Simplified Topography Research Articles

Topography effect on seismic waveform tomography: a quantitative study

SUMMARY In seismic tomography practices the Earth's surface is sometimes assumed to be either spherical or flat for convenience in forward modelling calculations. The effect of irregular surface topography on seismic wave propagation is thus ignored, resulting in biases in the phases and amplitudes of synthetic seismograms, which contribute to the residuals that are mapped into velocity structures in tomography inversions. In this study, we conduct a series of inversion experiments based on the adjoint waveform tomography method to quantitatively assess the topography effect on waveform-based inversion results. We first employ models with simplified topography to better highlight and quantify the topography effect. Results show that when topography effect is ignored in the forward modelling, it is mapped into velocity perturbations, leading to spurious velocity anomalies in tomography models. The strength of the spurious velocity anomaly is quasi-linearly related to locally averaged topography gradient. Our inversion experiments demonstrate that in places of strong topography variation, such as the Longmenshan Fault Zone region where the 2008 Mw 7.9 Wenchuan earthquake occurred, topography effect can lead to spurious relative velocity anomalies of up to 10 per cent, which cannot be ignored in waveform-based tomography inversions.

The Response of the Polar Vortex to Tropospheric Temperature Eddies in an Idealized General Circulation Model

Abstract A dry-core idealized general circulation model with a stratospheric polar vortex in the Northern Hemisphere is run with a combination of simplified topography and imposed tropospheric temperature perturbations, each located in the Northern Hemisphere with a zonal wavenumber of 1. The phase difference between the imposed temperature wave and the topography is varied to understand what effect this has on the occurrence of polar vortex displacements. Geometric moments are used to identify the centroid of the polar vortex for the purposes of classifying whether or not the polar vortex is displaced. Displacements of the polar vortex are a response to increased tropospheric wave activity. Compared to a model run with only topography, the likelihood of the polar vortex being displaced increases when the warm region is located west of the topography peak, and decreases when the cold region is west of the topography peak. This response from the polar vortex is due to the modulation of vertically propagating wave activity by the temperature forcing. When the southerly winds on the western side of the topographically forced anticyclone are collocated with warm- or cold-temperature forcing, the vertical wave activity flux in the troposphere becomes more positive or negative, respectively. This is in line with recent reanalysis studies that showed that anomalous warming west of the surface pressure high, in the climatological standing wave, precedes polar vortex disturbances.

Topography and structural heterogeneities in surface ground deformation: a simulation test for Somma-Vesuvius volcano

Abstract. We simulate the deformation of Somma-Vesuvius volcano due to some overpressure sources by means of a finite element 3D code. The main goal of these simulations is to investigate the influence of topography and structural heterogeneity on ground deformation. In our model the sources of deformation are embedded in an elastic linear isotropic medium and located at various depths. Geometry (shape and lateral extension) of the sources is mainly constrained by the results coming from recent seismic tomography studies. The structural heterogeneity has been modelled in terms of dynamic elastic parameters (Young's modulus) retrieved from previous seismic tomography and gravity studies. A high-resolution digital terrain model is used for the topography of the volcano subaerial edifice. Evidences from our results suggest that real topography and structural heterogeneities are key factors governing the ground deformation, which often turns being one of the most relevant problems in volcano monitoring. A large deviation from the axially symmetrical model of the displacement field is the main result of our modelling. Such an asymmetry is routinely unaccounted for when Mogi's simplistic modelling in a homogeneous medium with simplified topography is used. Our study clearly demonstrate that a better knowledge of deformation patterns can significantly help in the location of monitoring sensors as well as in the design of an efficient geodetic network.

Study of wind-direction effects on flow fields over two-dimensional hills using large eddy simulations

Numerous studies have been performed to investigate the characteristics of flow fields over simplified topographies to understand the basic trend of the flow field evolution as a function of hill shapes, hill slopes, ground roughness, and oncoming flow stratifications. However, wind direction, which has a key function in determining flow structures, has rarely been examined. Accordingly, the effects of wind direction on flow fields over two-dimensional (2D) hills are investigated in this study using large eddy simulations. It is found that when the approaching wind direction is not normal to the ridge line, the wind direction behind the 2D hill is not parallel to the approaching wind direction. The recirculation bubble downstream of the hill is weakened, and a significant spanwise velocity behind the hill is observed as the wind direction angle increases. The main direction of the streamwise correlation is found to be normal to the ridge line when the wind direction is 45° rather than parallel with the oncoming wind. A distinct peak in the spectrum of streamwise velocity is identified when the wind direction is 45°. This spectral peak is caused by the periodic shedding of an organized spiral flow structure.

