共查询到20条相似文献，搜索用时 500 毫秒
1.
利用我们建立的三维分层线性理论计算模式和中尺度数值模式ARPS, 分别研究了三维分层流动过双山脉地形产生的三维线性和非线性山脉重力波和大气船舶的结构特征及其形成机制.线性理论计算结果表明三维三层流动过双山脉地形时,两个山脉各自强迫出一个发散模态的山脉背风波,在第二个山脉背风面,三维三层流动过双山脉地形可以强迫出两个发散模态的拦截背风波,大大加强了对大气环流的拖曳作用.非线性数值模拟结果表明,流动过山所产生的非线性山脉重力波和大气船舶完全不同于三维分层线性理论计算模式所产生的山脉重力波和大气船舶的结构和特征,由于分层流体之间的非线性相互作用,三维三层流动过双山脉地形时,可在第二个山脉背风面激发4个发散模态的拦截背风波. 三维三层流动过双山脉地形所强迫的山脉重力波和大气船舶,具有同三维三层流动过孤立山脉所产生的山脉重力波和大气船舶完全不同的结构和特征,三维流动过双山脉地形对两个山脉之间的距离表现出极大的敏感性.对于相距较远的两个山脉,流动过双山脉所强迫的山脉重力波表现为4个发散模态的拦截背风波,波动的能量相对于相距较近的两个山脉能传播到更高的高度. 相似文献
2.
In this paper the boundary element algorithm which uses the time-convoluted traction kernels is applied to a numerical parametric study on the seismic behavior of three-dimensional Gaussian-shaped hills subjected to vertically propagating incident waves. All calculations were executed in the time-domain and the medium was assumed to have a linear elastic constitutive behavior. Results are discussed in both time and frequency domain with respect to the dimensionless parameters. It was shown that wave length and site geometry, including shape and dimension ratios and, to some extent, wave type are the key independent parameters governing hill amplification behavior. Comparing two- and three-dimensional hills with similar shape ratios, two-dimensional hill had greater characteristic periods, where the three-dimensional hill had greater maximum amplification potential. Three-dimensionality has a strong effect on the seismic responses of the hill; however the rate of seismic response variation with the three-dimensionality factor depends on the shape ratio. It was shown that two-dimensional behavior was dominant in low height three-dimensional hills, however, as the shape ratio increased, three-dimensionality effects appeared and the seismic response of the hill tends toward the axisymmetric three-dimensional hill. 相似文献
3.
The so called “valley effect” relates to the typical seismic response of basin shaped bedrock filled by quaternary sediments. It is an aspect of the renown “local seismic effect” that shall be taken into account when dealing with microzoning studies. Several experimental surveys and numerical simulations performed worldwide over the last 40 years, confirmed that valley responses under seismic excitations show common features in various geological contexts as far as the sedimentary valleys (e.g. alluvial and lacustrine plains), the intermountain valleys (e.g. alpine valleys) and graben shaped basins. Such features mainly depend on the basin geometry, referred to as the shape ratio SR, and the sediment and basin impedance contrast IC. Although researchers agree on the prominent role of local seismic effects for interpreting erratic damages caused by seismic shaking in urbanized areas, no fully shared strategies have been identified for taking into account valley effect within microzoning studies. In this paper, a numerical simulations on three models of trapezoidal shaped basins have been performed. These valley models relate to sediments and basins detected within the Tuscany Region territory during the VEL project. Results, in terms of the amplification index $\text{ F }_{\mathrm{A}}$ F A have been provided. Three “valley effect charts” for various SR and IC values have been propose for taking into account the local seismic effects due to the basin amplifications within microzoning maps. 相似文献
4.
Multiple levels evolved in Buddha–Christian's Cave and in Donaldson's–Bronson's–Twin Cave in a subtly different manner to that normally associated with the development of multilevel caves adjacent to entrenched river valleys. It is often assumed that lower cave levels are generated by successive phases of valley entrenchment; however, multiple levels may also be induced by progressive adjustments in groundwater flow paths. Such adjustments occur as the effects of a change in base level propagate up-basin. In the Mitchell Plain much of the late Pleistocene drop in the elevation of the principal drainage (the East Fork of the White River) was accommodated by entrenchment along the lower reaches of tributary valleys. Caves initially developed in the mid-sections of tributary basins at relatively shallow depths. In the vicinity of the cave exit, the piezometric surface gradually steepened and was eventually lowered through a combination of valley headwall retreat and the incremental concentration of flow through master conduits. Upper-level passages were abandoned once the original high-level flow paths were replaced by lower-level routes. 相似文献
5.
