Environmental Engineering Reference
In-Depth Information
Modern physical geography recognizes that many different physical systems can coexist
and interact in the real world. The systems involve different physical materials (minerals,
gases, liquids, organic material) operating over very different time scales and over very
different spatial scales. Some systems are dominated by negative feedback and will return
to their original state after a minor perturbation. This is
steady state equilibrium
in
Chorley and Kennedy's (1971) classification as shown in Figure 1.7a. Where part of the
landscape is undergoing a gradual and progressive change over the medium to long term
yet preserves equilibrium in the short term, the situation is described as
dynamic
equilibrium
, as shown in Figure 1.7b. An example would be a river which attempts to
maintain the relationship between channel geometry and discharge whilst at the same
time lowering its longitudinal profile.
The study of parts of the physical landscape in a systems framework has inevitably
brought forward many examples where there appears not to be an equilibrium situation,
neither steady state nor dynamic. Renwick (1992) classifies these situations into two
types,
disequilibrium
and
non-equilibrium
. Disequilibrium features are those that tend
towards equilibrium but have not had enough time to reach it. Either the perturbation has
been quite recent or the processes operate at a low intensity.
Non-equilibrium features do not appear to move towards any equilibrium even with
long periods of stability in the environment. These features change so rapidly and so
dramatically that it is difficult to identify an average or equilibrium condition. Non-
equilibrium features are inherently unstable. Three causes of their instability are
identified (Renwick 1992) and are illustrated in Figure 1.9. First, there is the situation
where a landscape or part of it is affected by thresholds or sudden changes in the
magnitude of the rates of processes, so that the processes change quickly over several
orders of magnitude. In other words, the threshold is a major discontinuity caused by
high-magnitude, low-frequency events. Infrequent and atypical weather events, high-
magnitude floods, large mass movements and tectonic events all cause a system to
become unstable and to shift across a critical threshold. The movement across the
threshold is irreversible, and negative feedback is no longer able to restore the system to
its original form. This condition has also been called dynamic
metastable equilibrium
(Chorley
et al.
1984).
Sometimes the change may be gradual and progressive, causing a gradual decrease in
resistance, rather than a dramatic increase in driving forces. Global warming is a good
example of this. The melting of the permafrost in arctic lands could cause a thermokarst
landscape to form which would not be able to return to its original state, even if the
climate returned to its original condition.
The second case of non-equilibrium occurs when positive feedback prevents a return
to equilibrium (Figure 1.9b). Examples of this can be found in many soil systems, in
coastal systems and in fluvial systems.
A third cause of nonequilibrium is chaos, or completely unpredictable and
unstructured behaviour. As turbulent flow is chaotic, examples of chaotic behaviour in
systems have been proposed for some fluvial features and for overland flow on hill
slopes. However, it is difficult to separate the effects of thresholds, positive feedback and
chaos in many non-equilibrium systems. Such is the complexity of the real world that
instability is likely to result from differing combinations of them. Chaos theory is proving
to be a stimulus for re-evaluating systems in physical geography, however. Other systems
