- ItemEntrainment and Transport of Well-Sorted and Mixed Sediment Under Wave Motion(Journal of Geophysical Research: Oceans, 2022-08-08) Rafati, Yashar; Hsu, Tian-Jian; Calantoni, Joseph; Puleo, JackEntrainment and suspension of sediment particles with the size distribution similar to a range of natural sands were simulated with a focus on the vertical size sorting and transport dynamics in response to different wave conditions. The simulations were performed using a two-phase Eulerian-Lagrangian model by combining the LIGGGHTS discrete element method solver for sediment and SedFoam solver for the fluid phase. The model was first validated for a range of sand grain sizes from 0.21 to 0.97 mm having well-sorted and mixed (bimodal) size distributions using laboratory oscillatory flow data. Three sediment bed configurations were studied under a wide range of velocity-skewed waves with different wave intensity and skewness. It was found that the bimodal distribution having only 30% of coarse fraction and 70% of medium fraction responds similar to a well-sorted coarse sand configuration. Sediment fluxes of the bimodal distribution were slightly higher than those of well-sorted coarse sand because of the pronounced inverse grading in the bimodal distribution. Furthermore, for the bimodal distribution the medium fraction acted as a relatively smooth foundation underneath the coarse fraction which facilitated the mobilization of the coarser particles. Under high energy wave conditions, the smoothing feature was exacerbated and further caused the formation of plug flow where a thick layer of intense sediment flux was observed. Model results also showed that under high skewness waves, phase-lag effect occurred in well-sorted medium sand which caused lower net onshore sediment transport rates but the effect was significantly reduced for mixed sediments. Key Points: - Transport rates of mixed sand with bimodal distribution are similar to those of well-sorted coarse sand - Plug flow formation depends on the particle size distribution and occurs for the bimodal distribution - Phase lags in sediment entrainment and sediment settling are important for predicting net transport rates Plain Language Summary: Sediment transport driven by shoreward propagating waves depends on the sediment particle size. Generally, coarse particles (greater than 0.5 mm diameter) respond directly to the wave motion due to being entrained and transported near the bed with faster settling whereas medium particles (smaller than 0.3 mm diameter) do not respond directly to the flow field due to sediment entrainment away from the bed and slower settling. Natural sediment in coastal zones has a variety of sediment sizes often classified as well-sorted (nearly uniform sizes) or poorly sorted (mixed sediment sizes). The response of well-sorted sediment particles can be characterized and predicted with a representative sediment diameter. However, the response of mixed sediment depends on the size fractions and the interaction of different size fractions with each other and with the flow field. Well-sorted and mixed sediment particles were simulated using a computational model with conditions representative of normal and storm waves. Mixed sediment with only 30% of the coarse fraction (70% of the medium fraction) responded similar to the well-sorted coarse sediment with slightly higher sediment fluxes due to the inverse vertical sorting (upward coarsening). Additionally, the medium particles serve as a smooth bed underneath coarse particles enhancing sediment entrainment.
- ItemBlock-structured, equal-workload, multi-grid-nesting interface for the Boussinesq wave model FUNWAVE-TVD (Total Variation Diminishing)(Geoscientific Model Development, 2022-07-18) Choi, Young-Kwang; Shi, Fengyan; Malej, Matt; Smith, Jane M.; Kirby, James T.; Grilli, Stephan T.We describe the development of a block-structured, equal-CPU-load (central processing unit), multi-grid-nesting interface for the Boussinesq wave model FUNWAVE-TVD (Fully Nonlinear Boussinesq Wave Model with Total Variation Diminishing Solver). The new model framework does not interfere with the core solver, and thus the core program, FUNWAVE-TVD, is still a standalone model used for a single grid. The nesting interface manages the time sequencing and two-way nesting processes between the parent grid and child grid with grid refinement in a hierarchical manner. Workload balance in the MPI-based (message passing interface) parallelization is handled by an equal-load scheme. A strategy of shared array allocation is applied for data management that allows for a large number of nested grids without creating additional memory allocations. Four model tests are conducted to verify the nesting algorithm with assessments of model accuracy and the robustness in the application in modeling transoceanic tsunamis and coastal effects.