LES study of turbulent flow fields over hilly terrains — Comparisons of inflow turbulence generation methods and SGS models

The performance of inflow turbulence generation methods and SGS models are crucial for accurate prediction of turbulent flow fields over hilly terrains using large eddy simulation (LES). It is thus necessary to evaluate the capacity of different inflow turbulence generation methods and SGS models. In this study, two types of inflow turbulence generation methods, wind tunnel replication (WTR) method and consistent discrete random flow generation (CDRFG) method, are used to simulate the approaching flows of turbulent flow fields over simplified topographies, including a smooth two-dimensional (2-D) ridge and a smooth three-dimensional (3-D) hill. The advantages and limitations of aforementioned methods are discussed in the context of flow patterns and characteristics of turbulent flow. The ability of three different subgrid-scale (SGS) models is systematically investigated: (a) standard Smagorinsky model (SM), (b) coherent structure Smagorinsky model (CSM) and (c) dynamic Lagrangian Smagorinsky model (DLSM). The numerical results obtained from the CSM and DLSM are in relatively poor agreement with those of the experiment, which is mainly caused by inaccurate estimation of the characteristics of turbulent flow near the ground surface of approaching flow. The size of the recirculation bubble predicted by the CSM and DLSM are smaller than that of the SM for both 2-D ridge and 3-D hill. It is found that the closed-wake region is formed behind the 2-D ridge, while the open-wake region is established on the leeward side of 3-D hill due to the existence of the secondary flow. The open-wake region is well reproduced by the SM rather than the CSM and DLSM. The non-Gaussian characteristics of wake flow and the topography-driven twist effects are also examined.

Lightning-induced voltages on overhead lines over irregular terrains

This paper analyzes lightning-induced voltages on distribution lines located on irregular terrains since it is common, in some places in the world, that real distribution systems are placed on those terrains. Simplified topographies were used in this analysis to approximate to those found on real distribution systems. The lightning-induced voltages were calculated with a full-wave analysis approach based on the 3D Finite-Difference Time-Domain (FDTD) method. It was found that lightning-induced voltages on overhead lines above irregular topographies, in the majority of cases, are greater than those found for flat terrain. The enhancement factor can reach values up to 5.2 times. In a few cases, factors below unity were found. These results may give a new insight on the design of overhead distribution lines for improving lightning performance on real distribution networks.

Multi-phase flow modeling of submarine landslides: Transformation from hyperconcentrated flows into turbidity currents

Sedimentary density flows are major agents of sediment transport on oceans. Occurring after submarine landslides, the sedimentary density flows experience a series of dilution processes. Such flows, then, transform from the hyperconcentrated flows into the concentrated ones, and finally turn into the turbidity currents. This study applies a sophisticated multi-phase flow model to simulate sedimentary density flows caused by submarine granular landslides over a simplified topography with an emphasis on their dilution mechanisms. This model takes account of the particle-particle interactions, the particle-fluid interactions, and the turbulent motions of fluid and particles in the momentum transports. The two effects on turbulence modulation are considered: one is due to the stratification and the other is caused by the correlation between the fluid and sediment turbulent motions. The numerical results suggest that the dilution mechanisms are highly related to Kelvin–Helmholtz vortices. In addition to erosion of the hyperconcentrated region by vortices, this study finds that strong stir in the concentrated region by the vortices is another important dilution mechanism. Compared with the outer regions of the vortices (with zero vorticity), the vortex cores (with non-zero vorticity) have less efficient dilution processes due to rigid-body rotations there. The vortex cores can trap the small particles for a while. The involved flow regimes are also presented. The numerical results reveal that the turbidity currents can be either turbulent or laminar when the Bagnold limit of concentration is adopted to define the boundary of turbidity currents.

Study of the flow fields over simplified topographies with different roughness conditions using large eddy simulations

The parameters influencing the wind turbine fatigue load calculations, such as two-point correlations Ruu, power spectrum density Su, turbulent length scale Lu, skewness Sku, and kurtosis Kuu of the wind are examined. Four simplified topographies, i.e., a 3D hill with smooth ground (3Ds), a 3D hill with rough ground (3Dr), a 2D ridge with smooth ground (2Ds), and a 2D ridge with rough ground (2Dr) are considered to investigate the influence from the shape of the topography and the ground roughness conditions. Ruu was found to vary considerably for different hill shapes and ground roughness conditions. Sku and Kuu peaked in the shear layer region in the smooth cases, but not in the rough cases. Su exhibited concentration in the wake in the 3Ds, 3Dr, and 2Ds cases, but not in the 2Dr case. In addition, a prominent increase in Lux was observed just above the summit of the smooth 3D hill. The flow fields were further visualized using the enstrophy and Q-criteria. Coherent turbulent structures were observed to exist in the wake in the 3Ds, 3Dr, and 2Ds cases, whereas the flow was highly mixed in the wake in the 2Dr case.

