Investigating and modeling changes in bed height during the deposition process of a mixed-size sediment

Authors: UCHIDA, TATSUHIKO - University Of Hiroshima; Papanicolaou, Athanasios; YICHENG, JIANG - University Of Hiroshima; INOUE, TAKUYA - University Of Hiroshima; HATONO, MISAKO - University Of Hiroshima

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2025
Publication Date: 5/8/2025
Citation: Uchida, T., Papanicolaou, A.N., Yicheng, J., Inoue, T., Hatono, M. 2025. Investigating and modeling changes in bed height during the deposition process of a mixed-size sediment. Journal of Hydraulic Engineering. https://doi.org/10.1061/JHEND8.HYENG-14273.
DOI: https://doi.org/10.1061/JHEND8.HYENG-14273

Interpretive Summary: Sediment and soil movement usually occur within the top layer of a sediment bed which is called active layer. Knowing the grain size composition of the active layer (top layer of the bed) is important for predicting accurately deposition height and thus net erosion fluxes. Current approaches fail to account for the changes in the grain size distribution within the active layer due to sediment infilling over time. This leads to erroneous predictions of sediment fluxes especially at the beginning of a storm event leading to over prediction of sediment fluxes by nearly 200% in some cases. This paper introduces the concept of dynamic available porosity that considers the changes in the porosity due to sediment infilling of voids over time. This tool can be a valuable asset in the hands of watershed managers aiming to predict sediment rating curves in agricultural watersheds under different treatments.

Technical Abstract: The original concept of the “active layer” as envisioned by Hirano has so far been adopted and mostly utilized in sediment transport studies with the caveat that bed thickness (or sediment volume including porosity) is treated as a conservative variable. However, previous studies reveal that bed thickness and porosity rarely remain unchanged, and the assertion of an active layer with a constant porosity impedes sediment transport prediction. The primary goal of this study is to enhance an existing Eulerian Sediment Deposition model, E-SDM, to account for dynamic changes in the available porosity as function of changes that occur in the net depositional height over time through effective simulation of mixed-size sediment dynamics, including vertical and lateral exchanges of particles between the sediment bed and the water column. The secondary goal is to validate the model performance for new experiments performed in air and water mediums. The contribution of this study is, therefore, twofold. Through the incorporation of the concept of “available porosity” (AP) into the E-SDM to provides for the first time the temporal variation of porosity (as well as vertical distribution within the layers) as function of the changes that occur in the net depositional height. This dynamic approach was not implemented before and offers an improvement over the Uchida et al. (2020) study. Second, the outputs of the enhanced E-SDM are compared with sedimentation experiments conducted with natural sediment experiments in air and water under various grain size distributions. The numerical model shows notable predicted results satisfying agreement between calculated results and measurements. The enhanced E-SDM with the available porosity model is found to well represent the observed penetration of finer sediment particles into the void space, and adequately explains the change in porosity due to GSD comparatively to conventional active layer theory which assumes constant thickness and porosity approaches. In this study, the authors provide a discussion of how a dynamic approach in estimating the available porosity improves depositional height estimation.