Limited Horizontal Extent Research Articles

Rainfall and weather conditions inducing intense landslide activity in northern Spain (Deba, Guipúzcoa)

ABSTRACT Deba area is intensely affected by frequent shallow landslides triggered by rainfall. This contribution explores the role of rainfall in landslide activity during a quite long time span (60 years), from a large network of rainfall gauges and a complete inventory of landslides. Out of 1,180 landslides inventoried, more than 50% occurred simultaneously in 6 known dates, corresponding to 6 episodes triggering multiple landslides; 3,241 rainfall episodes have been automatically recognized and characterized in terms of rainfall amount and duration, providing a representative dataset that covers a wide range of movement types and behaviors.The relationship between rainfall episodes driving multiple movements simultaneously has not been explored in depth so far in northern Spain. The extraordinary character of the triggering rainfall has been assessed and empirical rainfall thresholds (total amount, and mean intensity), producing multiple landslides, have been found and compared with others described in the literature. Also, the meteorological conditions associated to those extreme events have been recognized: multiple landslide occurrences are triggered by extreme convective rainfall: intense, short and with limited horizontal extent, as well as a marked summer-autumn seasonality. This weather pattern is more characteristic of Mediterranean areas than of mild marine west-coast climates.

Sedimentary characteristics and formation mechanisms of the Ordovician carbonate shoals in the Tarim Basin, NW China

The Ordovician carbonate shoals are extensively developed in the Tarim Basin, NW China, and are taken as important petroleum reservoirs. The sedimentary characteristics and controlling factors of these shoals are poorly studied. Multiple means, including field geological investigations, core observations, thin section identifications by scanning electron microscope and cathodoluminescence microscope, granulometric analyses, and geochemical analyses, are conducted. The classification of limestones and shoals in the Ordovician of the Tarim Basin is established. Four types of shoals are developed, namely, intraclastic shoals, bioclastic shoals, oolitic shoals, and algal shoals. It is demonstrated that these four shoal types are deposited in progradation sequences, among which, the low energy bioclastic shoals are in the sequence of retrogradation. Different sedimentary environments develop various sedimentary combination patterns: the platform margin facies mainly develop reef–shoal combinations followed by shoal–shoal combinations; the open platform facies mainly develop shoal–shoal combinations and then reef–shoal combinations; the restricted platform facies develops restricted shoal combinations. Intraclastic shoals are the most developed shoals, which are directly controlled by hydrodynamic force that caused by sea level changes and paleogeography that dominated by tectonic movement. In the platform margin facies, shoals are well-developed with large thickness and limited horizontal extent, and the main shoal type is bioclastic shoal, followed by algae shoal. In the open platform facies, Shoals usually present thin-layered with large horizontal extent. The main shoal type is intraclastic shoal that shows a gently moundy or sill-like structure, the followings are bioclastic shoals and oolitic shoals that developed in the highland near the platform margin facies. In the restricted platform facies, a small number of shoals are developed, which distributed in striped shape paralleling to the shoreline. This study provides great guidance for predicting shoals in different sedimentary facies and build good foundation for future shoal-related reservoirs assessment and prediction.

Combined loading of strip footings on sand-over-clay with layers of varying extents

This study describes the numerical analyses performed to investigate the bearing capacity of strip footings placed on granular (frictional) material overlying soft clay subjected to combined vertical-horizontal and vertical-moment loading. A plane-strain finite element limit analysis is used to estimate the limiting load combinations for two layer soil geometries where the top layer is either fully extended in the horizontal direction or it is of limited horizontal extent, representative of rock or gravel berms commonly used in offshore practice.Bearing capacity envelopes for combined vertical, horizontal and/or moment loading are well-documented in the literature for cases of footings resting on single sand or clay soil. For two-layer sand-over-clay soil with a horizontally extensive top layer, the vertical-horizontal and vertical-moment envelopes initially coincide with the envelope for a single sand layer at low vertical loads, but show an abrupt reduction in the horizontal or moment capacity as the vertical load increases beyond a certain critical vertical load. The critical vertical load is found to vary as a function of the thickness and the lateral extent of the upper layer. Relationships are presented to enable the vertical-horizontal and vertical-moment envelopes to be estimated based on the problem geometry and material properties.The findings in this study provide insight into the response of subsea foundations placed on rock or gravel of limited extent overlying a clay seabed as well as the general response of shallow foundations on two-layer profiles.

