Discharge by dilution gauging

This method of measuring the discharge in a stream or pipe is made by adding a chemical solution or tracer of known concentration to the flow and then measuring the dilution of the solution downstream where the chemical is completely mixed with the stream water (BS 3680-2A, 1995).

In Fig. 7.19, co, ci and c2 are chemical concentrations (e.g. gL-1). co is the 'background' concentration already present in the water (and may be negligible); ci is the d Constant injection method d Constant injection method

Fig. 7.19 Dilution gauging: two basic methods.

known concentration of tracer added to the stream at a constant rate q; and c2 is a sustained final concentration of the chemical in the well-mixed flow. Thus,


An alternative to this constant rate injection method is the 'gulp' injection, or integration, method. A known volume of the tracer V of concentration cj is added in bulk to the stream and, at the sampling point, the varying concentration, c2, is measured regularly during the passage of the tracer cloud. Then:

r ti

So that,

The chemical used should have a high solubility, be stable in water and be capable of accurate quantitative analysis in dilute concentrations. It should also be non-toxic to fish and other forms of river life, and be unaffected itself by sediment and other natural chemicals in the water. A favoured chemical is common salt (NaCl), which has a solubility of 3.6 kg in 10 L water at 15°C. This has the added advantage that a solution containing common salt at its water solubility has a high electrical conductivity. It allows electrical conductivity monitoring to be used instead of water sampling, with salt concentrations then being derived from a calibration. Fluorescent dyes such as Rhodamine WT have also been developed as tracers (BS 3680-2D, 1993) with the advantage of being easily detected at very low concentrations. This allows dilution gauging to be undertaken on large rivers where the volume of common salt tracer needed would be impractical and prohibited on environmental grounds.

Careful preparations are needed and the required mixing length, dependent on the state of the stream, must be assessed first. Several methods can be used to assess the minimum mixing length (L); two commonly used equations,

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