Formation and Development

The development of creek systems is dependent upon the tidal range, with macrotidal areas consisting generally of straight, steep channels lying perpendicular to the shoreline: microtidal and mesotidal marshes often have more complex creek patterns (Luternauer et al., 1995). Seven different network types have been identified, in relation to British saltmarshes (Pye and French, 1993). The sequence, progressing from linear to meandering-dendritic, is found to be associated with an increase in marsh maturity (Fig. 2.4). However, this sequence is based entirely upon saltmarsh creek systems. Research into creek formation over the intertidal flats has been focused mainly in estuarine environments, which experience only limited creek development on the mudflats and sandflats. Insufficient research has been undertaken over large intertidal zones with extensive mudflats/sandflats, where different processes and formations occur, compared to the estuarine environments.

Figure 2.4: A classification of tidal-creek networks in saltmarshes (from Allen, 2000).

The formation of a dendritic creek system over intertidal flats is mainly a result of mudflats not being efficient at carrying surface water away from the flat as the tidal water retreats (Whitehouse et al., 2000). Subtle variations in surface topography focus/dissipate the sheetflow and cause the initial stages of channel development (Horton, 1945, for river systems). The initiation of creek development, as found by Perillo (2003) for the Bahia Blanca Estuary, Argentina, was attributed to the burrowing of crabs. Elsewhere, sheetflow caused by heavy rainfall at low water has been found to impact and disperse cohesive substrate; in comparison, ordinary tidal flows did not exceed the critical shear stress

(Mwamba and Torres, 2002). Once a depression is created, then more flow will be attracted to the area, to create a larger bed shear stress and cause an increase in erosion (Whitehouse et al., 2000).

Sheetflow also intermittently controls the headward erosion of low-order intertidal creeks (Mwamba and Torres, 2002). Headward erosion rates, up to a few metres annually, have been noted from youthful back-barrier marshes in North Norfolk, where the ebb-tide probably facilitated the process (Steers, 1939, 1977, cited in Allen, 2000b). The evolution of a tidal creek network was investigated over the coastal plains of the Mary River, northern Australia (Knighton et al., 1992). From aerial photography it was found that, over 50 years, the creeks extended over 30 km inland; a number of the associated tributaries were noted to have had

headward retreat rates over 0.5 km yr-1. These rates have been attributed to the associated large tidal range (6m in nearby macro tidal gulf), relatively flat plains and the availability of pre-existing channel lines (in the form of palaeochannels).

The phases and processes associated with network and channel evolution in fluvial systems can be compared with those of creek evolution on intertidal flats (Glock, 1931; and Horton, 1945). Schumm et al. (1987) identified 3 models of channel evolution that had been

described. The first model suggested by Horton (1945), stated that on a steep, newly exposed, surface a series of parallel rills will develop. With time, cross-grading and micro-piracy would occur among the rills, producing an integrated dendritic network (Fig. 2.5(a)). The second model of network growth relies on the action of headward growth of the channel. Quite simply, this involves the channel growing in a headward direction, whilst also bifurcating (Fig. 2.5(b)). With time, this will fill the space available and form a fully developed dendritic network. The third model was based on the findings by Glock (1931), who showed that the drainage area becomes rapidly subdivided by channels, with the addition of tributaries filling any available space (Fig. 2.5(c)). The most common form of natural channel growth was found to be related to this third model (Schumm et al., 1987). As such, the network evolution can be subdivided into various phases: (i) initiation; (ii) extension by headward growth (elongation); (iii) tributary addition (elaboration); (iv) maximum extension; and (v & vi) integration through abstraction and capture.

Figure 2.5: Models of the drainage network growth over time (1-3 represent the development of each network over time): (a) replacement of parallel rills by an angular dendritic pattern; (b) expansion by headward growth; and (c) extension of rapidly growing, long channels, with tributaries added subsequently to fill in drainage networks (Schumm et al., 1987).

A number of variables have been identified as being the most important in the development of creek systems: the elevation and slope of the land; the nature of the sediment present; and the water and sediment yield (Leopold et al., 1964; Allen, 1977; Chang, 1985; Richards and Lane, 1997; Werritty, 1997; and Schumm et al., 2000). The direction and magnitude of flow, within the tidal creeks and over the marsh surface, are controlled by the complex interaction of the topography, the tidal stage and the effect of wind and waves (Davidson-Arnott et al., 2002). If these variables undergo a progressive/instantaneous change, then the channel may cross an extrinsic threshold and experience change. If an imposed disturbance causes the system to cross such a threshold, then new landforms may develop (Werritty, 1997). The initial landform assemblages are then deemed to have been “geomorphologically responsive” to the changes. The effectiveness (impact) of a destructive event, such as a flood, depends

upon the force exerted, the return period of the event and the magnitude of constructive or restorative processes that occur in the intervening period (Wolman and Gerson, 1978). When the surrounding area stabilises, then the creek may become adjusted to the new conditions, returning to a dynamic equilibrium (Brookes, 1996).

Once creeks are formed on the intertidal flats, then the movement of the sediment, in terms of erosion and deposition, is important in controlling their evolution. The changes in the channel morphology, due to erosion/deposition, affects the rate of sediment transport and,

consequently, the change in channel characteristics; this, in turn, affects the rate of

erosion/deposition, creating a feedback relationship between the changes in the process and form (Pickup, 1977). The effect of erosion/deposition in the channel characteristics is likely to vary widely, from creek to creek. Such erosion/deposition is dependent upon factors such as channel size, erosion resistance, angle of repose of the bed and bank sediment, the effects of vegetation, sediment load of the stream and the magnitude and frequency of the water discharge (Pickup, 1977).

Natural changes over intertidal flats need also to be taken into consideration; for example, the response of creeks to sea-level rise is a natural change, which represents a long-term trend towards increased energy dissipation (Pethick, 1992). This pattern occurs because, as sea- level rises, both the tidal prism and tidal energy increases within the creeks. Hence, as the flow moves to landwards, not all of the energy is dissipated. This effect leaves a significant flow velocity still present when the flood tide reaches the landward end of the creek, causing undermining of the creek banks and, eventually, their collapse. For any change in water and/or sediment discharge, channel metamorphosis will occur; this involves an adjustment of the width, depth, slope or planform of the creek (Harvey, 1977; and Petts, 1977). The frequencies of the major discharges and sediment-producing events are important in the channel morphology, as they determine the recovery time. In association with these processes, scour, which occurs in the channels due to flood and ebb currents, is a major process controlling the depth of the creeks and their overall hydraulic geometry; this is through its effect in initiating bank undercutting and collapse (Pye, 1992).