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that fills up during a small flooding event: so-called
bankfull discharge.
It is possible to estimate the average velocity of a
river stretch using a kinematic wave equation such
as Manning's (equation 5.5).
ESTIMATING STREAMFLOW
In the past thirty years probably the greatest effort
in hydrological research has gone into creating
numerical models to simulate streamflow. With
time these have developed into models simulating
all the processes in the hydrological cycle so that
far more than just streamflow can be estimated.
However, it is often streamflow that is seen as the
end-product of a model, a reflection of the impor-
tance streamflow has as a hydrological parameter.
These models are described in Chapter 6; this section
concentrates on direct estimates of streamflow.
23
kR
.
.
s
(5.5)
v
=
n
where v is velocity (m/s); k is a constant depending
on which units of measurement are being used
(1 for SI units, 1.49 for Imperial); R is the hydraulic
radius (m); s is the slope (m/m); and n is the
Manning roughness coefficient. Hydraulic radius is
the wetted perimeter of a river divided by the cross-
sectional area. In very wide channels this can be
approximated as mean depth (Goudie et al ., 1994).
The Manning roughness coefficient is estimated
from knowledge of the channel characteristics
(e.g. vegetation and bed characteristics) in a similar
manner to Chezy's roughness coefficient in Table
5.3. Tables of Manning roughness coefficient can
be found in Richards (1982), Maidment (1992),
Goudie et al . (1994), and in other fluvial geo-
morphological texts.
Physical or geomorphological
estimation techniques
The geomorphological approach to river systems
utilises the idea that the river channel is in equi-
librium with the flow regime. This suggests that
measures of the channel (e.g. depth/width ratio,
wetted perimeter , height to bankfull discharge )
can be used to estimate the streamflows in both a
historical and contemporary sense. Wharton (1995)
provides a review of these different techniques. This
is not a method that can be used to estimate the
discharge in a river at one particular time, but it can
be used to estimate parameters such as the mean
annual flood . Important parameters to consider are
the stream diameter, wetted perimeter and average
depth. This is particularly for the area of channel
Dilution gauging
Dilution gauging works on the principle that the
more water there is in a river the more it will dilute
a solute added into the river. There is a well-
Table 5.3 Chezy roughness coefficients for some typical streams
Type of channel
Chezy roughness coefficient
for a hydraulic radius of 1 m
Artificial concrete channel
71
Excavated gravel channel
40
Clean regular natural channel <30 m wide
33
Natural channel with some weeds or stones <30 m wide
29
Natural channel with sluggish weedy pools <30 m wide
14
Mountain streams with boulders
20
Streams larger than 30 m wide
40
Source : Adapted from Richards (1982)
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