Geology Reference
In-Depth Information
concentration is that there is a continuous call
on the shear stress exerted by overland flow in
order to provide the momentum to sediment
involved in its removal from the soil surface, this
acquired momentum being lost on return to the
soil surface. The 'saltation stress' involved in
this process reduces the effective shear stress.
The variant of GUEST named GUEPS (Yu &
Rose, 1997; Rose & Yu, 1998) incorporates these
refinements, together with the option to predict
soil loss in any given hydrological event, should
the erodibility
b
be known or can be estimated
from other information. In GUEPS the sediment
concentration at the transport limit is written
(Presbitero
et al
., 2005; Fentie
et al
., 1999) as:
Effective runoff rate
Total runoff amout
Topography
Soil properties
Sediment
concentration at
transport limit
Erodibility
β
Sediment
concentration
æ
s
ö
FSV
r
C Rb
(1
-
)
ç
÷
sr
f
-
è
ø
=
c
(11.19)
t
D K
e
with
Event soil loss
F C
(1
-
)
Fig. 11.2
Flowchart for soil erosion prediction using
GUEST technology (from Yu
et al
., 1997a, with
permission).
K = +
1
V
b
2
gD
0.62 FVS
(11.20)
æ
s
sr
ö
-
C Rb
(1
-
)
ç
÷
D
f
-
è
ø
e
reduced to an effective value,
f
e
, which can be cal-
culated by program GUDPRO (Lisle
et al
., 1995).
If a value of
b
is determined, or can be esti-
mated based on past experience, then it is possi-
ble to use program GUEST to predict sediment
loss in any selected or given hydrological scenario
or context. A flow chart indicating the connec-
tion between the various sources of information
involved in event soil loss prediction is shown in
Fig. 11.2.
where
F
is the fraction of the stream power effec-
tive in erosive processes,
f
e
is the effective depos-
itability (m s
−1
) and (1 −
C
) is the fraction of the
soil fully immersed in the flow for a given water
depth,
b
is a shape factor depending on the rill
geometry, and
are sediment and water
density (kg m
−3
), respectively. In Equation (11.19),
K
can be regarded as a modifying factor of the
order of 1. The second term of the modifying fac-
tor
K
(Equation (11.20) ) takes into account the
effects of the saltation stress, and the third term
increases the available stream power due to sedi-
ment concentration. Such an increase of the
stream power occurs because the density of the
water and sediment mixture increases as the sedi-
ment concentration increases. These two modify-
ing terms are significant when the sediment
concentration is high, say >200 kg m
−3
, and/or the
flow depth is low so that the depositability,
f
, is
σ
and
ρ
11.5
Experimental Methods Commonly
Used in GUEST-Based Projects
Event soil loss has often been measured in USLE-
based experimentation by collection of runoff in a
container, with any overflow subsampled by flow
splitting. The sediment collected in containers is
subsequently stirred vigorously and subsampled
