Chapter 1. Systems, geomorphic thresholds and dynamic equilibrium as essential elements of understanding wetland formation and
5. Systems theory and wetland geomorphology
threshold will be exceeded where erosion perpendicular to the channel will result in preferential water flow across the levees rather than down channel, resulting in an avulsion. System theory and concepts of equilibrium may therefore be used to further enhance the explanation of avulsion events.
The position of wetlands in the geomorphic landscape suggests that it is the mode of their formation that leads to incision and eventually system failure (as in Schumm 1979). Interestingly, this suggests that wetlands are inherently vulnerable to change, although the magnitude of vulnerability varies according to the rapidity at which a specific system is likely to reach its geomorphic threshold. Furthermore, where wetlands incise, erosion may be natural and therefore inevitable, by virtue of an intrinsic threshold, or it may be that an external factor has exposed a predisposed vulnerability, causing incision (Schumm 1979). Thus, changes to climate or human interference cannot always be blamed for sudden wetland erosion. However, when a system reaches approaches a threshold value, external stimuli may help the system to reach or cross that threshold, resulting in gradual or sudden change, such as that initiated by erosion. Importantly, the system must be close to the threshold for the change to occur at all.
The potential applications of geomorphic threshold concepts to wetland management and rehabilitation are enormous. The proximity of a wetland to a geomorphic threshold is an indication of its inherent vulnerability to change. In addition, the threshold concept provides insight into external parameters, such as changes in climate, changes in catchment land use or activities in the wetland, which may accelerate movement towards the threshold. Information about proximity to a geomorphic slope threshold may be useful in monitoring, planning and implementing both wetland management and rehabilitation.
5.2. Floodplain processes and dynamics
Nonlinear systems are systems in which the movement of energy and/or matter into and out of the system is not necessarily equal, and where relationships between these processes is not linear, suggesting that the system as a whole is never at equilibrium.
Dissipative systems occur where energy is dissipated within the system, such that order is maintained. Natural floodplain environments fall neatly within this category on a number of levels. As locations of sediment accumulation and storage, they are dissipative and nonlinear. Although the sediment load in a floodplain river fluctuates, the output of sediment from these systems tends to be more constant than inputs.
During floods, the input of sediment may greatly exceed outputs such that floodplains are depositional. However, during low flows, a floodplain may temporarily reach equilibrium as sediment inputs and outputs are roughly the same, unless the stream is
partially aggrading or eroding. Thus on a long-term basis, floodplains are depositional features rather than equilibrium features. In addition, floodplain sedimentation follows an ordered accumulation pattern that results in characteristic channel, levee and backswamp deposits, as has been documented by numerous authors (e.g. Pizzuto 1987, Marriott 1992, Asselman and Middelkoop 1995, Makaske et al. 2002, Törnqvist 1994, Magilligan 1992). The dissipation of high-energy floodwaters results in the development of physical order on the floodplain. Furthermore, the unequal distribution of entropy in nonequilibrium systems is exhibited by the concentration of entropy in river belts across the floodplain. So, it appears that floodplain environments can be characterized and explained in terms of systems theory terminology, but what does that actually mean, and how can the interpretation move forward?
The usefulness of systems theory arises when one begins to apply equilibrium and threshold concepts to a system. By design, geomorphic systems are directional in nature, in that irrespective of landform change and processes, all landscapes tend towards reducing relief. Nevertheless, the manner in which different systems do so, and how energy is managed and distributed within a system, plays a large role in altering system morphology. Even though, as nonlinear systems, most systems are assumed to be in disequilibrium, there is recognition that unless controlled by positive feedback mechanisms, there is a central tendency for systems to move towards equilibrium. In addition the traditional concept of equilibrium as some metaphorical endpoint has changed slightly, and movement towards equilibrium is increasingly envisaged as a series of ongoing mutual adjustments controlled by feedback mechanisms, which produce different results at different locations (Phillips 2002).
