Geology Reference
In-Depth Information
where
T
is the duration of runoff, and
Q
is recog-
nised to be a function of time (
Q
(
t
)). Then the
total soil loss (
SL
) during an erosion event is
given by:
Then from equations (11.16) and (11.17) it follows
that
2.5
æ
1.4
ö
S
Q
-
1
Q
=
ç
(m s
)
÷
(11.18)
e
S
Q
è
ø
b
-
2
SL
=SD
c
Q t
(kg m
)
(11.12)
t
Thus, use of
Q
e
provides an alternative method of
calculating
−
t
(and also
b
). Of course if
Q
is meas-
ured at small time intervals, there is no concep-
tual advantage in using equations (11.17) and
(11.18) rather than Equation (11.10) to calculate
−
t
and
b
, since the two methods are equivalent.
However, the major advantage of using
Q
e
as a
single hydrological driver is in allowing its pre-
diction in situations where
Q
(
t
) is not measured.
As shown in Yu
et al
. (1997a,b, 1999), and Yu and
Rose (1999),
Q
e
can be estimated with much less
weather data than is required to predict the com-
plete hydrograph of
Q
(
t
).
If the erodibility parameter
b
has been deter-
mined or can be estimated, then GUEST can be
used to explore predictive scenarios (Yu and Rose,
1997). Yu
et al
. (1997b) provide and illustrate
methodologies for six different soil erosion pre-
diction scenarios with decreasing quality of avail-
able data.
As described under experimental methods
(Section 11.5), in many applications of GUEST,
Q
was measured as a function of time (i.e.
Q
(
t
) ).
However, GUEST has also been applied when the
runoff rate is not measured. Yu
et al
. (1997a,b,
1999) and Yu and Rose (1999) describe a number
of alternative methods of employing GUEST
where runoff rate is not measured (as, for exam-
ple, in experiments using USLE-type methodol-
ogy). Most of these methods involve use of an
'effective runoff rate'
Q
e
, which is now derived.
It follows from Equation (11.8) and Manning's
equation that:
0.4
-
3
t
ckQ
=
(kg m
)
(11.13)
where
k
is a constant for any given plot character-
istic and soil, and is given by:
3/5
s
sr f
FS
æ
L S
2/3
1/2
ö
-
3.4
0.4
k
=
(kg m
s
)
(11.14)
ç
÷
(/
-
)
n
è
ø
11.4
Subsequent Development of GUEST
With sediment concentration at the transport
limit, the total soil loss during an erosion event is
equal to
During experience gained in processing very large
bodies of experimental data, some further refine-
ment of GUEST took place, although the effect
of such refinement on results obtained is notice-
able only in rather unusual experimental condi-
tions. These refinements (described in Yu &
Rose, 1999, and in Presbitero
et al
., 2005) include
recognition of the following factors, which are
probably of very small or possibly negligible
effect in many applications. Firstly, at very high
sediment concentrations, sediment can signifi-
cantly enhance fluid density above that of water
(see Chapter 12 for a case study of such situa-
tions). Secondly, at shallow depths of flow, some
of the larger water-stable aggregates may be inad-
equately submerged to participate in saltation.
Another factor associated with high sediment
0.4
-
2
S
kQ
Q t
D
(kg m
)
(11.15)
So, since
k
is a constant within any event, the
event average value of
c
t
is given by:
1.4
Q
ck
Q
S
-
3
=
(kg m
)
(11.16)
t
S
Thus a single effective steady state runoff rate
(
Q
e
), which is required to compute the average
sediment concentration at the transport limit
during an erosion event, can be invoked such
that:
0.4
-
3
ckQ
=
(kg m
)
(11.17)
t
e
