The air flow and heat transfer in gravel embankment in permafrost areas

A comparative numerical investigation of transient temperature profile and pore-air velocities in horizontal rock block embankments are conducted using the "gravels model", in which the embankment is composed of stones and air, and the "porous media model" respectively. As the velocities from the "gravels model" directly reflect the true flow of air and winter-time convection, in this paper it can be concluded that computational results from the "gravels model"are superior to the "porous media model". In addition, the "gravels model" has the advantages of reflecting the effect of the dimensions and collocation of gravels upon the temperature fields.Therefore, the computation of the gravels embankment is mainly based on the gravels model.Simulation results show that in summer, a clockwise circulation of the pore-air extends throughout most of the embankment. However its motion is very weak that results in relatively straight horizontal isotherm lines. And heat transfer is mainly maintained through conduction. But in winter, the pore-air velocities are higher and multiple vortexes are formed in the embankment.Natural convection then becomes the dominant influence on the isotherm shapes within the embankment. The isotherms are complex and alternative upward and downward flowing plumes exist. The winter-time convection can further reduce the temperature of the foundation soil beneath the gravel embankment. In addition, the effects of the gravel dimensions within the embankment have been analyzed and compared in the gravels model. It shows that in winter, large stones, e.g. 200 mm, lead to stronger vortexes than those of small stones, say 60 mm. Consequently, the zone of low-temperature beneath the large-stone embankment extends deeper into the ground.     

Numerical analysis for cooling effect of open boundary ripped-rock embankment on Qinghai-Tibetan railway

At present, the Qinghai-Tibetan railway is being built, and it will pass across more than 550-km perma-frost regions. Therefore, the key to the stability of therailway embankment lies in solving the permafrost problem. Because global warming and existence of railway tend to degrade the permafrost in these re-gions[1], more difficulties and problems are induced in the construction and maintenance of railway. In the area where the mean annual air temperature is higher than a certain value, the …     

The long wavelength component of mantle convection

Theoretical models of mantle convection predict that the major temperature fluctuations within the mantle are confined to narrow horizontal boundary layers and vertical plumes. These fluctuations result in heterogeneities in seismic body wave velocities which could, in principle, be detected by seismic tomographic techniques. However, recent tomographic images of the spatial variations of temperature in the mantle are considerably “out of focus” in that only the longest wavelength components can be resolved. To assess this partial recovery of the total tomographic image, theoretical temperature fields have been generated with a numerical model of high Rayleigh number mantle convection and then Fourier analysed in two dimensions. Upon re-synthesizing the model temperature fields, the Fourier series expansions were truncated at various levels of resolution. The truncated expansions, containing only the long wavelength components of the model temperature fields, are compared to both the complete field and the tomographic images of the mantle. At the current level of resolution it seems unlikely that seismic tomography could distinguish between layered and whole-mantle convection. Estimates, based on current tomographic data, of long wavelength fluctuations of temperature and surface topography are predicted, in the case of whole-mantle convection, to represent approximately 10% of the total temperature variation, and approximately 50% of the total topographic relief. Thus topography at the core-mantle boundary may be more accurately inferred from seismic tomography than may the characteristic lateral temperature fluctuation which drives the convective circulation.     

Upper mantle heterogeneities in Africa deduced from Rayleigh wave dispersion

Rayleigh-wave group velocities have been measured along 11 paths on the African continent, north of the equator. The results lead to a division of this part of the continent into three regions: cratonic areas, non-cratonic areas (“mobile zones”), and a region located to the east of the meridian 31°E (Red Sea neighbourhood, Ethiopia, Djibouti). The highest velocities are found beneath the cratonic areas, at ～ 200 km depth (～ 5%). At greater depths, the “mobile zones” exhibit higher velocities than the cratonic regions, but the difference is not as marked (～ 2%) and the resolution is poor. Velocities in the eastern region are similar to those found under young oceanic regions.     

