected instantaneously into the main flow of the
a single sample taken at a point far enough down- stream. The concentration is measured in samples
stream for full mixing to have occurred over the taken at frequent intervals from a point far enough
stream cross-section (Fig. 5.21). The degree of mixing downstream for full mixing to have occurred. Sampling
must be checked first by taking several samples across should begin before the tracer arrives at the sampling
the section. The method assumes steady discharge point and end after the cloud of tracer has passed. As