Abstract
A natural levee is a typical wedge-shaped deposit adjacent to a river channel. Given its location and distinctive features, the levee can serve as a key to revealing depositional processes of the coupled channel to floodplain system preserved in the rock record. Levee-floodplain topographic evolution is also closely linked to river avulsion processes which can spell a catastrophic flood. Nonetheless, the levee geometry and its aggradation pattern on the floodplain have not been fully incorporated in the study of avulsion. Here, we present a levee-building model using an advection settling of suspended sediment to reproduce the evolution of a fluvial levee over floods and to examine the effects of boundary conditions on levee geometry and grain-size trend. We further investigate river avulsion frequencies and styles (i.e., local vs. regional avulsion) associated with the grain-size distribution of supplied sediment and the overflow velocity into the floodplain, which control the levee geometry and especially the aggradation rate at the levee crest. In the modelling results, the levee develops 1) a concave-up profile, 2) exponentially decreasing grain size in the deposit, and 3) a relatively steeper shape for coarser sediment supply. The subsequent scaling analysis supports that the input grain size and levee profile slope are positively correlated with the avulsion frequency, whereas the overflow velocity is inversely proportional to the avulsion frequency. In connection with the avulsion styles and levee geometry, we suggest that steeper levee slopes tend to promote more local avulsions protecting abandoned channels from topographic healing, but gentler slopes of the levee are likely to lead to regional avulsions as abandoned channels with gentler levees are more vulnerable to the removal of topographic memory. The insights drawn from the current modelling work may thus have potential implications for reconstructing paleoenvironments in regard to river sediment transport and flood processes via levee deposits. Based on the roles of a levee on the avulsion frequency and style, the flood hazards triggered by river avulsions as well as the alluvial architecture in sedimentary records can be better assessed.
Highlights
The levee geometry and its aggradation pattern on the floodplain have not been fully incorporated in the study of avulsion
The subsequent scaling analysis supports that the input grain size and levee profile slope are positively correlated with the avulsion frequency, whereas the overflow velocity is inversely proportional to the avulsion frequency
In connection with the avulsion styles and levee geometry, we suggest that steeper levee slopes tend to promote more local avulsions protecting 20 abandoned channels from topographic healing, but gentler slopes of the levee are likely to lead to regional avulsions as abandoned channels with gentler levees are more vulnerable to the removal of topographic memory
Summary
Rivers overflow into floodplains, facilitating the deposition of suspended sediment on account of the loss of flow competence and transport capacity. The reduction in competence and capacity are responsible for the decreases in depositional rate and grain size away from the rivers and form distinctive wedge-shaped natural levees along the channel margins. 35 transport and depositional processes in floodplains have been carried out These models include James (1985) and Pizzuto (1987), which quantify the suspended sediment deposition and grain-size distribution across the channel margin to the floodplain and have been a great aid in comprehending the mechanisms of overbank configuration. The study by Mohrig et al (2000) suggested that comparing channel depth with levee crest height (i.e., normalized superelevation) can be regarded as an avulsion criterion According to those authors, the river would be avulsed when the levee. We develop an advection-settling, suspended sediment transport model to quantitatively determine what are the main controls on geometry, depositional rate, and grain-size sorting in fluvial levee evolution during flooding. We explore river avulsion frequencies and styles associated with the levees constructed differently under the various input sediment grain sizes and flood-flow discharges and demonstrate the contributions of levee deposits to the mechanisms governing the river avulsion behaviours with respect to their geomorphic and sedimentary processes
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