Environmental Engineering Reference
In-Depth Information
Fig. 3.16 Typical set up for the constant-rate injection method of tracer-dilution gaging. The
tracer solution is released in the center of a stream section using a Marriote bottle system. The
background value of tracer concentration is monitored at an upstream point, and the fully mixed
value of tracer concentration is monitored at a downstream point. The set up is the same for the
sudden injection method with the exception that the Mariotte bottle is not used because the tracer is
instantaneously poured into the stream (From Dingman (
2002
) modified with kind permission of
©
S. Lawrence Dingman
2002
. All Rights Reserved. The figure was created based on an illustra-
tion originally appearing in Gregory and Walling (
1973
:134))
tracer-dilution methods are generally more difficult to use than current-meter
methods, and under most conditions the results are less reliable. Therefore, these
methods should not be used when conditions are favorable for a current-meter
measurement (Rantz
1982
:212).
In a typical tracer-dilution measurement, a tracer solution is injected into the
stream and the stream discharge is estimated from measurements of the tracer-
solution concentration, tracer-solution injection rate, and tracer concentrations at a
sampling cross section downstream from the injection site (Fig.
3.16
). Two
methods are commonly used; the constant-rate injection and the sudden injection.
For both methods, it is assumed that the tracer is completely mixed at the down-
stream measurement point. In the constant-rate injection (CRI) method, a tracer of
known concentration
C
1
(kg/m) is injected at a constant rate
q
(m
3
/s) in the center of
a stream channel using a constant-flow device such as Mariotte bottle (e.g., Moore
2004
). If the injection is continued for a sufficiently long period, monitoring of
concentration at a downstream sampling cross section will show a plateau of
constant concentration
C
2
(kg/m). Discharge (
Q
) is estimated from the dilution
ratio by:
Q
¼
qC
1
ð
C
2
Þ
=
ð
C
2
C
b
Þ
(3.23)
where
C
b
(kg/m) is the background tracer concentration in the stream.
In the sudden-injection or slug-injection (SI) method, a slug of tracer solution is
instantaneously applied to a stream, and concentration is monitored at a down-
stream sampling cross section to generate a concentration-time curve (Fig.
3.17
).
The total mass of the injected tracer must equal the total mass of tracer going