Transferability of a calibrated numerical model of rock avalanche run‐out: Application to 20 rock avalanches on the Nuussuaq Peninsula, West Greenland

AbstractLong run‐out rock avalanches are one of the most hazardous geomorphic processes, and risk assessments of the threat they pose are often reliant on numerical modelling of their potential run‐out distance. The development of such models requires a thorough understanding of past flow behaviour inferred from deposits emplaced by previous events. Despite this, few records exist of multiple rock avalanches that occurred in conditions sufficiently consistent to develop a set of more generalised, and hence transferrable, rules. We conduct field and imagery‐based mapping and use numerical modelling to investigate the emplacement of 20 adjacent rock avalanches on the southern flanks of the Nuussuaq peninsula, West Greenland. The rock avalanches run out towards the Vaigat Strait, and are sourced from a range of coastal mountains of relatively uniform geology. We calibrate a three‐dimensional continuum dynamic flow code, VolcFlow, with data from a modern, well‐constrained event that occurred at Paatuut (ad 2000). The best‐fit model assumes a constant retarding stress with a collisional stress coefficient, simulating run‐out to within ±0.3% of that observed. This calibration was then used to model the emplacement of deposits from five other neighbouring rock avalanches before simulating the general characteristics of a further 14 rock avalanche deposits on simplified topography. Our findings illustrate that a single calibration of VolcFlow can account for the observed deposit morphology of a uniquely large collection of rock avalanche deposits, emplaced by a series of events spanning a large volume range. Although the prevailing approach of tuning models to a specific case may be useful for detailed back‐analysis of that event, we show that more generally applied models, even using a single pair of rheological parameters, can be used to model potential rock avalanches of varied volumes in a region and, therefore, to assess the risks that they pose. © 2018 John Wiley & Sons, Ltd.

Estimating the reproduction quality of precipitation over the north atlantic and influence of the hydrostatic approximation in the WRF–ARW atmospheric model

The Weather Research and Forecast numerical model (WRF) with the dynamic Advanced Research WRF (ARW) solver was used to simulate the winter (January 2016) and summer (July 2015) atmospheric state over the North Atlantic with a high (15 km) spatial resolution. The quality of precipitation modeling was validated by remote sensing Global Precipitation Measurements (GPM) data and atmospheric ERA-Interim reanalysis. Nonhydrostatic and hydrostatic equations for the vertical velocity were additionally used to investigate their influence on the accuracy of the precipitation modeling results. It was shown that the model in this configuration satisfactorily reproduces the precipitation field. No evidence of hydrostatic approximation was revealed (over a simulation domain with a resolution of 15 km, simplified topography, and parameterizations of convection and microphysical processes).

Reflection of Linear Internal Tides from Realistic Topography: The Tasman Continental Slope

AbstractThe reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of the mode-1 internal tide as mode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget.

Drainage System and Detailed Urban Topography: Towards Operational 1D-2D Modelling for Stormwater Management

Flash floods are increasingly occurring due to climate change and the dramatic increase of population over areas characterized by aging and undersized drainage systems. Many studies are dealing with the topic of urban pluvial flooding, but few of them thoroughly consider interactions between surface runoff and pipe flow. In addition, to get a better knowledge of the processes leading to flash floods in urban areas, the specificities of urban topography features have to be taken into account while modelling dual drainage, such as streets, sidewalks, road curbs, bridges, etc. This paper aims to present a detailed modelling approach at a district scale as a first step, in order to determine impacts of detailed and simplified topography implementation on intense runoff modelling. The second step aims to set up an optimized city scale runoff modelling and management. The coastal Mediterranean city (Nice, France) characterized by steep and low slopes areas suffered from an intense rainfall event in October 2015. This study-case enables to investigate on such problematic as a high-resolution (0.2m) photo-interpreted topographic dataset is available. Investigation based on Geographic Information System (GIS) is followed by 1D and 2D modelling. Results are presented by confronting outcomes given by GIS analysis and hydrodynamic modelling. It is shown that high-resolution data should be used carefully while dealing with urban hydrology and hydraulics. The reflection is driven towards a set of guidelines for modellers interested in helping stormwater operational management.