Compressional velocity in source regions of deep earthquakes: An application of the master earthquake technique

The technique of earthquake location relative to a master event is used to estimate near-source velocity and take-off angles for rays travelling to selected stations. Computations of a reconnaissance nature were carried out with arrival times of P and pP from deep earthquakes beneath the northwest corner of the Fiji plateau, the Peru-Brazil border region and the basin separating Fiji from the Tonga arc. These data yield estimates of compressional velocity of 11.2 ± 0.4, 11.4 ± 0.7 and 10.7 ± 0.3 km/sec respectively. Each of these velocities and the other parameters of each model space are essentially independent of their starting values. The corresponding depth ranges are 600–660, 580–650 and 540–600 km. These in-situ velocities are 5–10% higher than those of the Helmberger and Wiggins model. To account for such high velocities by a thermal effect alone would require an improbably high thermal contrast of 1000°C between “normal” mantle and the cooler earthquake zones. Spinels of proposed mantle composition would have compressional velocities of about 10.4 km/sec at temperatures that are taken as normal for these depths. If the high values of near-source velocity are explained by the addition of a post-spinel assemblage, then by implication this transformation occurs at shallower depths in those seismic zones than in the “normal” mantle.     

Study on wall-type gravel drains as liquefaction countermeasure for underground structures

One of the most dramatic causes of damage to engineering structures during earthquakes has been the development of soil liquefaction beneath and around the structures. In order to dissipate the excess pore water pressures near structures, gravel drains are usually employed. In this study, the use of recycled concrete crushed stones as gravel drain materials is addressed. In order to investigate the performance of wall-type gravel drains, two series of shaking table tests were performed. The test results showed that gravel drains, when appropriate grain size distribution is considered, effectively dissipate the excess pore water pressure underneath the structure, and consequently reduce the magnitude of uplift. To supplement the laboratory tests, finite element analyses were also performed. For specified structure, ground and earthquake conditions, there is a critical width of gravel drain at which no uplift of structure will occur. The results of the model tests and the finite element analyses were then employed in developing design charts for determining the critical width of gravel drain to prevent buoyant rise of structure when the surrounding soil mass liquefies.     

冬季亚洲中强震与夏季我国东部主雨带的准同纬性

根据1954～2003年50年亚洲中强震与我国地温资料,分析了冬季震中纬度—震涡中心纬度—冬季我国东部强地热涡中心纬度—夏季我国东部主雨带纬度4者之间的相互关系,发现冬季中强震与夏季主雨带之间具有较好的准同纬性. 这是每年汛期降水预报的一个指标. 文中也讨论了应用此预报指标时的一些特例.     

Ventifacts distribution in Qatar

This study attempts to investigate the distribution of ventifacts in Qatar. It is believed that ventifacts are confined to the areas within about 5 km of the Miocene or Mio-Pliocene Hofuf formations and the spreads of continental gravels derived from them. Three hypotheses were formulated: (1) Ventifacts in Qatar are confined to areas within about 5 km of the Hofuf formations and the spreads of continental gravels derived from them. The distribution of ventifacts within these areas varies according to the nature of the ground surface; (2) The most active ventifaction areas are where the continental gravels merge with the Eocene limestone because of the increase in saltation particle speed in these areas where bedrock or bare limestone is exposed; (3) The unit area ratio of ventifact to non-ventifact pebbles varies inversely with the total amount of pebbles. To test these hypotheses, nine land class categories were identified in the three major Hofuf formations. Line transects were carried out from randomly selected stations near the middle of the Hofuf formations. Along each transect systematic sampling was carried out at 200 m intervals. The data were processed using a WANG MVP 2200 computer with software developed for the project. It was found that ventifacts tend to concentrate on the outer edges of the continental gravels in areas of limestone outcrop and limestone pavement. Higher areas have big gravel counts and a low ratio of ventifacts while the low-lying plains have small gravel counts and a higher ratio of ventifacts. In certain areas ‘ventifact fields’ were found where the density of ventifacts was as high as 30 per m². Many of the ventifacts in these fields were buried beneath the surface suggesting that the ventifaction predates the present site conditions. Other high ventifact density areas were discovered where the ventifacts have collected in shallow depressions or hollows on the limestone plateaux. Water action has washed these ventifacts, a high proportion of which are dreikanters, into the hollows, where they have been partially buried in fine alluvial silts. These ‘ventifact graveyards’ are generally only a few metres wide but contain large numbers of fine specimens.     

