Geoscience Reference
In-Depth Information
10
Slope systems
John Wainwright and Richard E. Brazier
10.1
Introduction
proach, though, is that it does require consideration to
be given in terms of linkages and boundary conditions
between the different elements in a landscape.
More recent approaches have exploited digital eleva-
tion model (DEM) data to analyse the three-dimensional
form of slopes and their landscape position in more detail
(e.g. Pike, 2000), although they too suffer from an inabil-
ity to account for landscape dynamics. The latter require
a characterization of the land-forming processes and their
spatiotemporal variability. The following chapter consid-
ered these dynamics in terms of runoff generation and
overland-flow erosion processes, and it is useful to de-
velop these dynamics further in terms of the patterns in
which they occur on slopes, and are replaced or comple-
mented by other slope-forming and modifying processes
such as creep, mass movement and weathering (Dietrich
and Dunne, 1978; Selby, 1993) (Chapter 6). A general pro-
cess model for soil-covered and rock slopes is shown in
Figure 10.2, showing some of the complexities of under-
standing slope systems and their evolution. This chapter
will attempt to unravel some of these complexities in rela-
tion to specific types of slope system common in drylands.
Outwith the major sand seas, hillslopes are ubiquitous in
the arid zone. Even in some areas dominated by dunes,
surface stabilization and evolution are affected by slope
processes to a certain extent (e.g. Kidron, 1999; Kidron
and Yair, 2001; Parsons
et al
., 2003). The understanding of
slope systems is therefore fundamental to geomorphology
in general and to arid-zone geomorphology in particular.
While there have been a number of key texts relating to
slopes over the last 40 years (e.g. Carson and Kirkby,
1972; Young, 1972; Parsons, 1988; Selby, 1993), most
have been out of print for a long time, and more recent
overviews from the tectonic perspective of landform evo-
lution give little explicit reference to them (e.g. Burbank
and Anderson, 2001). This omission is problematic, as the
exact nature and rates of slope evolution have significant
feedbacks on landscape evolution as a whole.
Slope systems need to be evaluated, as do other geomor-
phic systems, in terms of linkages between process and
form. One issue in terms of slope form is the very wide
range of potential slope shapes and configurations that
can occur. Dalrymple, Blong and Conacher (1968) pro-
duced a useful descriptive approach based on nine types
of slope unit (Figure 10.1). Their qualitative model al-
lows any slope to be described in terms of these nine
types, and the different types may be repeated in any or-
der. Only plateau areas - type 1 - need to be present,
and as Parsons (1988) points out, the flexibility of the
model is also its limitation in terms of distinguishing any
particular slope. It also suffers from being static and two-
dimensional, thereby missing any important characteris-
tics relating to catchment shape, and the feedbacks, for
example, between drainage density, catchment response
and channel (type 9) behaviour. One advantage of the ap-
10.1.1
Contexts of slope systems
The last two decades have seen a renaissance of what
might be considered to be geological approaches to un-
derstanding landform evolution (e.g. Burbank and Ander-
son, 2001), in part linked into the development of Earth
System Science (Wainwright, 2009a, and see Chapter 1).
At the largest scale, the tectonic setting provides a fun-
damental control on the location of some arid areas, and
the relative stability of intracratonic surfaces when com-
pared to active continental margins or intermediate zones