Azimuth dependent wave amplification in alluvial valleys 总被引：1，自引：0，他引：1
An extension of the indirect boundary element method (IBEM) to three-dimensional scattering by two-dimensional alluvial valleys is presented. While the valley is two-dimensional, the incident plane waves can arrive outside the 2D plane so the scattering is three-dimensional with coupling of P---SV---SH waves. Such a method makes it possible to take earthquake location into account in the estimation of site effects in alluvial valleys. The method is validated by transparency tests, by comparison with 2D simulations, and by comparison with results of other authors. The advantage of the method is that is combines high accuracy with cost-efficiency in terms of computer-time. It is applied to theoretically estimate site effects across a simplified model of an alluvial valley in the French Alps where azimuth dependence of local amplification has been observed. A parametric study with simulations for a range of azimuths and incidence angles shows that (1) the local amplification depends strongly on both azimuth and incidence of the incoming waves, (2) the global pattern of amplification across the valley is very complex for all azimuths, and (3) it is not possible to predict the 3D response of the valley from 2D modeling. Theoretical spectral ratios are in approximate agreement with observed ones for a station in the center of the valley where the local structure justifies use of a simplified model for the comparison. 相似文献
6.
The seismic site response of alluvial valleys with limited width is evaluated using three engineering models. The models are based on the one-dimensional, two-dimensional and the frame model approaches. The objective is to analyse the effects of the main parameters governing surface motions and provide engineering guidance for predicting them. The limitations on the use of the one-dimensional model in site response evaluation in valleys are pointed out. The frame model, which accounts for the limited width of valley, gives response results that are in good agreement with the two-dimensional model results. It is found that the effect of the two-dimensional amplification is significant over a distance from the valley edges beyond which the response may be adequately represented by one-dimensional analysis. The soil amplification varies depending on the soil type, site location relative to the valley and the dominant period and amplitude of input rock record. © 1997 by John Wiley & Sons, Ltd. 相似文献
7.
相邻地形对地震动特性的影响分析 总被引：1，自引：0，他引：1
基于采用透射人工边界的显式动力有限元方法,研究了相邻凸起地形对地震动反应谱特性的影响,分析了相邻凸起之间距离的变化对地形效应的影响。研究结果表明：1）与单一凸起地形对地震动的放大效应相比,相邻地形的存在对地震动反应谱谱比曲线的形状影响不大,但是却对谱比的值具有较大影响,而且其影响程度的大小与地表观测点的位置有关;2）相邻凸起地形的存在对凸起平台段中点地震动高频成分的放大效应具有较大影响,随着相邻凸起之间距离的增大,该影响效应逐渐减弱,多个相邻凸起构成的组合地形对地震动的放大效应逐渐接近单一凸起地形。 相似文献
8.
基于黏弹性人工边界的显式动力有限元方法,研究了突出平台状山谷地形场地的地形效应,分析了山谷地形对地表地震动的影响.结果表明:(1)沿着地震动输入方向地表反应较其他方向上的反应强烈;(2)与单一凸起山体地形对地表地震动的放大效应相比,相邻地形的存在对地表地震动位移傅里叶谱谱比曲线的形状影响不大,但是对PGD有着较大的影响... 相似文献
9.
K. N. Jansson A. P. Stroeven G. Alm K. I. T. Dahlgren N. F. Glasser B. W. Goodfellow 《地球表面变化过程与地形》2011,36(3):408-418
In order to extend our knowledge of glacial relief production in mountainous areas new methods are required for landscape reconstructions on a temporal resolution of a glacial cycle and a spatial resolution that includes the most important terrain components. A generic data set and a 50 m resolution digital elevation model over a study area in northern Sweden and Norway (the present day landscape data set) were employed to portray spatial patterns of erosion by reconstructing the landscape over successive cycles of glacial erosion. A maximum‐value geographic information system (GIS) filtering technique using variable neighbourhoods was applied such that existing highpoints in the landscape were used as erosional anchor points for the reconstruction of past landscape topography. An inherent assumption, therefore, is that the highest surfaces have experienced insignificant down‐wearing over the Quaternary. Over multiple reconstruction cycles, proceeding backwards in time, the highest summits increase in area, valleys become shallower, and the valley pattern becomes increasingly simplified as large valleys become in‐filled from the sides. The sum of these changes reduces relief. The pattern of glacial erosion, which is to 60% correlated to slope angle and to 70% correlated to relative relief, is characterized by (i) an abrupt erosional boundary below preserved summit areas, (ii) enhanced erosion in narrow valleys, (iii) restricted erosion of smooth areas, independently of elevation, (iv) eradication of small‐scale irregularities, (v) restricted erosion on isolated hills in low‐relief terrain, and (vi) a valley widening independent of valley directions. The method outlined in this paper shows how basic GIS filtering techniques can mimic some of the observed patterns of glacial erosion and thereby help deduce the key controls on the processes that govern large‐scale landscape evolution beneath ice sheets. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
10.