- ItemNumerical modeling of intertidal mudflat profile evolution under waves and currents(Coastal Engineering Journal, 2022-06-25) Miranda, Paterno S.; Kobayashi, NobuhisaThe erosional and accretional profile changes of an intertidal mudflat are examined using available field data and the cross-shore numerical model CSHORE that is extended to allow for a mixture of sand and mud. The semidiurnal migration of the still water shoreline and surf zone is resolved numerically to predict the net cross-shore and longshore sediment transport rates influenced by the small cross-shore (undertow) and longshore currents induced by breaking waves of about 0.2 m height. Alongshore sediment loss or gain is included by approximating the alongshore sediment transport gradient using an equivalent alongshore length. The calibrated CSHORE reproduces the measured erosional (accretional) profile change of about 0.1 m (0.1 m) over a cross-shore distance of 950 m during the erosional (accretional) interval of 206 (195) days. The mudflat profile changes are equally affected by mud characteristics, the semidiurnal tide amplitude, and the wave height, period, and direction. In addition, the alongshore water level gradient and wind stress influence longshore current and sediment transport. This study shows the importance of sediment transport in the surf zone that may have been excluded in previous numerical modeling.
- ItemNumerical investigation of unsteady effects in oscillatory sheet flows(Journal of Fluid Mechanics, 2022-06-06) Mathieu, Antoine; Cheng, Zhen; Chauchat, Julien; Bonamy, Cyrille; Hsu, Tian-JianIn this paper, two-phase flow simulations of oscillatory sheet flow experimental configurations involving medium and fine sand using a turbulence-resolving two-fluid model are presented. The turbulence-resolving two-phase flow model reproduces the differences of behaviour observed between medium and fine sand whereas turbulence-averaged models require an almost systematic tuning of empirical model coefficients for turbulence–particle interactions. The two-fluid model explicitly resolves these interactions and can be used to study in detail the differences observed experimentally. Detailed analysis of concentration profiles, flow hydrodynamics, turbulent statistics and vertical mass balance allowed the confirmation that unsteady effects, namely phase-lag effect and enhanced boundary layer thickness, for fine sand are not only due to the small settling velocity of the particles relative to the wave period. The occurrence and intensity of unsteady effects are also affected by a complex interplay between flow instabilities, strong solid-phase Reynolds stress and turbulence attenuation caused by the presence of the particles.
- ItemSensitivity of tidal hydrodynamics to varying bathymetric configurations in a multi-inlet rapidly eroding salt marsh system: A numerical study(Earth Surface Processes and Landforms, 2021-12-22) Deb, Mithun; Abdolali, Ali; Kirby, James T.; Shi, Fengyan; Guiteras, Susan; McDowell, ConorWe describe the development of a high-resolution, two-dimensional hydrodynamic model for a multi-inlet rapidly eroding tidal wetland on the western shore of Delaware Bay, using the finite-volume, primitive equation community ocean model (FVCOM). Topo-bathymetric surveys, together with water surface and current velocity measurements during calm and stormy conditions, have been conducted to support model validation. The tested model is then used to quantify the tide-induced residual transport and asymmetry at major inlet entrances to determine the governing hydrodynamics. We chose a skewness method to calculate the tidal asymmetry and serve as a proxy for sediment transport estimates. The effects of the dredging of an artificial entrance channel and progressive channel deepening in shifting wetland hydrodynamics are shown by developing a scenario analysis. Model results show that the artificially dredged channel has altered the volume exchange at other inlet entrances and increased the net seaward export. The changes in the characteristic frequency of the frictional dissipation in the channel and the system's natural frequency are investigated using a simple ocean–inlet–bay analytical model. Subsequently, we have compared the channel friction scale to the inertia scale and observed that the new connection and gradual channel deepening reduce the overall frictional dominance. Ultimately, the study has shown how the short- and long-term channel bathymetry changes, mainly the artificially dredged channel and progressive channel deepening, can affect the connected system's net circulation and trigger internal marsh erosion.