On self-maintenance of clear-air turbulence

It is shown that thin patches of clear-air turbulence (CAT) of limited horizontal extent may be a self-maintaining phenomenon due to the relative decrease of Richardson number at the lower and upper edge of the patch.

Numerical analysis of a laterally loaded shaft constructed within an MSE wall

Drilled shafts have been widely used to support lateral loads from superstructures. For typical applications, design methods are available to generate lateral load versus displacement curves and to estimate ultimate lateral capacity and displacement of the drilled shaft under a certain lateral load. However, occasionally drilled shafts have to be constructed within the reinforced zones of MSE walls, for instance drilled shafts supporting sound walls, traffic signs, billboards, and other superstructures. Under these circumstances, existing design methods are not applicable because of: (1) the limited horizontal extent of the soil mass; (2) the resistance from reinforcement; and (3) the influence of MSE wall facing. In this regard, a full-scale field study was conducted to investigate the behavior of shafts within the MSE wall, subjected to lateral loads. The test wall was 43 m long and 6 m high and constructed with layers of uniaxial geogrid and selected backfill. Three-dimensional numerical analyses were performed prior to the construction of this test wall (i.e., Class-A prediction) to guide its design and after the field test using the actual material properties (i.e., Class-C prediction). The selected test shaft for the analyses in this study was located at 1.8 m behind the wall facing. The numerical results from the Class-A and Class-C predictions are compared with the field data. The study showed that the Class-A prediction provided useful information for the design of the test wall and development of field test details. The Class-C prediction improved the overall accuracy of the calculations and could serve as a reference for future study.

Sea‐land breeze development during a summer bora event along the north‐eastern Adriatic coast

AbstractThe interaction of a summer frontal bora and the sea‐land breeze along the north‐eastern Adriatic coast was investigated by means of numerical simulations and available observations. Available measurements (in situ, radiosonde, satellite images) provided model validation. The modelled wind field revealed several regions where the summer bora (weaker than 6 m s−1) allowed sea‐breeze development: in the western parts of the Istrian peninsula and Rijeka Bay and along the north‐western coast of the island of Rab. Along the western Istrian coast, the position of the narrow convergence zone that formed depended greatly on the balance between the bora jets northward and southward of Istria. In the case of a strong northern (Trieste) bora jet, the westerly Istrian onshore flow presented the superposition of the dominant swirled bora flow and local weak thermal flow. It collided then with the easterly bora flow within the zone. With weakening of the Trieste bora jet, the convergence zone was a result of the pure westerly sea breeze and the easterly bora wind. In general, during a bora event, sea breezes were somewhat later and shorter, with limited horizontal extent. The spatial position of the convergence zone caused by the bora and sea‐breeze collision was strongly curved. The orientation of the head (of the thermally‐induced flow) was more in the vertical causing larger horizontal pressure gradients and stronger daytime maximum wind speed than in undisturbed conditions. Except for the island of Rab, other lee‐side islands in the area investigated did not provide favourable conditions for the sea‐breeze formation. Within a bora wake near the island of Krk, onshore flow occurred as well, although not as a sea‐breeze flow, but as the bottom branch of the lee rotor that was associated with the hydraulic jump‐like feature in the lee of the Velika Kapela Mountain. Copyright © 2010 Royal Meteorological Society

