Linking levee construction with channel avulsion: Insights from an advection settling model

Abstract:

A natural levee is a typically wedge-shaped deposit of suspended sediment adjacent to a channel. Given their location and distinctive features, levees can serve as the key to revealing the depositional processes of the linked channel to the floodplain system preserved in the rock record. To date, however, quantitative analysis of the effect of flood flow characteristics and grainsize distribution on fluvial levee geometry and stratigraphy has been lacking. The river avulsion process accounting for fluvial levee-building evolution, also, has not been fully linked.

In this study, we present a levee-building model to provide an understanding of levees in terms of geomorphology and sedimentology. The model used an advection-settling of suspended sediment to reproduce the evolution of a fluvial levee during flooding. We investigated river avulsion frequency and style (i.e., local or regional avulsion) related with levees that were constructed differently under the various boundary conditions. The modeling results showed that the channel avulsion frequency is controlled by the geometry of a levee which is associated with the grain size distribution of supplied sediment and shear velocity of the channel. Levee depositional rate at the proximal location near the channel increased as shear velocity of the channel increased and thus mean particle size of suspended sediment from the flood became coarser. Consequently, the avulsion frequency of the channel became higher as the time taken for the channel to be super-elevated became shorter.

The modeling work has potential implications for reconstructing conditions of river sediment transport through levee morphology and sedimentology. Based on the understanding of the geomorphological effects of a levee, the relations between levee and channel properties can be applied to interpret the state of paleo-currents and basin filling history in the sedimentary record.