Page 195 - Fundamentals of Geomorphology

178 PROCESS AND FORM

Catenas in humid climates can be expected to develop (p. 25). Kirkby ran the model for three phases. First,
thicker soils in downslope positions but in semi-arid for a period, starting 500,000 years ago and ending
areas, where wash keeps soils thin except on the lowest 50,000 years ago, corresponding roughly to inland val-
gradients, catenas can be expected to have deeper soils ley development with a fixed base level under mainly
upslope and thinner soils downslope. periglacial conditions; second, for a period of cliff
A drawback with modelling long-term changes is retreat from 50,000 to 10,000 years ago; and, third,
the assumption that climate has remained constant. for a period of basal removal covering the last 10,000
However, it is possible to allow for climatic change in years (Figure 7.8). The observed upper convexities of
models. Kirkby (1984), for instance, included changes the slope profiles as surveyed by Savigear can, accord-
of climate in his model of cliff retreat in South Wales, ing to the model, only be formed during the periglacial
as originally studied by Savigear (1952) and often phase and require at least 100,000 years to form. They
quoted as an exemplar of location–time substitution are today relict features.

conservation, but now apply reasonably well established of soil erosion means reducing the rate of soil loss to
geomorphic transport laws (e.g. Dietrich and Perron approximately the rate that would exist under natu-
2006). Figure 7.9 shows how a three-dimensional hills- ral conditions. It is crucially important and depends
lope model explains the development of ridge-and-valley upon the implementation of suitable soil conservation
topography in soil-mantled terrain. strategies (Morgan 1995). Soil conservation strategies
demand a thorough understanding of the processes of
erosion and the ability to provide predictions of soil loss,
HUMANS AND HILLSLOPES which is where geomorphologists have a key role to play.
Factors affecting the rate of soil erosion include rainfall,
Hillslopes are the location of much human activity, runoff, wind, soil, slope, land cover, and the presence or
and their study has practical applications. Knowledge absence of conservation strategies.
of runoff and erosion on slopes is important for plan- Soil erosion is an area where process geomorphological
ning agricultural, recreational, and other activities. Land modelling has had a degree of success. One of the first and
management often calls for slopes designed for long-term most widely used empirical models was the Universal
stability. Mine tailing piles, especially those containing Soil Loss Equation (USLE) (Box 7.2). The USLE has
toxic materials, and the reclamation of strip-mined areas been widely used, especially in the USA, for predicting
also call for a stable slope design. This final section sheet and rill erosion in national assessments of soil ero-
will consider the effects of humans upon hillslope soil sion. However, empirical models predict soil erosion on a
erosion. single slope according to statistical relationships between
important factors and are rather approximate. Models
Soil erosion modelling based on the physics of soil erosion were developed dur-
ing the 1980s to provide better results. Two types of
Soil erosion has become a global issue because of its physically based model have evolved – lumped mod-
environmental consequences, including pollution and els and distributed models (see Huggett and Cheesman
sedimentation. Major pollution problems may occur 2002, 156–9). Lumped models are non-spatial, pre-
from relatively moderate and frequent erosion events in dicting the overall or average response of a watershed.
both temperate and tropical climates. In almost every Distributed models are spatial, which means that they
countryoftheworldunderalmostallland-covertypesthe predict the spatial distribution of runoff and sediment
control and prevention of erosion are needed. Prevention movement over the land surface during individual storm