The impact of different sediment concentrations and sediment transport formulas on the simulated floodplain processes

SummaryOverbank sedimentation is an important process in river floodplain ecosystems and is a component of the floodplain geomorphologic evolution. The impact of suspended sediment supply on floodplain processes is still unclear because sediment deposition can be influenced by many factors. We quantified the effect of sediment supply (suspended sediment) and transport formulas on simulated floodplain processes using a coupled two-dimensional hydrodynamic and sediment transport model (MIKE21C). Erosion and deposition depths, net sedimentation depth and total volume were quantified based on the last time step of the simulation period.The MIKE21C model was validated by comparing simulated water surface elevations to those from a one-dimensional hydrodynamic model. We compared the sediment transport model simulated suspended sediment concentrations (SSCs) to measured concentrations at a gage station. Erosion and deposition processes were simulated using five hydrograph scenarios as a function of high and low SSC and two sediment transport equations, Van Rijn (1984) and Engelund and Hansen (1967).A specific location could be an erosional or depositional zone at different time steps of the simulation. Thus, floodplain deposition is a discontinuous function of river discharge and varies spatially and temporally over the floodplain. Large flows with high SSC were more effective for floodplain deposition than lower discharges, which dominantly caused sediment scour. Coupled hydrodynamic and sediment transport models that account for feedback processes between topography and hydraulics should be given first preference for future floodplain restoration projects. From a restoration perspective, larger flows are required for greater floodplain deposition rates and maintenance of dynamic processes. The Engelund and Hansen (1967) equation simulated higher transport rates than the Van Rijn equation (1984). For future studies, transport equations should be selected based on the study objectives and field characteristics. The current model might be used to analyze the impact on floodplain processes from altered SSC due to Libby Dam operation.

► Hydrodynamic and sediment transport models to simulate sediment and hydraulics.
► Impact of suspended sediment concentration on floodplain processes.
► Sediment transport simulated with two different formulas.