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Dilution Stream flow Gauging
Dilution gauging methods measure stream flow on the basis of the rate of dispersal of an introduced tracer, with the concentration of the tracer monitored downstream The method is used in streams with highly turbulent flow conditions where conventional stream velocity measurements are difficult to apply. Chemical tracers such as table salt (NaCl) can be monitored using an electrical conductivity (EC) meter or a specific ion electrode. The mixing of fluorescent dyes such as Rhodamine WT can be monitored using a fluorometer. Alternatively, water samples can be collected at different times for laboratory analysis.
The slug-injection method involves introducing a tracer of known volume and concentration into the stream in a single increment or slug. The concentration of the tracer and the time of measurement is monitored at a downstream location, before and after the slug is introduced. The resulting concentration hydrograph maps the dispersed passage of the tracer slug at the measurement point, with stream discharge (Q) represented as the area under the curve and calculated by integration:
(Equation 1)
Where ∀ is the known volume of tracer (∫), ct is the tracer concentration of the introduced solution, c0 is the background concentration in the stream, c is the changing tracer concentration as measured downstream, and t1 and t2 are the initial and final times of measurement.
As the name suggests, the constant-injection method relies on the tracer solution being introduced into the stream at a constant rate. The tracer concentration at the downstream measurement point should rise and stabilise to a constant level. The stream flow ( Q) can be calculated from:
(Equation 2)
where c1 is the stabilised tracer concentration at the downstream measurement point, and Qt is the tracer injection rate.