Geology Reference
In-Depth Information
Catenas in humid climates can be expected to develop
thicker soils in downslope positions but in semi-arid
areas, where wash keeps soils thin except on the lowest
gradients, catenas can be expected to have deeper soils
upslope and thinner soils downslope.
A drawback with modelling long-term changes is
the assumption that climate has remained constant.
However, it is possible to allow for climatic change in
models. Kirkby (1984), for instance, included changes
of climate in his model of cliff retreat in South Wales,
as originally studied by Savigear (1952) and often
quoted as an exemplar of location-time substitution
(p. 25). Kirkby ran the model for three phases. First,
for a period, starting 500,000 years ago and ending
50,000 years ago, corresponding roughly to inland val-
ley development with a fixed base level under mainly
periglacial conditions; second, for a period of cliff
retreat from 50,000 to 10,000 years ago; and, third,
for a period of basal removal covering the last 10,000
years (Figure 7.8). The observed upper convexities of
the slope profiles as surveyed by Savigear can, accord-
ing to the model, only be formed during the periglacial
phase and require at least 100,000 years to form. They
are today relict features.
conservation, but now apply reasonably well established
geomorphic transport laws (e.g. Dietrich and Perron
2006). Figure 7.9 shows how a three-dimensional hills-
lope model explains the development of ridge-and-valley
topography in soil-mantled terrain.
of soil erosion means reducing the rate of soil loss to
approximately the rate that would exist under natu-
ral conditions. It is crucially important and depends
upon the implementation of suitable soil conservation
strategies (Morgan 1995). Soil conservation strategies
demand a thorough understanding of the processes of
erosion and the ability to provide predictions of soil loss,
which is where geomorphologists have a key role to play.
Factors affecting the rate of soil erosion include rainfall,
runoff, wind, soil, slope, land cover, and the presence or
absence of conservation strategies.
Soil erosion is an area where process geomorphological
modelling has had a degree of success. One of the first and
most widely used empirical models was the Universal
Soil Loss Equation (USLE) (Box 7.2). The USLE has
been widely used, especially in the USA, for predicting
sheet and rill erosion in national assessments of soil ero-
sion. However, empirical models predict soil erosion on a
single slope according to statistical relationships between
important factors and are rather approximate. Models
based on the physics of soil erosion were developed dur-
ing the 1980s to provide better results. Two types of
physically based model have evolved - lumped mod-
els and distributed models (see Huggett and Cheesman
2002, 156-9). Lumped models are non-spatial, pre-
dicting the overall or average response of a watershed.
Distributed models are spatial, which means that they
predict the spatial distribution of runoff and sediment
movement over the land surface during individual storm
HUMANS AND HILLSLOPES
Hillslopes are the location of much human activity,
and their study has practical applications. Knowledge
of runoff and erosion on slopes is important for plan-
ning agricultural, recreational, and other activities. Land
management often calls for slopes designed for long-term
stability. Mine tailing piles, especially those containing
toxic materials, and the reclamation of strip-mined areas
also call for a stable slope design. This final section
will consider the effects of humans upon hillslope soil
erosion.
Soil erosion modelling
Soil erosion has become a global issue because of its
environmental consequences, including pollution and
sedimentation. Major pollution problems may occur
from relatively moderate and frequent erosion events in
both temperate and tropical climates. In almost every
country of the world under almost all land-cover types the
control and prevention of erosion are needed. Prevention
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