Dynamical analysis of sea-breeze hodograph rotation in Sardinia

Abstract. This study investigates the diurnal evolution of sea-breeze (SB) rotation over an island at the middle latitudes. Earlier research on sea breezes in Sardinia shows that the onshore winds around various coasts of the island exhibit both the theoretically predicted clockwise rotation as well as seemingly anomalous anticlockwise rotation. A non-hydrostatic fully compressible numerical model (WRF) is used to simulate wind fields on and around the island on previously studied sea-breeze days, and is shown to capture the circulation on all coasts accurately. Diurnal rotation of wind is examined, and patterns of clockwise and anticlockwise rotation are identified. A dynamical analysis is performed by extracting individual forcing terms from the horizontal momentum equations. Analysis of several regions around the island shows that the direction of rotation is a result of a complex interaction between near-surface and synoptic pressure gradient, Coriolis and advection forcings. An idealized simulation is performed over an artificial island with dramatically simplified topography yet similar dimensions and latitude to Sardinia. Dynamical analysis of the idealized case reveals a rather different pattern of hodograph rotation to the real Sardinia, yet similar underlying dynamics. The research provides new insights into the dynamics underlying sea-breeze hodograph rotation, especially in coastal zones with a complex topography and/or coastline.

Runout and deposit morphology of Bingham fluid as a function of initial volume: implication for debris flow modelling

Debris flow modelling has become an important tool for assessing the related hazard so as to undertake appropriate mitigation actions and reduce the associated risk. Volume values are key input data for landslides numerical modelling. This work analyses the influence of the uncertainties related to initial volume and initial mass morphology (using the aspect ratio as a parameter) on the spreading of Bingham fluid. The dependency of this effect on slope changes is also analysed using three interesting landscape configurations: a horizontal plane, a simplified bilinear topography with increasing slope angle, and a real topography (Colima volcano landscape). We use the smoothed particle hydrodynamics (SPH) model to carry out this analysis. The SPH model is a depth-integrated model, which was previously validated by reproducing problems with analytical results (1D break dam). The initial aspect ratio is a primordial control parameter of the spreading in the case of a quasi-horizontal plane or very gentle slope, but this effect dissipates when the slope angle increases and the flow dynamics become essentially controlled by its volume. Even if most hydrodynamic models using different rheological approach can successfully reproduce real events, input data uncertainties and model sensitivity should be taken into account.

A model for fluid flow in non‐fully filled tribological interfaces – Part 2: Simulations

AbstractThe paper presents simulation results of an algorithm for non‐fully filled gaps, as it was introduced in part one of this work. The model considers the roughness of two solid surfaces moving relative to each other in reference to its effect on the fluid displacement. Due to the lack of fluid in the gap, the area where a hydrodynamic pressure can build up is rather small. However approaching asperities can build up local pressure. The non‐linear effect of the filling ratio on the pressure build‐up and the fluid flow is shown using a simplified topography of two ellipsoidal asperities. Depending on the filling ratio the fluid is squished out of the decreasing gap volume between the two asperities. So the amount of solid to solid contact is reduced by the fluid flow. The paper also discusses the behaviour of a multi asperity contact by means of two Gaussian height distributed surfaces sliding over each other. It is discussed how the starved regime influences the local pressure distribution and the gap geometry. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Suitability of simple rheological laws for the numerical simulation of dense pyroclastic flows and long-runout volcanic avalanches

[1] The rheology of volcanic rock avalanches and dense pyroclastic flows is complex, and it is difficult at present to constrain the physics of their processes. The problem lies in defining the most suitable parameters for simulating the behavior of these natural flows. Existing models are often based on the Coulomb rheology, sometimes with a velocity-dependent stress (e.g., Voellmy), but other laws have also been used. Here I explore the characteristics of flows, and their deposits, obtained on simplified topographies by varying source conditions and rheology. The Coulomb rheology, irrespective of whether there is a velocity-dependent stress, forms cone-shaped deposits that do not resemble those of natural long-runout events. A purely viscous or a purely turbulent flow can achieve realistic velocities and thicknesses but cannot form a deposit on slopes. The plastic rheology, with (e.g., Bingham) or without a velocity-dependent stress, is more suitable for the simulation of dense pyroclastic flows and long-runout volcanic avalanches. With this rheology, numerical flows form by pulses, which are often observed during natural flow emplacement. The flows exhibit realistic velocities and deposits of realistic thicknesses. The plastic rheology is also able to generate the frontal lobes and lateral levees which are commonly observed in the field. With the plastic rheology, levee formation occurs at the flow front due to a divergence of the driving stresses at the edges. Once formed, the levees then channel the remaining flow mass. The results should help future modelers of volcanic flows with their choice of which mechanical law corresponds best to the event they are studying.

