Linking levee-building process with channel avulsion: geomorphic analysis for assessing avulsion frequency and style

A natural levee is a typical wedge-shaped deposit adjacent to a river channel. Given the location and distinctive features, the levee can serve as a key to revealing depositional processes of the coupled channel to the floodplain system preserved in the rock record. Levee-floodplain topographic evolution is also closely linked to river avulsion processes which can spell a catastrophic flooding. Nonetheless, the levee geometry and its aggradation pattern on 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 grain-size distribution of supplied sediment and overflow velocity into floodplain that control the levee geometry and especially the aggradation rate at the levee crest. In the modeling results, levee develops 1) a concave-up profile, 2) exponentially decreasing grain size in the deposits, and 3) a relatively steeper shape for coarser sediment supply with longer time for approaching to the equilibrium state in grain size. The subsequent scaling analysis supports that the input grain size and levee profile slope are positively correlated with avulsion frequency, whereas the overflow velocity is inversely proportional to avulsion frequency. In connection with 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 levee are likely to lead to regional avulsions as abandoned channels with gentler levees are more vulnerable to removal of topographic memory. The insights drawn from the current modeling work, thus may have potential implications for reconstructing paleoenvironments in regard to river sediment transport and flood processes via levee deposits. Based on roles of a levee on the avulsion frequency and style, the flood hazards triggered by river avulsion as well as the alluvial architectures in the sedimentary record can be better assessed.