An indirect boundary integral method to obtain the three-dimensional response of an infinitely long, layered, viscoelastic valley of arbitrary cross-section embedded in a layered viscoelastic half-space is presented. The valley is excited by homogeneous plane waves impinging at an oblique angle with respect to the axis of the valley. The method and associated computer programs are tested by comparison with available results in the limiting two-dimensional case of incidence normal to the axis of the valley. Additional comparisons with previous three-dimensional results obtained by a hybrid finite element-boundary integral method for cylindrical valleys subjected to obliquely incident waves show large differences. However, the results obtained here for an infinitely long valley appear to be in some agreement with earlier results for an elongated prolate semi-ellipsoidal valley and with results obtained by a discrete wavenumber boundary element approach. An extensive bibliography on the dynamic response of valleys is also presented. 相似文献
11.
In this numerical study the effects of basin edge on the dynamic behavior of the clayey basins are investigated. For this purpose a range of bedrock inclinations at the valley sides from slighter 10° and 20° to steeper 30° and 40°, and three types of stiff, medium plasticity and soft clay materials are selected. The results of the 2D analyses show that not only the amplification pattern of different clay types is different, but also it differs for each material type under different motion intensities. Also, the frequency domain results show that different parts of the valleys are sensitive to different periods. It was seen that under two-dimensional conditions the amplification of clay types other than soft clay could be higher. Finally, the results of this research show the important effect of motion intensity on the 2D behavior of valleys. 相似文献
12.
It is well-known that the response of a site to a seismic solicitation depends on local topographical and geotechnical characteristics. Many aspects of seismic site effect still need to be studied in more detail and they can be incorporated in the seismic norms after quantification. The purpose of this paper is to contribute to establishment of a simple method to include complex site effects in a building code. Horizontal ground movements in various points of two-dimensional (2D) irregular configurations subjected to synthetic SV waves of vertical incidence are calculated. The parametric studies are achieved by means of HYBRID program combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The results are shown in the form of pseudo-acceleration response spectra. For the empty valleys, we can classify the spectral response according to a unique geometric criterion: the “surface/angle” ratio, where surface is the area of the valley opening, and angle denotes the angle between the slope and horizontal line in the above corner. To assess the influence of the 2D effect on the spectral response of filled valleys, the response of alluvial basins are compared with the response of one-dimensional columns of soil. Finally, an offset criterion is proposed to choose a relevant computation method for the spectral acceleration at the surface of alluvial basins. 相似文献
13.
The geological formations that occur around Sagar consist of Upper Rewa quartzitic sandstones of Vindhvan age and nine Deccan Trap flows with three main inter-trappean bands. Most of the villages around Sagar depend upon the flow No. 5 for their water supplies. This flow forms valleys which may occur either in between the Vindhyan hills or Vindhyan hills and Trap hills, or Trap hills. To evaluate the geo-hydrological conditions of this flow under different topographic and stratigraphic controls, detailed geologic, water-shed and water table maps were prepared and analysed, after taking into consideration rain fall data for a period of sixty years. The villages selected for studies include the farm-lands of Richonda which occur in between the Vindhyan hills and Kudari, which occurs in between the Trap and Vindhyan hills. Villages like Patkui and Bhainsa, occurring on either side of the surface water divide of the same flow, are also taken into consideration. Water table maps for these villages were prepared once before the onset of rains and the second time immediately after the rains, on a scale of 16″ to a mile at 1 foot contour intervals. From such studies made on this flow, the following conclusions have been arrived at:
- Where a flow occurs in adjacent Deccan Trap valleys separated by a long continuous Vindhyan ridge, the Vindhyan ridge may act as an underground barrier, separating the ground water body of the flow into two distinct units, and the upper unit may give rise to springs on that valley side of the Vindhyan having the lower elevation.
- Where the Vindhyan ridge loses its height and disappears below the flow, the adjacent separate water bodies of this flow merge into a single water body. The portion of the Vindhyan ridge that occurs below the ground still continues to act as barrier for the water bodies of the lower flows.
