Episodes of vertical accretion and catastrophic stripping: A model of disequilibrium flood-plain development

Abstract

This paper describes flood plains formed episodically by vertical accretion along high-energy, laterally stable channels, in southeastern Australia. Overbank deposition gradually builds a flood plain of fine-textured alluvium over a period of hundreds or thousands of years, following which catastrophic erosion by a single large flood, or a series of more moderate floods, strips the flood plain to a basal lag deposit from which it slowly reforms. This periodic destruction appears due to the progressive development of large levee banks and flood-plain surfaces of highly variable relief. As the levees and flood plain grow, overbank flow is gradually displaced from the broad flood plain into the main channel and flood-plain backchannels, with a resulting concentration of erosional energy. Eventually, high flows greatly exceed erosional thresholds, and wholesale scour of the channel boundary and flood plain occurs. Vertical-accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals, even along the same river. Some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime. The almost random but catastrophic nature of this flood-plain erosion means that sediment supply and transport are highly variable and probably impossible to predict. This model of flood-plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial, coarsegrained, traction-load flood plains along high-energy rivers in narrow gorges to extensive low-gradient flood plains in which alluvial stratigraphy is dominated by fine-grained overbank deposition.