How muddy meanders work: erosion and deposition of mixed sand and mud in an active meandering river

It has long been assumed that in actively meandering rivers vegetation provides the strength to eroding outer banks and accreting inner banks that enables sustained meandering to occur. The evidence of well-developed fluvial meanders on Mars indicates that the process can occur without the help of stabilizing plant cover. By virtue of cohesive forces, mud can provide strength, but the mechanism of accretion along inner banks in active currents in meanders is poorly understood. The Quinn River in Nevada (slope = 0.00015, bankfull width = 19.8m), an actively meandering river with sparse vegetation on the floodplain, provides an opportunity to explore these issues in depth. The river reworks silt and clay deposits of paleo-Lake Lahontan, which generates a muddy load that combines with fine sand from upstream sources. Stratigraphic sections across the inner bank show that lateral accretion deposits consist of alternating sand-rich and mud-dominated interbeds. Both lidar and drone photos clearly display higher elevated floodplain at the upstream side of the bend and lower topography at downstream inner bank, which provide clues to the deposition and erosion pattern on the floodplain. Flow field measurements through a bend and suspended sediment measurements at a modest discharge are used to constrain the application of Delft3D to predict patterns of erosion and deposition. By default Delft3D treats mud and sand transport calculations separately, yet we find all mud deposits are sandy, and all sand deposits are muddy. By modifying settling velocities to include sand in the cohesive mud transport, and increasing settling velocities of clay to account for flocculation, Delft3D predicts deposition that sensibly varies with flow stage. The local condition of cohesive sediment erosion or deposition is accurately predicted from the dimensionless ratio R, which depends on boundary shear stress, size-dependent sediment concentration and settling velocity, and entrainment parameters. In shallow overbank simulations, the sand, silt and clay deposit together to build an elevated inner bank deposit, as suggested in the lidar map. However, in an extreme flood condition, flow cuts channels across this deposit. Further work is underway to predict meander growth and cutoff in this sandy mud depositing system.