Comparison between CALIPSO and MIPAS observations of polar stratospheric clouds

Polar stratospheric cloud (PSC) detection, top height, and composition as derived from measurements by the infrared emission limb sounder Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat and the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar have been compared. The comparisons are based on coincident observations from the 2006 and 2007 Antarctic and the 2006/2007 and 2007/2008 Arctic winters. During the middle of the Antarctic season, very good agreement in common PSC detection (around 90%) has been found. At the beginning and end of the Antarctic PSC season and in the Arctic, the frequency of common PSC detection is generally less (60–70%) which can be explained by cloud inhomogeneity and viewing geometry differences. MIPAS PSC top heights are about 0–2 km lower than CALIPSO top heights with larger offsets at higher altitudes. The negative bias of MIPAS PSC top heights can be modeled under the assumptions of limited horizontal cloud extent and a field‐of‐view–dependent sensitivity. The comparisons further show a high degree of consistency between PSC composition derived from the fundamentally different classification approaches of the two instruments. Remaining differences can be explained considering the physical limitations of each approach and the definition of composition boundaries within the classification scheme of each instrument.

Retrieval of Urban Boundary Layer Structures from Doppler Lidar Data. Part II: Proper Orthogonal Decomposition

Abstract The proper orthogonal decomposition technique is applied to 74 snapshots of 3D wind and temperature fields to study turbulent coherent structures and their interplay in the urban boundary layer over Oklahoma City, Oklahoma. These snapshots of data are extracted from single-lidar data via a four-dimensional variational data assimilation technique. The total velocities and fluctuating temperature are used to construct the data matrix for the decomposition; thus the first eigenmode represents the temporal mean of these data. Roll vortices with a wavelength–height ratio of 3.2 are identified in the first, most energetic eigenmode and are attributed to the inflection-point instability. The second and third spatial eigenmodes also exhibit roll characteristics with different time and length scales, resulting in clockwise- and counterclockwise-rotating roll vortices above the airport and the central business districts. Their positive correlation with temperature fluctuation suggests that those roll structures are driven by thermal as well as wind shear. Their limited horizontal extent seems to coincide with the path of the Oklahoma River. With decreasing rank, coherent structures undergo a transition from roll to polygon patterns. A localized downdraft or updraft located above a cluster of restaurants is captured by the fourth eigenmode. In the capping inversion layer, gravity wave eigenmodes are observed and may be attributed to convection waves. The representation of instantaneous snapshots by high-ranking eigenmodes is then examined by reconstruction of reduced-order fields. It is found that the first four eigenmodes are sufficient to capture the overall characteristics of the 74 snapshots of data.

The effects of wind shear on cirrus: A large‐eddy model and radar case‐study

AbstractWind shear is an almost ubiquitous feature of the troposphere at cirrus altitudes, but there have been few studies focused on investigating its effects on cirrus clouds. In this study we consider a case of strongly sheared frontal ice cloud, which was observed by the Chilbolton radar. This was simulated using the Met Office large‐eddy model (LEM), which included a fully integrated radiation code. It is well known that there can be significant variations between results from different cirrus models, which makes a comparison of the simulations with observations important. So, radar observations of ice water content (IWC) were simulated directly within the framework of the LEM, and true and simulated IWC observations were compared, as well as their Fourier transforms and probability density functions (p.d.f.s). This showed that the LEM was capturing reasonably well the horizontally averaged IWC profile, and also at upper levels the variability in the IWC field, at scales of less than approximately 14 km. At lower levels the IWC field was too homogeneous.Varying the shear within the LEM simulations showed that shear had little effect on the mean IWC profile, but it increased the mixing and so the homogeneity of the IWC field. P.d.f.s of IWC derived from the simulated radar data compared well with observed p.d.f.s, except that the largest IWCs were not captured and there was too little variance at lower levels. Comparing p.d.f.s of IWC and total water for runs with and without shear clearly showed the effects of shear‐induced mixing. LEM results also showed that the variation of the correlation of IWC with vertical separation is shear dependent and initially linear, with or without shear. In this case, shear had little effect on the top‐of‐atmosphere and surface fluxes, or the within‐cloud heating rate profile, but this would be significantly different for a more patchy cloud.A modified case‐study, which allowed Kelvin–Helmholtz wave breaking, showed that wave breaking can significantly affect the microphysical processes, by increasing nucleation, deposition and sublimation rates. Wave breaking can occur in thin layers of limited horizontal extent, which would not be resolved by a global model. These effects were most significant when there was no large‐scale uplift and the vertical velocities from the wave breaking formed a cirrus cloud which did not otherwise occur. Copyright © 2005 Royal Meteorological Society

