Geology Reference
In-Depth Information
F
w
accounts for the effect of a surface water layer
in protecting the soil from raindrop impact
(dimensionless);
t
is the overland flow shear
stress (N m
−2
); and
t
c
is the critical shear stress for
initiation of soil particle motion (N m
−2
). The soil
erodibility coefficients
k
r
and
k
f
increase in value
as the soil becomes easier to erode (i.e. sandy soils
have larger values than clayey soils). However,
they cannot yet be determined from a directly
measurable soil property and therefore require
calibration (e.g. Wicks
et al
., 1992; Adams &
Elliott, 2006).
A simple estimate of bank erosion by channel
flow is made as follows:
uration conditions simulated by SHETRAN,
using factor of safety analysis. This includes an
allowance for the effect of vegetation root cohe-
sion. For each landslide the volume of eroded
material is determined and, depending on condi-
tions, is routed down the hillslope as a debris
flow. The proportion of the material reaching the
channel network is then calculated and fed to the
SHETRAN sediment transport component for
routing to the basin outlet.
The central feature of the landslide model is
the use of derived relationships (based on a topo-
graphic index) to link the SHETRAN grid resolu-
tion (which may be as large as 1 or 2 km), at which
the basin hydrology and sediment yield are mod-
elled, to a subgrid resolution (typically around
10-100 m) at which landslide occurrence and ero-
sion is modelled. Through this dual resolution
design, the model is able to represent landsliding
at a physically realistic scale while remaining
applicable at the basin scales (up to 500 km
2
)
likely to be of interest, for example feeding a
reservoir.
Full detail of the landslide component is pro-
vided in Burton and Bathurst (1998). However, it
will be helpful to know that the critical soil satu-
ration conditions for landslide occurrence are
determined using the one-dimensional, infinite-
slope factor of safety equation:-
Ek
æ
ö
t
=
b
-
1
for
tt
>
(14.3a)
ç
÷
b
b
b
bc
t
è
ø
bc
E
b
=
0 for
t
b
≤
t
bc
(14.3b)
where
E
b
is the rate of detachment of material per
unit area of bank (kg m
−2
s
−1
);
k
b
is the bank erod-
ibility coefficient (kg m
−2
s
−1
);
t
b
is the shear stress
acting on the bank (N m
−2
); and
t
bc
is the critical
shear stress for initiation of motion of bank mate-
rial (N m
−2
).
Although the SHETRAN contaminant trans-
port component is not presented here, it may be
noted that it allows for the transport of contami-
nants adsorbed to sediment particles (Ewen,
1995).
[
]
()
(
)
é
ù
2
CC
+
Lm
-
tan
f
s
r
+
(14.4)
ê
ú
g
d
sin 2
b
tan
b
ê
ú
ë
û
14.3.3 SHETRAN landslide erosion
and sediment yield component
w
FS
=
L
Through its integrated surface and subsurface
representation of river basins, SHETRAN pro-
vides not only the overland and channel flows
needed for modelling the transport of eroded soil,
but also soil moisture conditions and hence a
basis for simulating rain- and snowmelt-triggered
landsliding. The SHETRAN landslide component
was thus developed to simulate the erosion and
sediment yield associated with shallow landslides
at the basin scale (Burton & Bathurst, 1998). The
occurrence of shallow landslides is determined as
a function of the time- and space-varying soil sat-
where
q
g
g
(
)
L
=
o
+
m
sat
+
1
-
m
m
(14.5)
g
d
g
g
w
w
w
and
FS
is the factor of safety (
FS
< 1, unsafe;
FS
1,
safe);
C
s
is the effective soil cohesion;
C
r
is the
root cohesion;
f
is the effective angle of internal
friction of soil on an impermeable layer;
d
is the
soil depth above the failure plane;
b
is the slope
angle;
q
o
is the vegetative surcharge per unit plan
area;
g
sat
is the weight density of the saturated
≥
