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Open access publications by faculty, staff, postdocs, and graduate students from the Center for Applied Coastal Research.


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    Modeling Lobe-And-Cleft Instabilities on a River Plume
    (Journal of Geophysical Research: Oceans, 2024-05-13) Shi, Fengyan; Simpson, Alexandra; Hsu, Tian-Jian
    Abstract 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, 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.
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    Non-Equilibrium Scour Evolution around an Emerged Structure Exposed to a Transient Wave
    (Journal of Marine Science and Engineering, 2024-06-05) Velioglu Sogut, Deniz; Sogut, Erdinc; Farhadzadeh, Ali; Hsu, Tian-Jian
    The present study evaluates the performance of two numerical approaches in estimating non-equilibrium scour patterns around a non-slender square structure subjected to a transient wave, by comparing numerical findings with experimental data. This study also investigates the impact of the structure’s positioning on bed evolution, analyzing configurations where the structure is either attached to the sidewall or positioned at the centerline of the wave flume. The first numerical method treats sediment particles as a distinct continuum phase, directly solving the continuity and momentum equations for both sediment and fluid phases. The second method estimates sediment transport using the quadratic law of bottom shear stress, yielding robust predictions of bed evolution through meticulous calibration and validation. The findings reveal that both methods underestimate vortex-induced near-bed vertical velocities. Deposits formed along vortex trajectories are overestimated by the first method, while the second method satisfactorily predicts the bed evolution beneath these paths. Scour holes caused by wave impingement tend to backfill as the flow intensity diminishes. The second method cannot sufficiently capture this backfilling, whereas the first method adequately reflects the phenomenon. Overall, this study highlights significant variations in the predictive capabilities of both methods in regard to the evolution of non-equilibrium scour at low Keulegan–Carpenter numbers.
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    Biophysical flocculation reduces variability of cohesive sediment settling velocity
    (Communications Earth & Environment, 2023-04-24) Ye, L.; Penaloza-Giraldo, J. A.; Manning, A. J.; Holyoke, J.; Hsu, T.-J.
    Biophysical cohesion, introduced predominantly by Extracellular Polymeric Substances (EPS) during mineral flocculation in subaqueous environments, plays important role in morphodynamics, biogeochemical cycles and ecosystem processes. However, the mechanism of how EPS functioning with cohesive particles and affects settling behaviors remain poorly understood. We measure initial flocculation rate, floc size and settling velocity of mineral and artificial EPS (Xanthan gum) mixtures. Combining results from these and previous studies demonstrate coherent intensification of EPS-related flocculation compare with those of pure mineral and oil-mineral mixtures. Importantly, the presence of EPS fundamentally changes floc structure and reduces variability of settling velocity. Measured data shows that ratios of microfloc and macrofloc settling velocity for pure mineral flocs is 3.9 but greatly reduced to a lowest value of 1.6 due to biological EPS addition. The low variability of settling velocity due to EPS participation explains the seemingly inconsistent results previously observed between field and laboratory studies.
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    Fourth-order stability analysis for capillary-gravity waves on finite-depth currents with constant vorticity
    (Physics of Fluids, 2023-02-01) Dhar, A. K.; Kirby, James T.
    We derive a fourth-order nonlinear evolution equation (NLEE) for narrow-banded Stokes wave in finite depth in the presence of surface tension and a mean flow with constant vorticity. The two-dimensional capillary-gravity wave motion on the surface of finite depth is considered here. The analysis is limited to one horizontal dimension, parallel to the direction of wave propagation, in order to take advantage of a formulation using potential flow theory. This evolution equation is then employed to examine the effect of vorticity on the Benjamin–Feir instability (BFI) of the Stokes capillary-gravity wave trains. It is found that the vorticity modifies significantly the modulational instability and in the case of finite depth, the combined effect of vorticity and capillarity is to enhance the instability growth rate influenced by capillarity when the vorticity is negative. The key point is that the present fourth-order analysis exhibits considerable deviations in the stability properties from the third-order analysis and gives better results consistent with the exact numerical results. Furthermore, the influence of linear shear current on Peregrine breather (PB) is studied.
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    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, Fengyan
    Hydrodynamic 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.
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    Momentum Balance Analysis of Spherical Objects and Long-Term Field Observations of Unexploded Ordnance (UXO) in the Swash Zone
    (Journal of Marine Science and Engineering, 2023-01-03) Cristaudo, Demetra; Gross, Benedict M.; Puleo, Jack A.