The design and testing of small, low-cost GPS-tracked surface drifters

We have designed and constructed drifters appropriate for use in regions characterized by limited horizontal extent. The drifters follow the upper 30 cm of surface currents, but can be modified to follow water at any depth. The drifters store their latitude and longitude internally, and transmit their current latitude and longitude to a handheld Global Positioning System (GPS)-radio receiver, making their location and subsequent retrieval straight forward A field test of six drifters in Hog Island Bay, Virginia, United States, in August 2003 was successful and led to several design improvements. With simple construction and a total materials cost of under U.S.$200 this design will make drifters an accessible part of interdisciplinary experiments, provide a potentially valuable educational tool, and make experiments that require large numbers of drifters more cost-effective.

Thermospheric responses to gravity waves arising from mesoscale convective complexes

We employ a linear model of the responses to local body forces to estimate the spectra of gravity waves arising due to vertical motions within mesoscale convective complexes (MCCs) at equatorial latitudes. Ray tracing methods are then used to propagate these spectra through model wind and temperature fields and to anticipate their thermospheric effects. We find that gravity wave forcing by MCCs is dominated by large vertical motions of limited horizontal extent, that individual convective cells within an MCC effectively act as independent sources of gravity waves if they are separated by two or more diameters, and that vertical body forces create higher-frequency gravity waves than comparable horizontal body forces. Ray tracing reveals that of the gravity waves excited by MCCs, only the high-frequency, long-vertical-wavelength portions of the refracted spectrum (above the shear) can penetrate to high altitudes, that variable winds impose significant anisotropy on the surviving gravity waves, that refraction and dissipation determine the portion of the MCC wave spectrum important in the lower thermosphere, and that large net momentum flux divergence may lead to strong local body forcing well into the thermosphere. Gravity waves excited by MCCs and having sufficiently large vertical wavelengths and group velocities above the shears at lower altitudes can penetrate to altitudes at which they may be expected to contribute to the seeding of equatorial spread F.

LocalizedS‐inversion of time‐domain electromagnetic data

Interpretation of time‐domain electromagnetic (TDEM) data over inhomogeneous geological structures is a challenging problem of geophysical exploration. The most widely used approach of interpreting TDEM data is based on the smooth 1‐D layered resistivity inversion. We have developed an effective technique of fast TDEM inversion based on thin‐sheet conductance approximation that we call S‐inversion. In this paper we extend the S‐inversion technique, approximating the conductivity cross‐section by adding a local inhomogeneous disk with an excess conductance ΔS to the horizontal conductive thin sheet used in S‐inversion. Localized S‐inversion determines the distribution of this excess conductance as a function of a depth and a horizontal coordinate. This new method takes into account the limited horizontal extent of the inhomogeneities, localizing inversion. The numerical modeling results and inversion of practical TDEM data demonstrate that the method resolves local geological targets better than traditional 1‐D inversion and original S‐inversion. The method can be applied to interpret both ground and airborne TDEM data sets.

Scattering layer thickness and position estimated by radar frequency domain interferometry: 1. Effects of the limited horizontal extent and advection of the scattering layers.