- Where the flow occurs over a large area, but at places is overlaid by younger flows, giving rise to hills with distinct water-shed characteristics, the water body of the flow is generally continuous on either side of the hills, immediately after the rains. This, however, gets disrupted into separate water bodies during summer months and it is found to recede in the slope direction; nevertheless, the trend of recession is controlled locally by the levels at which the porous zone of a flow occurs.
- Where the continuity of the flow is disrupted due to denudation, producing valleys, the continuity of the water table of that flow is also interrupted.
- On either side of a distinct surface water divide of a flow, the ground water bodies occur as separate units in the flow.
14.
《地震科学（英文版）》2019,(Z1)
A two and a half dimensional(2.5 D) multidomain indirect boundary element method(IBEM) is developed to study the wave scattering of obliquely incident P-, SVand SH-waves by a hill-valley staggered topography in a multi-layered half-space. The IBEM algorithm includes using2.5 D full-space and half-space Green's functions to construct scattered fields in decomposed closed and opened half-space regions, respectively, and using the dynamic stiffness method to solve the free fields. All regions are finally integrated by introducing the compatibility conditions to obtain the total wave fields. The proposed 2.5 D IBEM has the flexibility in dealing with complex boundaries by directly applying the fictitious loads on the regions' boundaries, with a less storage requirement compared to the full 3 D models. Besides, by combining the specific advantages of the two kinds of Green's functions, the method is well suitable for handling coupled topographies with high accuracy. The method is validated by comparison with published results for a single valley as well as a single hill topography. The effects of height-to-width ratio of hill and layering on dynamic responses are further parametrically investigated by numerical implementations in frequency domain. Results show that the interaction between valley and hills can lead to a more significant amplification within the valley region, and dynamic responses are deeply influenced by the height-towidth of hill, simultaneously depending on incident angle and frequency. Besides, the site effects become more complex when the stratification feature is taken into account. 相似文献
15.
DIRK GOOSSENS 《地球表面变化过程与地形》1996,21(3):205-216
Wind tunnel experiments of aeolian dust deposition on topographic scale models of ranges of hills were conducted. Different hill sizes and hill spacings were used, and comparisons with the deposition patterns over single, isolated hills were made. Dust profiles over ranges of hills differ from the profiles over identical, but isolated hills. On isolated hills the sedimentation maximum on the windward hillslope is always single and located on the concave part of the slope. In the case of ranges of hills, the maximum is either single or double, with the second peak on the convex part of the windward slope in the latter case. The local sedimentation maximum on the convex leeslope, which is rather unimportant on isolated hills, is much more developed in multiple-hill topography. Also, dust deposition on the leeslopes is significantly higher in multiple-hill topography than on isolated hills. Dust patterns on ranges of hills may be affected by the dust shadow created by the most upstream-located hill. If hills succeed each other quickly, they are located within the shadow zone and are protected from important dust deposition. The plume of high air dust concentration that is created by a hill largely determines the dust pattern on the next hill. As a result of the supply of dust from above by the descending plume, areas that are normally devoid of dust now experience significant dust deposition. 相似文献
16.
Boundary layer characteristics and turbulent exchange mechanisms in highly complex terrain 总被引：2，自引：0，他引：2
Mathias W. Rotach Marco Andretta Pierluigi Calanca Andreas P. Weigel Alexandra Weiss 《Acta Geophysica》2008,56(1):194-219
The Mesoscale Alpine Programme’s Riviera project investigated the turbulence structure and related exchange processes in an
Alpine valley by combining a detailed experimental campaign with high-resolution numerical modelling. The present contribution
reviews published material on the Riviera Valley’s boundary layer structure and discusses new material on the near-surface
turbulence structure. The general conclusion of the project is that despite the large spatial variability of turbulence characteristics
and the crucial influence of topography at all scales, the physical processes can accurately be understood and modelled. Nevertheless,
many of the “text book characteristics” like the interaction between the valley and slope wind systems or the erosion of the
nocturnal valley inversion need reconsideration, at least for small non-ideal valleys like the Riviera Valley. The project
has identified new areas of research such as post-processing methods for turbulence variables in complex terrain and new approaches
for the surface energy balance when advection is non-negligible. The exchange of moisture and heat between the valley atmosphere
and the free troposphere is dominated by local “secondary” circulations due to the curvature of the valley axis. Because many
curved valleys exist, and operational models still have rather poor resolution, parameterization of these processes may be
required. 相似文献
17.
