Browsing by Author "Shi, Fengyan"
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Item A surface porosity approach for eliminating artificial ponding in coastal salt marsh simulations(Coastal Engineering, 2022-11-23) Deb, Mithun; Kirby, James T.; Abdolali, Ali; Shi, FengyanHydrodynamic processes over marsh topography are significantly affected by surface defects such as cuts and rills on channel berms and platforms. These meter-scale features are often missing in the model representation due to the spatial resolution available from data sources, as well as incomplete resolution in the model grid itself. To minimize the artificial hydraulic isolation in the numerical models, we propose implementing an effective porosity algorithm on the marsh surface by considering the fine-scale topography over marsh depressions that control the drainage process. The modification is carried out to eliminate artificial ponding effects observed in model simulations in Bombay Hook National Wildlife Refuge, DE, USA using the original FVCOM code. Results from the revised and original FVCOM models are compared with pressure gauge data collected from an isolated depression in the marsh platform. The new implementations for proper wetting and drying are efficient and accurate for hydrodynamic modeling inside a complex salt-marsh system, which constitutes a major breakthrough in the context of increasing need for better understanding of physical and morphological changes in valuable coastal ecosystems.Item Block-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.Item Modeling Lobe-And-Cleft Instabilities on a River Plume(Journal of Geophysical Research: Oceans, 2024-05-13) Shi, Fengyan; Simpson, Alexandra; Hsu, Tian-JianAbstract The lobe-and-cleft instability is a widely recognized mechanism leading to along-front structure on density current fronts. Early studies based on laboratory and numerical simulations suggested that the lobe-and-cleft instability is due to convective instability in the nose of gravity currents traveling over a nonslip boundary. Horner-Devine and Chickadel (2017, https://doi.org/10.1002/2017gl072997) reported the presence of lobe-and-cleft instabilities at the Merrimack River, which are generated at the river front in the absence of a no-slip boundary. Hence, the observed lobe-and-cleft instabilities must be due to other mechanisms. In this study, we carried out non-hydrostatic large eddy simulations of a riverine outflow into an idealized 3D domain. With a fine grid resolution of 0.15 × 0.31 m in two horizontal directions and about 0.125 m in the vertical direction, the model reproduced the lobe-and-cleft feature, with the magnitude and size of lobes consistent with the field observation. The model results revealed that instabilities start from the primary Kelvin-Helmholtz instability, followed by the secondary instability through stretching and tilting, generating counter-rotating streamwise vortices in the plume and at the plume head. The upwelling associated with streamwise vortex cells brings a slower flow to the plume surface, resulting in lobe-and-cleft patterns at the front and positive and negative vertical vorticity at the plume surface. The model also predicted a lobe width of about two to three times the plume thickness, consistent with the field observation and the lobe/cleft spacing associated with pairs of counter-rotating streamwise vortices. Modeled turbulent dissipation rate shows a trend of exponential decay from 10−4 to 10−3 m2/s3 at the frontal head to 10−7 to 10−6 m2/s3 behind the front, similar to the findings in the previous field studies. Key Points A non-hydrostatic large eddy simulation model is applied to reproduce lobe-and-cleft instabilities observed at the Merrimack River Model results reveal that instabilities originate from the Kelvin-Helmholtz instability, followed by the secondary instability, generating counter-rotating streamwise vortices at the front Modeled turbulent dissipation rate shows an exponential decay with increasing distance away from the front, consistent with field measurements Plain Language Summary Density currents are ubiquitous in nature and they play a key role in many important processes, such as weather pattern, ocean temperature and ecosystem, and sediment transport. The lobe-and-cleft instability is a mechanism that leads to along-front structure on density current fronts. These instabilities are prominent features for identifying the existence of density currents and they are also responsible for kinetic energy dissipation and mixing associated with the density currents. Although lobe-and-cleft instabilities have been observed in river plume fronts, their generation mechanisms remain unclear. In this study, we used a computer model to simulate the phenomena in an idealized domain similar to field observation. The model was able to reproduce the lobe-and-cleft feature that was observed in the field. We found that the instabilities were initiated from the primary Kelvin-Helmholtz instability and were followed by the secondary instability through stretching and tilting. This generates contour-rotating streamwise vortices in the plume and extends to the plume head. The lobe-and-clefts feature is caused by the upwelling associated with these streamwise vortex cells, which bring a slower flow to the plume surface.Item Sensitivity 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.Item Tsunamis caused by submarine slope failures along western Great Bahama Bank(Nature Publishing Group, 2016-11-04) Schnyder, Jara S.D.; Eberli, Gregor P.; Kirby, James T.; Shi, Fengyan; Tehranirad, Babak; Mulder, Thierry; Ducassou, Emmanuelle; Hebbeln, Dierk; Wintersteller, Paul; Jara S.D. Schnyder, Gregor P. Eberli, James T. Kirby, Fengyan Shi, Babak Tehranirad, Thierry Mulder, Emmanuelle Ducassou, Dierk Hebbeln & Paul Wintersteller; Kirby, James T.; Shi, Fengyan; Tehranirad, BabakSubmarine slope failures are a likely cause for tsunami generation along the East Coast of the United States. Among potential source areas for such tsunamis are submarine landslides and margin collapses of Bahamian platforms. Numerical models of past events, which have been identified using high-resolution multibeam bathymetric data, reveal possible tsunami impact on Bimini, the Florida Keys, and northern Cuba. Tsunamis caused by slope failures with terminal landslide velocity of 20 ms−1 will either dissipate while traveling through the Straits of Florida, or generate a maximum wave of 1.5 m at the Florida coast. Modeling a worst-case scenario with a calculated terminal landslide velocity generates a wave of 4.5 m height. The modeled margin collapse in southwestern Great Bahama Bank potentially has a high impact on northern Cuba, with wave heights between 3.3 to 9.5 m depending on the collapse velocity. The short distance and travel time from the source areas to densely populated coastal areas would make the Florida Keys and Miami vulnerable to such low-probability but high-impact events.