Frequency domain interferometry (FDI) is a mesosphere‐stratosphere‐troposphere radar technique used for probing thin‐layered structures of the atmosphere. The position and thickness of a scattering layer embedded within the illuminated volume can be deduced from the complex coherence of received signals at two closely spaced frequencies. This technique has permitted us to identify layered structures (called “FDI layers”) with thicknesses estimated to be around 50–200 m in the troposphere‐stratosphere. However, its application needs very restrictive hypotheses. For example, the layers are assumed to have a large extent in the horizontal plane with respect to the horizontal extent defined by the ‐6‐dB angular width of the two‐way antenna pattern. This hypothesis seems not always to be verified in light of observations with other radar techniques. In this paper, theoretical calculations are performed in order to estimate the consequences of the limited extent of the scattering structure in the horizontal plane on the FDI parameters, i.e. the vertical extent and position in height of the single layer. The first analysis concerns the effects of the limited extent by itself, and the second one deals with the consequences of the advection of the scattering structure by the wind. It is shown that in some limiting cases, substantial biases in the FDI parameters can occur. Finally, this work stresses the necessity to confront the FDI data with other technique measurement results or to complete it with space domain interferometry data. A future paper will be devoted to the study of the effects of the tilt of a scattering layer on the thickness and position estimations.

Anoxic sediments—an important environment for PGE; An overview

Anoxic sediments and metasediments have been recently recognized as important host environments for the platinum group elements (PGE). Economically significant PGE concentrations have been reported from marine, metal-rich black shales in China, Yukon (Canada) and Poland. These ore-bearing horizons are characterized by extremely restricted vertical, and sometimes larger horizontal extents. They are associated with the evolution of intracontinental rifting structures without any magmatic activity, suggesting a possible driving mechanism for mineralization processes. Complex PGE associations, including a significant relationship of Pd and Pt with Ni, Bi, Se, Y and also with Sb, Cu, Co, Au, U, and F are suggested using the results of a new statistical evaluation of available geochemical data. These occurrences show more common features with other hydrothermal PGE deposits. The rift-volcanic-related, marine metal-rich black shales of the Czech Republic and Finland, however, display lower PGE values. The mineralized layers are much thicker with a typically limited horizontal extent showing a close association with volcanic centres. Pd reveals a strong correlation with Ni, Zn, Au and V; Pt exhibits a highly significant link with the Cu and Mo phases. Low average Ni Cu ratios and PGE+Au chondrite-normalized patterns are similar to those of some magmatic PGE deposits hosted in mafic and ultramafic rocks. It is very likely that organic matter played an important role in the genesis of PGE accumulations in metal-rich black shales in both rift and rift-volcanic-related environments.

Transient electromagnetic modeling of shallow A‐type sections with 3-D inhomogeneities

A hard rock area underlain by granitic/gneissic or basaltic rocks often has an A‐type three‐layer geoelectric section in which resistivity increases with depth. The middle layer of moderate resistivity caused by fracturing/fissuring that lies between the surface‐weathered layer and the substratum of unfractured rock is not a good target for a direct current (DC) resistivity sounding since it is generally suppressed in the observations. Moreover, its definition requires expanding the electrode spacing to a length several times the depth of the target layer, and this may be a drawback if the target layer is either laterally variable or limited in its horizontal extent. We first studied the transient electric field of a horizontal electric dipole (HED) source excited by a step turn‐off current for a 1-D model of an A‐type geoelectric section. The results of this theoretical study are presented as graphs of normalized apparent resistivity versus a time‐related dimensionless parameter. Irrespective of the separation between the transmitter and receiver dipoles, these transient sounding curves become similar to the corresponding Schlumberger sounding curves at late time. Hence the transient electric field measurement offers the possibility of sounding at a fixed transmitter‐receiver spacing that may be shorter than the target depth. Also, at early times, for a certain ratio of the dipole separation to the target depth, there is a dramatic increase in the resolution of the response. Thus, it is possible to resolve suppressed layers of an A‐type section in this type of sounding. A study of the effects of transmitter ramp time and receiver bandwidth on the transient apparent resistivity curves shows that a very fast current shut‐off and wideband measurement are required to realize all the possibilities suggested by this modeling. Some 3-D transient electromagnetic (TEM) modeling was also done to simulate (1) a lateral variation in the resistivity of the middle layer of an A‐type section and (2) a weak zone of limited horizontal extent in the substratum of a two‐layer section. We observed that the 3-D inclusion has less effect at late time but is more pronounced at early time. In view of the above results, we conclude that the transient E‐field sounding with a grounded wire source can be used in place of a conventional DC resistivity sounding to overcome the problem of poor resolution due to the suppression of the intermediate layer in a geoelectric section where the resistivity increases with depth. As such, it has a potential application in groundwater as well as geotechnical surveys, because together with the overlying weathered layer, the fractured rock constitutes the aquifer in hard rocks.

