Geology Reference
In-Depth Information
In
transport-limited processes
, the rate of soil and rock
transport limits the delivery of sediment to streams.
In other words, the supply of sediment exceeds the capac-
ity to remove it, and transport processes and their spatial
variation dictate hillslope form. Soil creep, gelifluc-
tion, through-wash, rainflow, rainsplash, and rillwash are
all hillslope processes limited by transporting capacity.
On
supply-limited
(or
weathering-limited
)
hillslopes
,
the rate of sediment production by weathering and ero-
sional detachment (through overland flow and mass
movement) limits the delivery of sediment to streams.
In other words, weathering and erosional processes dic-
tate hillslope form. Leaching of solutes, landsliding,
debris avalanches, debris flows, and rockfall are all
hillslope processes limited by sediment supply.
The distinction between transport-limited and supply-
limited process is often blurred. Nonetheless, it is an
important distinction because it affects the long-term
evolution of hillslopes. Hillslopes and landscapes dom-
inated by transport-limited removal typically carry a
thick soil layer supporting vegetation, and slope gradi-
ents tend to reduce with time. Hillslopes and landscapes
dominated by supply-limited removal often bear thin
soils with little vegetation cover, and characteristically
steep slopes tend to retreat maintaining a sharp gradi-
ent. Mathematical models of hillslope evolution support
these findings, suggesting that the wearing back or wear-
ing down of the mid-slope depends upon the processes
in operation. As a generalization, surface wash processes
lead to a back-wearing of slopes, whereas creep pro-
cesses lead to a down-wearing of slopes (e.g. Nash 1981).
Nonetheless, the pattern of slope retreat and slope decline
is crucially dependent on conditions at the slope base, and
especially on the transport capacity of streams.
A study of young fault scarps formed in alluvium
in north-central Nevada, USA, showed that hillslope
processes change as the scarps age (Wallace 1977)
(Figure 7.5). The original fault scarps stand at 50
◦
to 70
◦
. At this stage, mass wasting is the dominant
process, a free face develops at the scarp top, which
retreats through debris fall, and material accumulates
lower down. Later, the scarp slope adopts the angle of
repose of the debris, which is about 35
◦
. At this gentler
gradient, wash erosion dominates hillslope development
and further slope decline occurs.
Stage 1: Fault controlled
5°
Fault scarp
60°
5°
Stage 2: Gravity and debris controlled
Free face (slope
replacement begins)
Debris slope
Wash slope
Stage 3: Debris controlled
35°
Stage 4: Debris and wash controlled
35°
Stage 5: Wash controlled
Slope
decline
operative
8-25°
5-10°
Figure 7.5
Proposed sequence of change on a fault scarp
developed in alluvium, Nevada, USA. The changes are
incremental, the dashed line shown at each stage
representing the hillslope profile at the previous stage.
Source:
Adapted from Wallace (1977)
Hillslope development
Slope processes fashion hillsides over hundreds of thou-
sands to millions of years. It is therefore impossible
to
study
hillslope
evolution
directly.
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