Tide‐surge Interaction Intensified by the Taiwan Strait

The Taiwan Strait is a long and wide shelf‐channel where the hydrodynamics is extremely complex, being characterized by strong tides, and where storm surges frequently occur during the typhoon season. Obvious oscillations due to tide‐surge interaction were observed by tide gauges along the northern Fujian coast, the west bank of the Taiwan Strait, during Typhoon Dan (1999). Numerical experiments indicate that nonlinear bottom friction (described by the quadratic formula) is a major factor to predict these oscillations while the nonlinear advective terms and the shallow water effect have little contribution. It is found that the tide‐surge interaction in the northern portion of the Taiwan Strait is intensified by the strait. Simulations based on simplified topographies with and without the island of Taiwan show that, in the presence of the island, the channel effect strengthens tidal currents and tends to align the major axes of tidal ellipses along the channel direction. Storm‐induced currents are also strengthened by the channel. The pattern of strong tidal currents and storm‐induced currents along the channel direction enhances tide‐surge interaction via the nonlinear bottom friction, resulting in the obvious oscillations along the northern Fujian coast.

Experimental study of real roughness attenuation in concentrated contacts

The understanding of the processes involved in the in-contact deformation of surface roughness represents one of key factors in increasing lubrication capabilities of highly loaded machine components. Two main approaches have been developed in an effort to understand the changes of initial surface topography within highly loaded contacts to provide detailed information about lubrication film thickness and pressure distribution in the vicinity of roughness features. The first approach considers the real surface topography while the other uses the simplified topography features. Numerical solutions based on measured topography data can provide the film thickness and pressure distribution around asperities of realistic scale; nevertheless, obtained results are typically limited to the specific topography configuration measured from a very small area of rubbing surface. That is why some researchers have considered harmonic features of various wavelength and amplitudes to explain the behaviour of real roughness. This study is focused on the experimental validation of an approach based on Fourier decomposition of the surface roughness into harmonic components. Two optical measurement techniques—phase shifting interferometry and thin film colorimetric interferometry are combined to provide the undeformed surface topography and film thickness data within the elastohydrodynamic contact formed between a smooth disk and a ball having a real rough surface. The results obtained under pure rolling conditions not only confirmed the general principle that roughness deformation is component dependent and that long wavelengths deform more than short wavelengths, also the observed deformation for different components agreed well with the data predicted by the theory.

Influence of topography on ground deformation at Mt. Vesuvius (Italy) by finite element modelling

Ground deformations are observed in connection with volcanic activity, and therefore, geodetic monitoring can provide significant indication of changes of equilibrium conditions. The aim of this paper is to study the deformation of Mount Vesuvius (Italy) caused by overpressure sources at various depths, using a commercial (Ansys) 3D finite element code, in the framework of linear elastic isotropic material behavior. Both homogenous and heterogeneous media with carbonate basement were analyzed to understand the influence of topography on the ground deformations. The topography of the Somma-Vesuvius was taken into account, using a digital terrain model, and the carbonate basement was schematically modelled by assuming two horizontal layers with different Young moduli. The presence of a strong deviation from axially symmetric pattern of the displacement field, and of small subsidence areas, was found. These characteristics are completely unknown from the simple Mogi model and by simplified topography model, as verified by ad hoc simulations. These preliminary results, showing areas of the volcanic edifice experiencing high deformation, can improve the determination of the sources of deformations, i.e. the most relevant problem in the volcano monitoring. Moreover, the knowledge of the deformation pattern, including the topography effects, can provide significant indications to optimize the location of sensors and the characteristics needed to design an efficient and reliable geodetic monitoring network able to detect shallow intrusion events.

Mathematical Modelling of Atmospheric Flow over a Shelter‐belt

AbstractThe paper presents a mathematical and numerical study of the flow with pollution dispersion in the atmospheric boundary layer (ABL) over a simplified topography with a shelter‐belt placed before the pollutant (coal dust) source. The flow is supposed to be viscous, incompressible, turbulent and stationary. A two different numerical models are briefly mentioned. The shelter‐belt is supposed to be a solid and impermeable obstacle significantly changing the flow‐field and hence it gives the possibility of influencing the level of the pollutant concentrations in the downstream region. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)