Particle path characteristics at the large gravel‐bed river Danube: results from a tracer study and numerical modelling

A tracer study performed on a 3 km long reach of the Danube River in Austria is presented. Forty artificial stones of three different sizes (intermediate b‐axis: 25 mm, 40 mm, 70 mm) were produced and a coded radio acoustic transmitter was implanted. The measurement system had to be improved to be applicable to large rivers with water depths up to 12 m. The positions of the stones were observed approximately once a week, depending on hydrology, over a period of at least one year by radio‐tracking from a boat, including a 15 year flood event. Transport paths and velocities, as well as the incipient motion of bedload transport, could be monitored for the first time on a large gravel‐bed river. The particle paths were found to be mostly bankline‐parallel, even though the stones passed a 30° river bend. The median of the transverse particle displacement was found to be 4% of the longitudinal displacement. Calculations considering both transverse slope and transverse flow velocities showed transverse transport to be 6·6% of the longitudinal transport indicating that marginal lateral transport is mainly influenced by morphology. A three‐dimensional (3D) numerical model using a stochastic particle tracing approach was validated with the data, indicating that the observed positions are well reproduced by the model. Within the observation period, 74% of all stones passed the reach. With more than 1000 detections, particle transport could be characterized by a mean travel velocity of about 10 m per day (variable for the different grain sizes); single tracer stones were transported up to 1000 m during a single flood event. Size‐selective behaviour could be shown and the incipient motion of the large 70 mm gravel was detected at lower discharges than predicted by commonly used uniform bedload transport formulae. Copyright © 2012 John Wiley & Sons, Ltd.     

Quantifying River‐Groundwater Interactions of New Zealand's Gravel‐Bed Rivers: The Wairau Plain

New Zealand's gravel‐bed rivers have deposited coarse, highly conductive gravel aquifers that are predominantly fed by river water. Managing their groundwater resources is challenging because the recharge mechanisms in these rivers are poorly understood and recharge rates are difficult to predict, particularly under a more variable future climate. To understand the river‐groundwater exchange processes in gravel‐bed rivers, we investigate the Wairau Plain Aquifer using a three‐dimensional groundwater flow model which was calibrated using targeted field observations, “soft” information from experts of the local water authority, parameter regularization techniques, and the model‐independent parameter estimation software PEST. The uncertainty of simulated river‐aquifer exchange flows, groundwater heads, spring flows, and mean transit times were evaluated using Null‐space Monte‐Carlo methods. Our analysis suggests that the river is hydraulically perched (losing) above the regional water table in its upper reaches and is gaining downstream where marine sediments overlay unconfined gravels. River recharge rates are on average 7.3 m³/s, but are highly dynamic in time and variable in space. Although the river discharge regularly hits 1000 m³/s, the net exchange flow rarely exceeds 12 m³/s and seems to be limited by the physical constraints of unit‐gradient flux under disconnected rivers. An important finding for the management of the aquifer is that changes in aquifer storage are mainly affected by the frequency and duration of low‐flow periods in the river. We hypothesize that the new insights into the river‐groundwater exchange mechanisms of the presented case study are transferable to other rivers with similar characteristics.     

The velocity structure of the upper mantle in the transition zone from the East European Platform to Western Europe from seismic noise data

The surface wave tomography from ambient seismic noise recorded at stations in Western Europe (WE) and on the East European Platform (EEP) revealed the structure of the crust and upper mantle in the transitional zone from the Precambrian platform to the younger geological units in Western Europe. The Tornquist-Teisseyre Line separating these structures is clearly traced as a transition zone from the high velocities beneath EEP to the low velocities beneath WE in the crust and upper mantle, which extends to a depth of 150?C170 km. Below 200 km the relationship between the velocities beneath EEP and WE becomes the opposite. A similar relationship between the velocities in the upper mantle down to a depth of 300 km is observed on the southern boundary, where EEP borders on the northern segment of the Alpine-Himalayan seismic belt.     