Thus, of interest, is the rate and frequency of change to a system, which may be conceptualized through equilibrium theory. Phillips (2002) stresses that multiple types of equilibria are likely to coexist in a landscape at any one time, and that such equilibria are likely to be transient and unstable. Nevertheless, how does infilling of the Mfolozi floodplain occur? Gradually and equally over time, or sporadically in large, generous bursts? Is equilibrium likely to tend towards the system being dynamic, or metastable or punctuated? If the evolution is punctuated, how does the system alter to accommodate such change?
The concept of bifurcations is useful in terms of floodplain evolution, particularly in the context of avulsions. A bifurcation occurs when the system reaches a critical threshold
and must reorganize. Generally, successive bifurcations occur if one system parameter is increased (Huggett 1988). Since the system is complex, there are multiple ways in which the system could reorganize and multiple trajectories upon which evolution could continue. In nonlinear dynamical system theory, the bifurcation is considered to be both deterministic and probabilistic. Between bifurcations, deterministic (universal) laws control the behavior of the system. However, at the threshold when a bifurcation occurs, fluctuations, which are chance like, may control the trajectory of change. As an example, avulsions are probable once the alluvial ridge reaches a certain height of super-elevation above the floodplain, or cross-valley gradients reach a certain threshold. Much research has centered on determining avulsion probabilities and resulting alluvial architecture based on physical variables (e.g. Törnqvist and Bridge 2002, Morozova and Smith 2000, Slingerland and Smith 1998, Mackey and Bridge 1995, Heller and Paola 1996, Smith et al. 1989, Bridge and Leeder 1979). However, the exact location of an avulsion in both time and space is essentially chance-like in character and may be completely unpredictable. This chance-like occurrence determines the trajectory of continued floodplain evolution, with sometimes unknown or unpredicted implications. Following such a bifurcation, different parts of the system may take different lengths of time to adjust. For instance, the new gradient of the stream eventually alters stream capacity and competence, as well as channel pattern and morphology, but all of these act over different time scales. As a result, there may be old (inherited) and new elements within the same landscape. This character of dissipative systems is termed susceptibility to change by Huggett (1988).
The extent to which a bifurcation is system-altering depends on the extent to which the original system is forced to change or adopt a new equilibrium. Benson (1984) arranges these disturbances on a continuum from crisis to cataclysm. A crisis occurs when an event causes sudden alteration of a system’s principal structures, but through the absorption of this stress into its subsystems, the system survives. A catastrophe is an event when the sub-systems fail, but the system still survives, while a cataclysm is the complete destruction of both systems and subsystems. Benson (1984) suggests that system stress is primarily absorbed through the use of redundant pathways that were once integral to the system, such as the adoption of former channel courses to dissipate floodwater during a large flood event.
Alluvial fans within floodplain settings are more complex in terms of equilibrium history and tendency. The rapid transformation of such systems has led some authors to label them nonequilibrium features, in that they do not appear to display a tendency to move towards equilibrium (e.g. Renwick 1992). This is largely because of the effect of thresholds on the evolution of these systems, which tends to hasten system processes.
It can however be argued that alluvial fans behave in a similar manner to floodplains (i.e. reach a gradient threshold and avulse), but on a much more rapid time scale due to higher sedimentation rates. This does not seem an adequate reason to suggest that there is no equilibrium tendency in alluvial fans.
Deciphering floodplain processes and evolution involves the sifting of multiple layers using various components of geomorphic system theory. It appears likely that evolution of such coastal systems will comprise both elements of gradualism and sudden disturbance overlain over time. For instance, stepwise adjustments of sea level may be seen to be in line with jumps in the context of metastable equilibria.
Contrastingly, tectonic adjustment could be both gradual and catastrophic, as was the case following the occurrence of an earthquake in St. Lucia in 1932 (Krige and Venter 1933). In addition to the infrequent disturbances to the system caused by tectonic activity, infrequent, large rainfall events such as caused by occasional tropical cyclones that veer unusually far south with recurrence intervals of 300 years such as Cyclone Domoina may superimpose behaviour that could be typical of systems with punctuated equilibrium. This particular aspect is rather promising since the estuaries on which Cooper’s (1993, 1994, 2001) theory was based, are primarily those of KwaZulu-Natal.