4:30 PM - 4:45 PM
[SCG64-11] Stratigraphic feedbacks on free and forced alternate bar morphodynamics
Keywords:stratigraphy, morphodynamics, bars
As rivers aggrade, they develop subsurface stratigraphy consisting of heterogeneous grain-size distributions in the downstream, cross-stream, and vertical directions. During subsequent periods of degradation, grain-size heterogeneity stored in stratigraphy may be exhumed and potentially feedback on the processes that drive morphodynamic evolution. Here we investigate these feedbacks by implementing the ability to store, track, and access bed stratigraphy in the two-dimensional morphodynamic model FaSTMECH. We use a modified active layer approach, in which during aggradation the active layer and bedload are released to and stored in the highest stratigraphy layer. During degradation, the active layer takes on the sediment properties stored in stratigraphy. We simulate two straight-channel scenarios: one with an obstruction at the upstream end to force formation of fixed alternate bars, and one without an upstream obstruction where freely migrating alternate bars can form. Each scenario was modeled with and without stratigraphy enabled to isolate its effect on the system’s dynamics. The simulations with an obstruction showed minimal differences in sediment sorting and bed morphology between runs with and without stratigraphy enabled. However, the free bar simulation with stratigraphy enabled developed bars that were coarser, higher, and wider than the bars from the simulation without stratigraphy. The cyclical periods of aggradation and degradation associated with bar migration result in surface-subsurface interactions producing a corridor of fine bed material connecting the pools, facilitating sediment transport and allowing bars to grow and become coarser. Our results indicate that autogenic stratigraphy can have an important influence on the development and evolution of migrating alternate bars.