Particle transport in gravel-bed rivers: Revisiting passive tracer data

Data from tracer experiments were compiled and analysed in order to explore the role of geomorphological, hydrological and sedimentological constraints on fluvial gravel transport in gravel-bed rivers. A large data set from 217 transport episodes of tagged stones were compiled from 33 scientific papers. Our analyses showed that while magnitude of peak discharge is a major control on gravel transport and mobility, tracer travel distances show some scale dependence on the morphological configuration of the channel. Our results also highlight differences in the way tracers are displaced between step–pool and riffle and pool channels. The riffle–pool sequence seems to be a more efficient trap for travelling gravels than the step–pool pair. In addition, in step–pool channels there are clear differences in tracer transport between observations of first displacements after tracer seeding (unconstrained-stone conditions), and second and subsequent observations of tracer displacements (constrained-stone conditions). The comparison between tracer experiments under constrained conditions and those under unconstrained conditions also highlights the importance of bed state and structures in gravel mobility. The results of this study confirm that sediment transport in gravel-bed rivers is a complex process, whereby sedimentological and geomorphological controls are superimposed on the hydraulic forcing. © 2018 John Wiley & Sons, Ltd.     

Crust and upper mantle structure of the Ailao Shan-Red River fault zone and adjacent regions

Using arrival data of the body waves recorded by seismic stations, we reconstructed the velocity structure of the crust and upper mantle beneath the southeastern edge of the Tibetan Plateau and the northwestern continental margin of the South China Sea through a travel time tomography technique. The result revealed the apparent tectonic variation along the Ailao Shan-Red River fault zone and its adjacent regions. High velocities are observed in the upper and middle crust beneath the Ailao Shan-Red River fault zone and they reflect the character of the fast uplifting and cooling of the metamorphic belt after the ductile shearing of the fault zone, while low velocities in the lower crust and near the Moho imply a relatively active crust-mantle boundary beneath the fault zone. On the west of the fault zone, the large-scale low velocities in the uppermost mantle beneath western Yunnan prove the influence of the mantle heat flow on volcano, hot spring and magma activities, however, the upper mantle on the eas     

Evaluating fine sediment mobilization and storage in a gravel‐bed river using controlled reservoir releases

Two controlled flow events were generated by releasing water from a reservoir into the Olewiger Bach, located near Trier, Germany. This controlled release of near bank‐full flows allowed an investigation of the fine sediment (<63 µm) mobilized from channel storage. Both a winter (November) and a summer (June) release event were generated, each having very different antecedent flow conditions. The characteristics of the release hydrographs and the associated sediment transport indicated a reverse hysteresis with more mass, but smaller grain sizes, moving on the falling limb. Fine sediment stored to a depth of 10 cm in the gravels decreased following the release events, indicating the dynamic nature and importance of channel‐stored sediments as source materials during high flow events. Sediment traps, filled with clean natural gravel, were buried in riffles before the release of the reservoir water and the total mass of fine sediment collected by the traps was measured following the events. Twice the mass of fine sediment was retained by the gravel traps compared with the natural gravels, which may be due to their altered porosity. Although the amount of fine sediment collected by the traps was not significantly related to measures of gravel structure, it was found to be significantly correlated to measures of local flow velocity and Froude number. A portion of the traps were fitted with lids to restrict surface exchange of water and sediment. These collected the highest amounts of event‐mobilized sediments, indicating that inter‐gravel lateral flows, not just surface infiltration of sediments, are important in replenishing and redistributing the channel‐stored fines. These findings regarding the magnitude and direction of fine sediment movement in gravel beds are significant in both a geomorphic and a biological context. Copyright © 2006 John Wiley & Sons, Ltd.     