G.K. Korotaev 《Surveys in Geophysics》1997,18(6):567-618
The dynamics of a single vortex on a beta-plane is discussed in this paper. A barotropic, an equivalent barotropic, one-and-a half and two-layer models are considered. The momentum and energy balances are used to describe the evolution of a vortex. A quasi-stationary balance of the Rossby, Zhukovsky-Kutta forces and the force induced by Rossby-wave radiation, describes the dynamics of the barotropic vortex. A net Coriolis force occurs if the fluid is stratified. The difference between the dynamics of cyclones and anticyclones results directly from the Coriolis force acting on a single vortex in a stratified fluid.All vortices radiate Rossby waves in the quasigeostrophic approximation but intense anticyclones propagate steadily in a one-and-a half layer model. A critical amplitude that bounds radiating and steadily propagating anticyclones is found. Steady propagation of anticyclones in general is impossible in a two-layer fluid due to the radiation of a barotropic Rossby-wave. Some solutions of solitary wave type which are known for a two-layer model, survive owing to wave interference.A single vortex can extract energy from a Rossby wave if synchronism conditions are satisfied. The wave interference again plays a crucial role in this case. The wave interference also determines the energy exchange of vortices located at larger distances. If the distance between the vortices is shorter than the length of the radiated waves, modon may be formed due to a small energy loss.The unbounded monotonic variation of the planetary vorticity is a characteristic feature of a beta-plane approximation. As a result, a single vortex propagates up to a 'rest latitude' where it disappears. The evolution of a single barotropic vortex over bottom topography provides another example of a background vorticity distribution with a local extremum above hills (valleys) or ridges (troughs). Physics of its movement differs from a beta-plane case, but if a vortex lies over broad topography, equations are similar and the evolution of a vortex manifests the same typical features. Particularly, a cyclonic vortex tends to drift to the top of a hill or a ridge. An anticyclonic vortex, on the contrary, slides to the bottom of a valley or a trough.An interaction of a barotropic vortex with a broad mean flow is tractable qualitatively on the basis of previous results. Numerical examples illustrating absorption of a small vortex by a larger one and a vortex movement across the flow, are direct analogies of the vortex evolution over a hill and a ridge, respectively. At the same time, strong influence of strain drastically changes the vortex structure. 相似文献
18.
研究黄土丘陵河谷场地在地震作用下强地面运动特征的变化情况,可以揭示强震对该类场地上震害的触发机理。结合黄土高原的地貌特征,建立具有代表性的动力数值分析模型,通过输入不同幅值、频谱特性和持续时间的地震波,对起伏地形和覆盖黄土层共同影响下的黄土河谷场地进行地震反应分析。结果表明:黄土层和地形耦合作用控制了地表的PGA变化,使其趋于复杂,在同一输入波不同振幅作用下,与基岩河谷各测点相比,黄土覆盖河谷场地的地震动频谱幅值均有所增加,并且频谱主峰均向高频移动。在不同地震波输入下,场地不同部位的固有频率受地形高程和土层影响;而地震动大小和频谱幅值不仅与场地的基本频谱和地形起伏有关,也与输入地震波的频谱成分相关。输入波PGA与地震频谱特征都不变时,同一场地输出的地震频谱形状具有相似的特征,随着地震持时增长,能量向场地基本频率附近集中,从而可能导致场地上相应频率建筑物震动幅值增加,造成累积破坏。 相似文献
19.
A numerical method is used for calculating the two-dimensional scattering of incident SH waves to try to explain some of the amplification patterns observed from recent data of the Mexico City's accelerometric array. The method is briefly presented and its efficiency is tested against analytical and other numerical solutions for canyons and alluvial valleys.
Spectral ratios computed for transition and lake-bed zones of the Mexico City valley with respect to the average motion at hill-zone sites are also presented. The one-dimensional model is used to explain the amplifications observed at a site where the valley is relatively shallow, while the two-dimensional approach is employed at another site at the centre of the valley where irregular amplification patterns have been observed. Results in the time domain are also shown. 相似文献
20.
《Soil Dynamics and Earthquake Engineering》1995,14(3):199-210
The seismic site response of alluvial valleys with limited width is studied. The intent of this investigation is to integrate the seismological and engineering perspectives to gain physical insight into the dynamic behaviour of alluvial valleys. A simplified engineering model (frame model) is developed to predict the nonlinear seismic response of symmetrical valleys. The proposed model is one-dimensional and accounts for the limited width of the valley. The frame model identifies the significant vibration modes and their variation in the horizontal and vertical directions. Sensitivity analyses are performed on the valley response to evaluate the effect of the uncertainty in establishing the dynamic soil properties. Response results from the frame model are compared with those calculated using one-dimensional and two-dimensional finite-element models. The proposed frame model response shows good agreement with finite-element model results. 相似文献