Geoscience Reference
In-Depth Information
established relationship between the amount of the
tracer found naturally in the stream (
C
o
), the
concentration of tracer put into the river (
C
t
), the
concentration of tracer measured downstream after
mixing (
C
d
), and the stream discharge (
Q
). The type
of tracer used is dependent on the equipment
available; the main point is that it must be
detectable in solution and non-harmful to the
aquatic flora and fauna. A simple tracer that is often
used is a solution of table salt (NaCl), a conductivity
meter being employed to detect the salt solution.
There are two different ways of carrying out
dilution gauging that use slightly different
equations. The first puts a known volume of tracer
into the river and measures the concentration of the
'slug' of tracer as it passes by the measurement
point. This is referred to as gulp dilution gauging.
The equation for calculating flow by this method is
shown in equation 5.6.
Probably the most difficult part of dilution gaug-
ing is calculating the distance downstream between
where the tracer is injected and the river concen-
tration measuring point (the mixing distance). This
can be estimated using equation 5.8.
2
+
07
.
Cw
g
w
d
(5.8)
z
=
L
013
.
C
z
where
L
= mixing distance (m)
C
Z
= Chezy's
roughness coefficient
(see
Table 5.3)
w
= average stream width (m)
≈
9.8 m/s
2
)
g
= gravity constant (
d
= average depth of flow (m)
FLOODS
The term
flood
is difficult to define except in the
most general of terms. In a river a flood is normally
considered to be an inundation of land adjacent
to a river caused by a period of abnormally large
discharge or encroachment by the sea (see cover
photograph, Figure 5.13, and Plate 6), but even this
definition is fraught with inaccuracy. Flooding may
occur from sources other than rivers (e.g. the sea and
lakes), and 'abnormal' is difficult to pin down,
particularly within a timeframe. Floods come to our
attention through the amount of damage that they
cause and for this reason they are often rated on a
cost basis rather than on hydrological criteria.
Hydrological and monetary assessments of flooding
often differ markedly because the economic valua-
tion is highly dependent on location. If the area of
land inundated by a flooding river is in an expensive
region with large infrastructure then the cost will
be considerably higher than, say, for agricultural
land. Two examples of large-scale floods during the
1990s illustrate this point. In 1998 floods in China
caused an estimated US$20 billion of damage with
over 15 million people being displaced and 3,000
lives lost (Smith, 2001). This flood was on a similar
scale to one that occurred in the same region during
1954. A much larger flood (in a hydrological sense)
CV
CCt
Q
=
t
−
( )
(5.6)
∑
∆
d
o
where
Q
is the unknown streamflow,
C
is the
concentration of tracer either in the slug (
t
),
downstream (
d
), or background in the stream (
o
);
t
is the time interval. The denominator of this
equation is the sum of measured concentrations of
tracer downstream.
The second method uses a continuous injection
of tracer into the river and measures the concen-
tration downstream. The continuous injection
method is better than the slug injection method as
it measures the concentration over a greater length
of time, however it requires a large volume of the
tracer. Using the formula listed below the stream
discharge can be calculated using equation 5.7.
−
−
0
Qq
CC
CC
t
d
(5.7)
=
d
where
q
is the flow rate of the injected tracer (i.e.
injection rate) and all other terms are as for the gulp
injection method.
