Models of Floodplain Formation

Floodplain formation is a process of sequestration of part of the sediment load of a river into alluvial deposits adjacent to the channel. Sediment deposition results from the movement of high energy sediment-laden flow from the main flow thread of the channel to lower energy environments on the floodplain. Vertically accreted and lat- erally accreted floodplain deposits represent the two common depositional styles, and result from different hydraulic behaviour of the flow within the channel and on the floodplain. In this study, the definition of the floodplain is taken from Leopold et al. (1964), who define it as “the surface being constructed by the existing stream”. The emphasis of this definition is on construction by sediment deposition, which implies inundation of the floodplain by flood flows on a regular or periodic basis, and differs from other approaches using vegetation characteristics or bench morphology as the definitive criteria (Williams, 1978; Woodyer, 1968).

The inundation frequency of the floodplain can be highly variable from site to site, controlled partially by the hydrologic regime and partially by the morphology of the channel and hence its evolutionary history. It can be expressed as the recurrence inter- val of the bankfull discharge rate calculated from observations of the flood history of the river, typically the annual maximum series. Data from North America presented by Wolman and Leopold (1957) and Leopold et al. (1964) suggested bankfull discharge occurred about once every 1 to 2 years. A subsequent study from the Cumberland Basin near Sydney showed the return period of bankfull discharge to lie in the range

of 4 to 10 year range (Pickup and Warner, 1976) whilst Williams (1978) observed an even larger range from one year to many decades.

Varying definitions of the bank full level and non-equilibrium channel morphology (eg. Brooks et al., 2003) partially contributes this variability. However, it is proposed here that an alluvial surface with a mean return period of 100, 50 or even 20 years constitues a terrace, or at least an abandoned floodplain, whilst an alluvial surface inundated annually would most likely represent a transient sediment bar or shoal rather than a true floodplain.

2.3.1.1 Floodplain Formation by Lateral Accretion

The process of lateral accretion of sediments to form a floodplain is generally associ- ated with meandering rivers and can produce the distinctive ridge and swale topogra- phy of such flood plain surfaces through formation of scroll bars. Sediments aggrade on the inner bank (point bar) of meanders, transported there by helical flow within the channel (Thorne and Furbish, 1995) scouring bedload from the thalweg up onto the point bars (Allen, 1970). Deposition upon the point bar may occur as a single layer diminishing in thickness from the channel to the floodplain (Wolman and Leopold, 1957) or as discrete ridge to form a scroll bar through the action of secondary flow separation currents (Nanson, 1980). Lateral floodplain accretion is dependent upon channel migration. Nanson and Hickin (1986) note that the rate at which river bends migrate across the valley floor appears to be related to the curvature of the channel.

A condition for this style of floodplain deposition to occur is that the valley width should be substantially wider than the radius of curvature of the meander bends in order to allow for free migration of the channel across the valley floor. The mean- der wavelength and the radius of curvature have been widely found to be 10-14 and 2-3 times channel width, respectively, and channel width commonly scales with dis- charge (Leopold et al., 1964; Williams, 1986; Knighton, 1998). Hence, the capacity

for meanders to form, migrate, and lay laterally accreted floodplain deposits is in part determined by the magnitude of the river’s discharge in relation to its valley setting.

2.3.1.2 Floodplain Formation by Vertical Accretion

In many of the valleys east of Australia’s Great Dividing Range, vertically accreted floodplains predominate. This arguably arises from the narrow, deeply incised valleys that inhibit meander development and migration. The process of vertical accretion of sediments essentially involves an advective transfer of flow momentum and sediment from the deep, high energy flow thread within the channel to overbank regions during floods. If flow energy on these overbank regions is lower than that of the channel, deposition of sediment will occur (Pizzuto, 1987). Irregular floodplain topography (Nanson, 1986; Walker et al., 1997) and features such as crevasse splays often form. Nonetheless, a consistent decline in grain size away from the channel is a charac- teristic of vertically accreted overbank deposits and reflects the declining capacity of the flow to transport sediment with increasing distance from the channel (Kesel et al., 1974; Cazanacli and Smith, 1998), though flood chutes can leave coarse sediment de- posits upon a floodplain. Levees, defined as prismatic sedimentary bodies adjacent to the channel and consisting of triangular cross section raised above the adjacent flood plain and aligned parallel to the channel (Brierley et al., 1997), are common features on vertically accreted floodplains. Levees typically experience high deposition rates (Kesel et al., 1974), but low lying flood basins further from the channel which are commonly richer in silt and clay sized particles can also aggrade rapidly (Asselman and Middelkoop, 1995; Cazanacli and Smith, 1998).

Vertically accreted floodplains have been observed in low energy semi-confined stream environments along the New South Wales coast by Nanson and Young (1981). They attributed their formation to a strong reduction in channel capacity downstream, progressively forcing ever greater flood flows out of the channels and onto the flood-

plains where sediment settles from the water column. By contrast, Cazanacli and Smith (1998) note the presence of vertically accreted floodplains along unconfined lowlands where they occur as part of a frequently avulsing channel complex flowing into a lake. The process of vertical accretion of floodplains in this case gradually leads to a decrease in relief of abandoned channels as low lying former channels fill with fine grained sediments.

Of significance to this study are the vertically accreted floodplains found in rel- atively narrow and moderately steep valleys. Brakenridge (1984) described a suite of vertically accreted flood plain deposits along the Duck River in Tennessee. The river flows over bedrock along the study reach and the channel occupies one third to one fifth of the valley floor and is strongly controlled in planform by bedrock val- ley walls. This is a setting equivalent to that found for many rivers east of the Great Dividing Range in Australia. Floodplain sediments within the valley were built up through vertical accretion of suspended load and lateral accretion of river banks and in-channel bars. Detailed reconstructions of floodplain stratigraphy and chronology revealed a sequence of sedimentary units, sometimes expressed vertically as terraces stacked side by side across the valley floor, though with occasional overarching sedi- mentary drapes. Brakenridge interpreted this depositional record as reflecting periodic occurrences of either channel widening or switching of the channel’s position laterally across the valley floor.