Reprocessing and reinterpretation of COCORP southern Appalachian profiles

Reprocessing of COCORP southern Appalachian data was focused on basic seismic evidence for continuation of sediments beneath a master decollement. The most important evidence is a nearly continuous series of subhorizontal reflections extending from the Valley and Ridge province into the Piedmont province. Continuity of subhorizontal reflections becomes tenuous in the Inner Piedmont. Careful reprocessing has yielded evidence for termination of strong reflections beneath the allochthon and the beginning of a relatively weak and complex series of “events”. Termination of sedimentary rocks beneath the Piedmont is interpreted from true amplitude seismic data. A zone of detachment continues southeast of the sediment termination as far as the master decollement root zone. Research on stacking velocities has indicated that complex velocity structures could create apparent low stacking velocities. This phenomenon may occur in the Charlotte belt of Georgia. Bouguer gravity can be modeled as a former craton of normal density with an accreted margin of very slightly higher density. Variation in crustal thickness also contributes to the Bouguer gravity gradient. No continuous large-scale overthrust is needed southeast of the interpreted master decollement root zone located beneath the Kings Mountain belt and Charlotte belt.     

The application of auto-temperature-controlled ventilation embankment in Qinghai-Tibet Railway

Auto-temperature-controlled ventilation embankment is an effective engineering measure for “cooling roadbed”. Practice proves that this new method can sufficiently make use of natural cold energy. It has the advantages of higher efficiency, better cooling effect and feasibility in engineering practice, and wider application in various environment, etc. And also, it is comparatively cheap in project cost. Through practice in the field for half a year, the testing results show that, with the application of auto-temperature-controlled system, the artificial permafrost table has been raised by 65 cm. The artificial permafrost table was basically at the embankment bottom, and the action of freeze-thaw circle on engineering stability was effectively avoided. In the month with highest ground temperature, in the scope with 1–4 m in depth, including the majority of the embankment and the upper part in the original seasonal layer, the ground temperature decreased by 0.7°C. Through thermal flux calculation in the original seasonal layer, in the month with the maximum thermal flux coming into permafrost, it is found that the thermal flux reduces nearly by half. Coming into the cooling period for nearly a month, the ground temperature in entire auto-temperature-controlled embankment is close to zero, and the foundation is at negative temperature. But in a large region in the embankment and foundation the ground temperature was over 0°C and varied from 0°C to0.39°C in ordinary ventilation embankment.     

Distribution of Mantle Up-welling Determined from Plate Motions: A Case for Large-scale Bénard Convection

—?The number and geometric distribution of putative mantle up-welling centers and the associated convection cell boundaries are determined from the lithospheric plate motions as given by the 14 Euler poles of the observational NUVEL model. For an assumed distribution of up-welling centers (called “cell-cores”) the corresponding cell boundaries are constructed by a Voronoi division of the spherical surface; the resulting polygons are called “Bénard cells.” By assuming the flow-kinematics within a cell, the viscous coupling between the flow and the plates is estimated, and the Euler poles for the plates are computed under the assumption of zero-net-torque. The positions of the cell-cores are optimized for the HS2-NUVEL1 Euler poles by a method of successive approximation (“subplex”); convergence to one of many local minima occurred typically after ～20,000 iterations. Cell-cores associated with the fourteen HS2-NUVEL1 Euler poles converge to a relatively small number of locations (8 to 10, depending on interpretation), irrespective of the number of convection cells submitted for optimized distribution (from 6 to 50). These locations are correlated with low seismic propagation velocities in tomography, uniformly occur within hotspot provinces, and may specifically be associated with the Hawaiian, Iceland, Reunion/Kerguelen (Indian Ocean), Easter Island, Melanesia/Society Islands (South Pacific), Azores/Cape Verde/Canary Islands, Tristan da Cunha (South Atlantic), Balleny Islands, and possibly Yellowstone hotspots. It is shown that arbitrary Euler poles cannot occur in association with mantle Bénard convection, irrespective of the number and the distribution of convection cells. Nevertheless, eight of the observational Euler poles – including the five that are accurately determined in HS2-NUVEL1 (Australia, Cocos, Juan de Fuca, Pacific, and Philippine) – are “Bénard-valid” (i.e., can be explained by our Bénard model). Five of the remaining six observational poles must be relocated within their error-ellipses to become Bénard-valid; the Eurasia pole alone appears to be in error by ～115°, and may actually lie near 40°N, 154°E. The collective results strongly suggest Bénard-like mantle convection cells, and that basal shear tractions are the primary factor in determining the directions of the plate motions as given by the Euler poles. The magnitudes of the computed Euler vectors show, however, that basal shear cannot be the exclusive driving force of plate tectonics, and suggest force contributions (of comparable magnitude for perhaps half of the plates) from the lithosphere itself, specifically subducting slab-pull and (continental) collision drag, which are provisionally evaluated. The relationship of the putative mantle Bénard polygons to dynamic chaos and turbulent flow is discussed.     

