Geology Reference
In-Depth Information
was developed significantly for flow-driven
erosion by Hairsine and Rose (1992a,b), and by
Hairsine and Rose (1991) for rainfall-driven ero-
sion. This original version of program GUEST,
referred to as Type A, was described by Misra and
Rose (1996), and incorporated the simultaneous
effects of rainfall, runoff, and deposition on sedi-
ment concentration. The Type A version of
GUEST combined the steady-state solutions of
the Hairsine and Rose theories for both rainfall
and flow-driven types of erosion processes acting
together. It was assumed that the sediment con-
centration produced by rainfall could simply be
added to that produced by overland flow whose
stream power exceeded a threshold value.
The theory on which GUEST is based begins
with a governing equation for soil erosion, trans-
port and deposition:
processes can simply be added together. In the
practical applications referred to in this chapter,
the steady-state situation is assumed, so that the
time-variant term in Equation (11.1) is ignored.
The effectiveness of this Type A version of
GUEST was much enhanced by its implementation
as a Fortran-based computer program (Misra &
Rose, 1989, 1992), and this was used to assess
parameter sensitivity for both rainfall and runoff-
driven processes (Misra & Rose, 1996), and the rela-
tionship between erodibility parameters and soil
strength (Misra & Rose, 1995). This version of
GUEST was also used both to assess the relative
importance of the range of factors governing soil
loss, and their interaction, and in evaluating the
likely effectiveness of soil conservation options.
Furthermore, this computer program greatly facili-
tated extraction of soil erodibility parameters from
experimental data collected when erosion was due
to either, or both, rainfall-driven or flow-driven
processes. Following extensive experimental inves-
tigation it was shown that in many situations of
significant soil erosion, flow-driven erosion was
dominant over that due to rainfall, at least in sedi-
ment mass terms. This is not, however, to down-
grade the significance of rainfall-driven processes,
especially in structural breakdown and chemical
enrichment, and in low slope contexts where
rainfall-driven processes can dominate.
In soil erosion investigations, a newly designed
facility called the Griffith University Tilting Flume
Simulated Rainfall Facility (or GUTSR) (Misra &
Rose, 1995) provided valuable data on erosion con-
sequences for a wide range of soil types and condi-
tions and erosion contexts (e.g. Proffitt & Rose,
1991a,b; Proffitt
et al
., 1991, 1993a,b; Hairsine &
Rose, 1991, 1992a,b; Misra & Rose, 1995, 1996).
Experiments using this facility showed that even
during constant flow-driven situations, sediment
concentration can fluctuate with time between
recognizable upper and lower limits. The upper
limit can be associated with the so-called 'trans-
port limit' (Foster, 1982). The lower limit, where
the strength of the soil matrix controls sediment
concentration, has been termed the 'source limit'
(Hairsine & Rose, 1992a,b). The transport limit can
be achieved, for example, when rill wall collapse
provides a ready supply of weak, eroded sediment.
¶
(
cD
)
¶
(
cq
)
i
+
i
=
ee rr d
+
+ +
-
(11.1)
i
ri
i
ri
i
¶
t
¶
x
where
D
is water depth (m),
q
is unit discharge
(flow rate per unit flow width, m
2
s
−1
),
c
i
is the
sediment concentration for particle size class
i
(kg
m
−3
),
e
i
and
e
ri
are rates of rainfall detachment and
re-detachment (kg m
−2
s
−1
),
r
i
and
r
ri
are rates of
flow entrainment and re-entrainment (kg m
−2
s
−1
),
and
d
i
is the rate of deposition (kg m
−2
s
−1
). The
detachment and entrainment terms are related to
the process of dislodging primary particles and
aggregates from the original soil. The dislocated
particles and aggregates are continuously being
returned to the soil surface under gravity. The
processes when these loose, deposited materials
are detached once again by the rain or entrained
by the flow are called re-detachment and re-
entrainment, respectively. Equation (11.1) is
based on a mass balance for individual particle
size classes. It is necessary to separate sediment
into different classes according to their particle
size because the associated settling velocity,
which characterizes the rate of deposition, is
closely related to particle size. While interactions
between rainfall-driven and flow-driven erosion
processes have been the subject of subsequent
investigation (e.g. Asadi
et al
., 2007), Equation
(11.1) assumes that the effects of both erosion
