Environmental Engineering Reference
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Fig. 5.4 Specific stream power plot showing channel response to straightening in several British rivers. Modified from
Brookes and Wishart (2006).
in excess of 25 to 35W/m 2 were likely to respond
through erosion (Fig. 5.4). However, it should be
noted that subsequent experience has shown that
specific stream power cannot be used in isolation
to infer either the potential for post-project adjust-
ment, or the type of instability likely to occur.
In the case of catchment-scale sediment as-
sessment, the purpose of screening is to allow
prioritization of reaches for further investigation,
with the limited resources being targeted on the
riskier locations, and more advanced, quantita-
tive investigations being reserved for those loca-
tions and situations that really merit such
advanced treatment. The advantage of stream
power analysis in this context is that it provides
a fairly rapid and low-cost means of assessing
sediment issues throughout an entire drainage
network, using readily accessible variables (spe-
cifically, the reach-averaged discharge, width and
slope, all at bankfull stage). However, while these
are in essence simple variables to characterize,
experience has shown that the accurate measure-
ment of slope is crucial and that care must be
exercised in selecting a slope value that reliably
represents bankfull conditions in the study reach
(Mant 2008).
springs from papers by Bagnold (1966, 1980). The
most widely used index of stream power is the
specific stream power, or stream power per unit
area of the bed ( v ), defined by:
v ¼ r
gQS
w
ð 5 : 1 Þ
where, water density (kg/m 3 ), g
¼ acceleration
¼ discharge (m 3 /s), S
due to gravity, Q
¼ water sur-
face slope and w
¼ a representative channel width;
the units of stream power are watts per square
metre (W/m 2 ). This represents the stream power
used by Bagnold (1966) in his sediment transport
analyses, although he omitted the gravitational
constant, g, in order that the units of specific
stream power should match those of sediment
transport per unit stream width (kg/m/s).
Brookes (1987a, 1987b) applied the stream pow-
er concept to investigate post-project readjust-
ment of straightened river channels in some
European countries. He found that straightened
reaches in rivers with less than 15 to 25W/m 2 of
specific stream power at the bankfull stage tended
to respond morphologically through processes led
by deposition, while those in streams with powers
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