Concerning the origin of enhanced sodium layers

A mechanism is proposed for the formation of the thin layers of enhanced sodium concentration observed in the atmospheric sodium layer. The proposed mechanism involves the wind‐shear distortion of initially thick sodium clouds of limited horizontal extent, originally produced by the deposition of material of extraterrestrial origin. This mechanism explains almost all the observed characteristics of the enhanced layers, including their height of occurrence, small vertical extent, vertical motion, rapid growth and decay and highly variable duration.

The reflection of wave energy by undulations on the seabed

The problem of the reflection of incident surface water waves by undulations on the seabed is treated on the basis of linear perturbation theory. The flow is assumed to be two-dimensional, uniplanar and nonseparating, and the undulations are assumed to be of limited horizontal extent, enabling results for the reflected and transmitted waves to be obtained by Fourier transform methods. A steady state formulation of the problem is adopted, and this provides general results which are applied to the case of a patch of sinusoidal undulations on the bed. The principal conclusions are that the reflection coefficient is both oscillatory in the ratio of the length of the patch to the surface wavelength and also, as expected, critically dependent on the ratio of the surface to bed wavelengths. In addition, the transmitted wave may experience a small phase shift, even in cases of zero reflection. Examples are presented which indicate that relatively few bottom undulations, with wavenumber equal to approximately twice the surface wavenumber, may give rise to a very substantial reflected wave. A possible consequence of this is a coupling between ripple growth and wave reflection, which may be important in problems of coastal protection.

Wind-Induced Mixing in Stratified Fluid

In this note a simple model is offered for mixed-layer deepening caused by wind action at the free surface of a density-stratified fluid. Solar radiation, cooling advection, and Coriolis effects are not considered. To include an aspect of mixing in waters of limited horizontal extent, a steamwise pressure gradient is taken into account.

Coal in Antarctica: ABSTRACT

Coal has been reported from numerous locations in Antarctica beginning about 70 years ago. Only since about 1960, however, have data become available on petrologic and analytic studies. Nearly all Antarctic coal deposits are in the Transantarctic Mountains of East Antarctica, that portion of the continent lying mostly south of Africa, Asia, and Australia. All appear to be of Permian age, and most have been altered by contact thermal metamorphism; they range in rank from low-volatile bituminous to semianthracite. The coal beds generally lack marine deposits and underclay. In addition to those in the Transantarctic Mountains, coal deposits have been reported from the Prince Charles Mountains. Coals in the Prince Charles Mountains mostly are unaffected by thermal metamorphis and have a rank of high-volatile bituminous. Most coal deposits in Antarctica have very limited horizontal extent; beds range to 3 or 4 m in thickness, but are generally thinner. Except for some deposits in the coastal parts of the Transantarctic Mountains and those in the Prince Charles Mountains, coal is found mainly in the interior of the continent. The location of most coal poses a major transportation problem for potential mines. Some coal could possibly be mined and used locally as an energy source for heating or power production. Another potential problem, yet to be resolved, is the matter of territorial claims, which might dictate the ownership of mineral deposits. End_of_Article - Last_Page 532------------

Reservoir Mechanism

The physical problem dealt with in this article is the unsteady groundwater flow in an isotropic, nonhomogenous aquifer of limited horizontal extent and arbitrary boundary shape. The unsteady ground-water level is described by a differential equation which is solved by the Galerkin finite element method, combined with orthogonal eigenfunctions. Unlike many numerical procedures the physical properties of the aquifer are not lost. Boundary conditions are provided by nature in the form of watertight rocks, rivers, lakes or any kind of constant water level in hydraulic contact with the aquifer, or given boundary inflow or outflow.