Investigating and Modeling Changes in Bed Height during the Deposition Process of a Mixed-Size Sediment

Abstract

The original concept of the active layer as envisioned by Hirano has so far been adopted and mostly utilized in studies with the caveat that the volume of the sediment bed including porosity is a conservative variable. However, studies conducted by researchers in the past few decades have revealed that the bed porosity rarely remains unchanged, as well as the bed height and grain size distribution (GSD), and the assertion of an active layer with a constant porosity impedes sediment transport prediction. Moreover, at present, the active layer thickness, i.e., the topmost layer composed of varying grain size sediment that is eroded, transported, and deposited, is determined empirically based on the representative grain size such as the maximum grain size while ignoring the role of GSD on changes in porosity. This paper first highlights the essential problems that are found in the conventional continuity equation with the empirically determined active layer thickness when compared to the continuity equation that is derived from the spatially averaged continuity equation of sediment particles with kinematic boundary conditions of the riverbed, considering the changes in available porosity (AP) for each particle class. Then, this study verifies and validates the Eulerian sediment deposition model (E-SDM) using the AP concept for the prediction of the temporal changes in the depositional heights of sediment particles for different sediment deposition conditions through comparisons with newly performed column experiments in air and static water. The outputs of the E-SDM explained the change in porosity due to GSD, satisfying the agreement between calculated results and measurements, indicating the model effectiveness and efficiency as a new tool for modeling changes in bed height during the deposition process of a mixed-size sediment. The effects of grain size distribution (GSD) on fluvial landforms and dynamics are still not well understood. A major obstacle in sediment transport simulation in rivers is the lack of a conservation equation that accounts for variations in porosity and depositional heights of sediment particles. A new sedimentation model, the Eulerian sediment deposition model (E-SDM), has been developed based on the concept of the available porosity. E-SDM successfully explains changes in the sedimentation process and porosity due to differences in GSD and sedimentation conditions. This study contributes significantly to the field of hydraulic engineering and geomorphology for predicting changes in deposition height and riverbed morphology considering spatial and temporal variations in the porosity structure. The E-SDM model can be applied to improve sediment transport simulations and predictions of riverbed morphology. Further research can explore the application of available porosity in other fields, such as concrete, soil mechanics, and powder engineering. The development of E-SDM can also inform the study of other complex systems involving multiphase physics including different size particles and fluid.