Major element chemistry of Galapagos Rift Zone magmas and their phenocrysts

Basalts dredged from the Galapagos Rift Zone between 85°W and 100°W were analyzed by electron microprobe to determine the chemistry of the glass exteriors and included phenocrysts, microphenocrysts and quench minerals. The basalts come from both “normal” mid-ocean ridge segments and from ridge segments that cross the Galapagos Platform. The basalts fall into two chemical and geographical groups. Group A basalts come from outside the central region of the Galapagos Platform (i.e., outside 89–92.5°W) and are chemically similar to basalts from “normal” ocean ridge segments. Group B basalts come from the center of the Galapagos Platform (89–92.5°W) and are enriched in incompatible elements like “plume-influenced” basalts from the Mid-Atlantic Ridge. The spinel, olivine, plagioclase and clinopyroxene phenocrysts in both groups of basalts are low-pressure, equilibrium phases, but the chemical difference among basalts from within each group indicates high-pressure fractional crystallization is also responsible for the chemical evolution of some of these basalts. Presently, no crystallization or partial melting model can relate the chemistry of the two groups of basalts and the compositional influence of a large-ion-lithophile elements and water-rich mantle beneath the Galapagos Platform is a viable alternate hypothesis. The eruption temperature of magmas from the “normal” ridge segments, as determined by olivine-liquid thermometry is 1217±10°C, suggesting steady-state conditions but on the Galapagos Platform the eruption temperatures are lower and more variable than on the “normal” ridge (1186°C±30°C) suggesting a more complex plumbing system and the absence of a steady-state magma chamber beneath the platform.     

INVESTIGATION OF DAMAGES CAUSED BY ABRASION AND IMPACT OF GRAVEL BED LOAD ON HYDRAULIC STRUCTURES

When gravel bed load deposits are sluiced from hydraulic structures, the mo- tion of bed load over the apron, as is well known, may consist of sliding, rolling and saltation. At times, saltation may become so intensive that gravels have been observed to jump clear out of the water surface. The movement of gravel bed load may cause serious damages to the hydraulic structures through abrasion and im- pact. The extent of damage depends upon a number of factors, such as rock prop- erties of gravels, flow conditions, configuration as well as the strength of materials of hydraulic structures. In most cases the movement of gravel bed load takes place in ribbons in plan, consequently, the lateral distribution of damages is uneven, and protective measures to be adopted against abrasion and impact should be applied to the key part most frequently attacked by the gravel bed load. Possible repair should be anticipated in the layout of structures in order to facilitate the operation.     

大三江盆地及邻区地壳结构研究

收集了大三江盆地及其邻区区域地震台网及多个流动台阵的连续波形及远震事件资料,采用背景噪声层析成像和接收函数叠加方法,分别获得了研究区三维S波速度结构、基底及莫霍面深度和泊松比.结果显示:浅层速度结构较好地反映了地表地形及地质特征,三江盆地呈明显的低速,虎林和勃利等小型盆地的S波速度也相对较低,而小兴安岭、张广才岭等则呈...