    Military activity has resulted in unexploded ordnance (UXO) existing in the nearshore. Understanding and predicting UXO behavior is important for object identification, and management. Here, two studies (laboratory and fieldwork) have been conducted to observe UXO surrogates in the swash zone and relate burial and migration to the underlying forcing conditions. A small-scale laboratory dam-break study was conducted to quantify migration of varying density spherical objects at different locations on a sloping, mobile, sandy bed. A moment balance was applied to derive two data-driven relationships to: (1) predict moments from the cross-shore flow velocity with predictions confined within a factor of two; (2) predict upslope or downslope migration from the moment. Fitting coefficients for the upslope and downslope relationships vary as a function of density, initial position, and burial. A field study was also conducted to investigate long-term behavior of eight varieties of UXO surrogates. Of the 129 observations, 56% were mobilized of which 76% were directed offshore. Burial/exposure was mostly related to far-field beach accretion/erosion (67%). However, scouring processes were also observed. Data showed that migration is likely a short-term process and most munitions will ultimately scour into a mobile bed.
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    Layout and design optimization of ocean wave energy converters: A scoping review of state-of-the-art canonical, hybrid, cooperative, and combinatorial optimization methods
    (Energy Reports, 2022-11-23) Golbaz, Danial; Asadi, Rojin; Amini, Erfan; Mehdipour, Hossein; Nasiri, Mahdieh; Etaati, Bahareh; Naeeni, Seyed Taghi Omid; Neshat, Mehdi; Mirjalili, Seyedali; Gandomi, Amir H.
    Ocean Wave energy is becoming a prominent technology, which is considered a vital renewable energy resource to achieve the Net-zero Emissions Plan by 2050. It is also projected to be commercialized widely and become a part of the industry that alters conventional energy technologies in the near future. However, wave energy technologies are not entirely yet developed and mature enough, so various criteria must be optimized to enter the energy market. In order to maximize the performance of wave energy converters (WECs) components, three challenges are mostly considered: Geometry, Power Take-off (PTO) parameters, and WECs’ layout. As each of such challenges plays a meaningful role in harnessing the maximum power output, this paper systematically reviews applied state-of-the-art optimization techniques, including standard, hybrid, cooperative, bi-level and combinatorial strategies. Due to the importance of fidelity and computational cost in numerical methods, we also discuss approaches to analyzing WECs interactions’ developments. Moreover, the benefits and drawbacks of the popular optimization methods applied to improve WEC parameters’ performance are summarized, briefly discussing their key characteristics. According to the scoping review, using a combination of bio-inspired algorithms and local search as a hybrid algorithm can outperform the other techniques in layout optimization in terms of convergence rate. A review of the geometry of WECs has emphasized the indispensability of optimizing and balancing design parameters with cost issues in multimodal and large-scale problems.
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    Entrainment 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, Jack
    Entrainment 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.
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    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.
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    Numerical modeling of intertidal mudflat profile evolution under waves and currents
    (Coastal Engineering Journal, 2022-06-25) Miranda, Paterno S.; Kobayashi, Nobuhisa
    The 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.
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    Numerical 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-Jian
    In 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.
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    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, Conor
    We 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.
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    Tsunamis caused by submarine slope failures along western Great Bahama Bank
    (Nature Publishing Group, 10/7/16) 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. Schnyder1, Gregor P. Eberli1, James T. Kirby2, Fengyan Shi2, Babak Tehranirad2, Thierry Mulder3, Emmanuelle Ducassou3, Dierk Hebbeln4 & Paul Wintersteller4; Kirby Jr, James T; Shi, Fengyan
    Submarine 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.
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    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, Babak
    Submarine 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.
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    Lattice Boltzmann approach for hydro-acoustic waves generated by tsunamigenic sea bottom displacement
    (Elsevier, 2016-09-23) Prestininzi, P.; Abdolali, A.; Montessori, A.; Kirby, J. T.; La Rocca, M.; P. Prestininzi, A. Abdolali, A. Montessori, J. T. Kirby, M. La Rocca; Abdolali, A.
    Tsunami waves are generated by sea bottom failures, landslides and faults. The concurrent generation of hydro-acoustic waves (HAW), which travel much faster than the tsunami, has received much attention, motivated by their possible exploitation as precursors of tsunamis. This feature makes the detection of HAW particularly well-suited for building an early-warning system. Accuracy and efficiency of the modelling approaches for HAW thus play a pivotal role in the design of such systems. Here, we present a Lattice Boltzmann Method (LBM) for the generation and propagation of HAW resulting from tsunamigenic ground motions and verify it against commonly employed modelling solutions. LBM is well known for providing fast and accurate solutions to both hydrodynamics and acoustics problems, thus it naturally becomes a candidate as a comprehensive computational tool for modelling generation and propagation of HAW.
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