Linking levee-building processes with channel avulsion: geomorphic analysis for assessing avulsion frequency and channel reoccupation

[摘要] 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 cause catastrophic floods. 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 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 the grain-size trend of the levee deposit. We further investigate river avulsion frequencies and potential channel reoccupation associated with the grain-size distribution of overbank sediment flux and the overflow velocity into the floodplain, both of which can control the levee geometry, especially the aggradation rate at the levee crest. In the modeling results, the levee develops (1) a concave-up profile, (2) an exponential decrease in grain size of the deposit away from the main channel, and (3) a relatively steeper shape for coarser sediment supply and vice versa. The subsequent scaling analysis supports that the input grain size to the floodplain 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 relatively steeper levee slopes tend to promote more reoccupations of preexisting floodplain channels as protecting abandoned channels from topographic healing, but relatively gentler levees are likely to create a new avulsion channel as their remnant channels are more vulnerable to the removal of topographic memory. The insights drawn from the current modeling work may thus have potential implications for reconstructing paleoenvironments in regard to river sediment transport and flood dynamics via levee deposits. Based on the roles of natural levees on the avulsion frequency and channel reoccupation, the flood hazards triggered by river avulsions as well as the alluvial architecture in sedimentary records can be better assessed.

[学科分类] 土壤学