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

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.