Geoscience Reference
In-Depth Information
the other at 80 per cent of depth) and average the
two.
Where there is no velocity meter available it may
be possible to make a very rough estimate of stream
velocity using a float in the stream (i.e. the time
it takes to cover a measured distance). When using
this method allowance must be made for the fact
that the float is travelling on the surface of the
stream at a faster rate than water closer to the stream
bed.
The velocity-area method is an effective tech-
nique for measuring streamflow in small rivers,
but its reliability is heavily dependent on the
sampling strategy. The technique is also less reliable
in small, turbid streams with a rough bed (e.g.
mountain streams). Under these circumstances other
streamflow estimation techniques such as
dilution
gauging
may be more applicable (see streamflow
estimation section).
ship is derived through a series of velocity-area
measurements at a particular site while at the same
time recording the stage with a stilling well (see
Figure 5.9). As can be seen in Figure 5.8, the rating
curve is non-linear, a reflection of the river bank
profile. As the river fills up between banks it takes
a greater volume of water to cause a change in stage
than at low levels.
70
60
50
40
30
20
10
0
0
0.5
1
1.5
Stage (m)
Figure 5.8
A rating curve for the river North Esk in
Scotland based on stage (height) and discharge
measurements from 1963-1990.
Continuous streamflow
measurement
The methods of instantaneous streamflow measure-
ment described above only allow a single measure-
ment to be taken at a location. Although this can be
repeated at a future date it requires a continuous
measurement technique to give the data for a hydro-
graph. There are three different techniques that
can be used for this method: stage discharge rela-
tionships, flumes and weirs, and ultrasonic flow
gauging.
Figure 5.9
Stilling well to provide a continuous
measurement of river stage (height). The height of
water is measured in the well immediately adjacent to
the river.
Stage vs discharge relationship
River
stage
is another term for the water level or
height. Where multiple discharge measurements
have been taken (i.e. repeat measurements using
velocity-area method) it is possible to draw a rela-
tionship between river stage and discharge: the
so-called
rating curve
. An example of a rating curve
is shown in Figure 5.8. This has the advantage of
allowing continuous measurement of river stage
(a relatively simple task) that can then be equated
to the actual discharge. The stage discharge relation-
An accurate stage vs discharge relationship is
dependent on frequent and accurate measurement
of river discharge, and a static river bed profile. If
the river bed profile changes (e.g. during a large
flood event it may get scoured out or new sediment
deposited), the stage vs discharge relationship will
change and the historic relationship will no longer
be valid. This assumption of a static river bed profile
