Aerodynamic entrainment threshold: effects of boundary layer flow conditions   总被引：4，自引：0，他引：4  

JON J. WILLIAMS  GRAEME R. BUTTERFIELD†  DAVID G. CLARK‡ 《Sedimentology》1994,41(2):309-328

Experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions. New methods are proposed for (1) the unambiguous determination of aerodynamic threshold for any grain population and (2) approximation of the probability density function (PDF) distributions of threshold shear velocity for aerodynamic entrainment. In wind tunnel experiments, the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgements and subsequent erosion from narrow strips of loose, finely fractionated ballotini were recorded photographically as wind speed was increased. Boundary layer parameters, including average threshold shear velocity (U_*t), were calculated using the momentum integral method. Direct observations show that sporadic oscillation of grains preceded dislodgement. At slightly higher velocities most grains rolled over their neighbours before entering saltation. Initial entrainment in spatially semi-organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold. These observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process. For each grain fraction, values of U_*t were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased. A probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate. Analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the PDF for U_*t required by the model and for future stochastic treatment of the threshold condition.  相似文献   

The Effects of Fe3? and Mn3? on Aluminum Silicate Phase Relations in North-Central New Mexico, U.S.A.     

GRAMBLING  JEFFREY A.; WILLIAMS  MICHAEL L. 《Journal of Petrology》1985,26(2):324-354

Aluminum silicate triple-point occurrences are common in metamorphicrocks of northern New Mexico. The three polymorphs show extensivesolid solution, with Fe and Mn substituting for Al. Mineraltextures, the spatial distribution of phases, and the systematicpartitioning of Fe and Mn indicate that the kyanite-andalusite-sillimaniteassemblages crystallized in equilibrium. The compositions ofminerals in the three-phase assemblage vary across the studyarea, recording regional variations in the pressures and temperaturesof metamorphism. The highest-pressure rocks, exposed at RioMora, contain kyanite at higher elevations and sillimanite atlower elevations. A sub-horizontal isograd separates the twominerals. Kyanite and sillimanite have nearly identical Fe contentwhich varies systematically with X_Fe2O3 in hematite or ilmenite.Andalusite occurs only along a single manganiferous layer, incrystals rich in MnAlSiO₅ and saturated in FeAlSiO₅. Triple-pointassemblages can be found wherever the folded manganiferous layercrosses the unfolded kyanite-sillimanite isograd. The TruchasRange, preserving slightly lower pressures of metamorphism,shows kyanite-andalusite-sillimanite in rocks with titaniferoushematite. Andalusite is enriched in Fe relative to kyanite andsillimanite, but no polymorphs contain Mn. Rocks with lowerX_Fe2O3 in hematite have kyanite and sillimanite without andalusite,whereas rocks with pure hematite contain only andalusite. Theshallowest erosional levels are preserved in the western PicurisRange where the three polymorphs occur as pure minerals in ilmenite-bearingrocks. Hematitic samples contain only andalusite which showsextensive solid solution of both Fe and Mn. The assemblage kyanite-andalusite-sillimanite is not invariant.Iron and manganese both add degrees of freedom. These transitionmetals have stabilized the three-phase assemblage, in apparentchemical equilibrium, across a P-T interval of 500-540 ?C, 3?8-4?6kb in rocks from New Mexico. The saturation level of FeAlSiO5in andalusite does not vary with Mn content but does vary withpressure and temperature. Calculations indicate that a 2-3 kbdecrease in pressure or a 25-50 ?C increase in temperature resultsin a 1 mole per cent increase in X_FeAlSiO5 in iron-saturatedandalusite.  相似文献   

Thermoluminescence dating of Dimlington Stadial deposits in eastern England   总被引：2，自引：0，他引：2  

ANN G. WINTLE  JOHN A. CATT 《Boreas: An International Journal of Quaternary Research》1985,14(3):231-234

The loess component of a solifluction deposit of the Dimlington Stadial exposed at the inland site of Eppleworth in eastern England gave a thermoluminescence date of 17.5 ± 1.6 × 10³ years. The solifluction deposit is overlain by a slightly weathered till correlated with the Skipsea Till of coastal exposures. which lies between organic horizons with radiocarbon dates of 18,500–18,240 B.P. and 13,045 B.P. Although the till must have been deposited during the Dimlington Stadial (after 18,240 B.P. at Dimlington and after 17,500 B.P. at Epplcworth), it gave apparent TL dates of 42.1 ± 3.6 × 10³ years at Eppleworth and 102 ± 9 × 10³ years at Dimlington, indicating that the components of the till were not exposed to light immediately before deposition.  相似文献   

Deposition of the Bear Gulch Limestone: a Carboniferous Plattenkalk from central Montana   总被引：1，自引：0，他引：1  

LORETTA ANN WILLIAMS 《Sedimentology》1983,30(6):843-860

The Carboniferous Bear Gulch Limestone of the central Montana Big Snowy Trough is a lithographic limestone analogous to the plattenkalk carbonates of the Mesozoic European/Middle Eastern Tethyan belt. It contains an excellently preserved chitinous and phosphatic marine fauna. Petrographic and field studies synthesized with palaeontological data provide insights into depositional history. Tectonic activity initiated development of small en echelon basins. Rims of these basins were sites of high primary productivity. Photosynthesis by subtidal cyanobacterial mats and algae caused in situ precipitation of micrite. Cyanobacteria and algae also provided a food source for benthic and nektonic organisms, which provided food for predators. Respiration in turn provided carbon dioxide for continued photosynthesis. Thus a balanced ecosystem evolved. Productivity in the water column used up available oxygen, resulting in dysaerobic to anaerobic conditions in the bottom waters and sediments. This plus high sedimentation rates of carbonate ooze transported downslope, aided in preservation of whole-body fossils. There are no likely modern analogues known for the Bear Gulch Limestone. However, comparisons with Mesozoic plattenkalke demonstrate that it is most closely analogous to those deposited in shallow water back-reef areas of the Tethyan carbonate platform, rather than to those deposited in the deeper water shelf-edge environment. This is in agreement with the water depth of roughly 40 m estimated for the Bear Gulch Limestone.  相似文献   

Deformation and metamorphism in the Bard area of the Sesia Lanzo Zone, Western Alps, during subduction and uplift   总被引：2，自引：0，他引：2  

P. F. WILLIAMS  R. COMPAGNONI 《Journal of Metamorphic Geology》1983,1(2):117-140

The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia-Lanzo Zone (Western Alps). Both complexes have undergone high-pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub-elements. This interpretation is supported by structural evidence.
The early Alpine (90-70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is >1500 MPa (>15kbar) and 550°C and in the GM is < 1500-1300 MPa (< 15-13 kbar) and 500-550°C. We suggest that the rocks were probably in an active Benioff zone during this time.
From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north-west.
A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.  相似文献   

FLOOD DEPOSITS OF THE SAND-BED EPHEMERAL STREAMS OF CENTRAL AUSTRALIA   总被引：4，自引：0，他引：4  

GEORGE E. WILLIAMS 《Sedimentology》1971,17(1-2):1-40

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SOME ASPECTS OF THE EOLIAN SALTATION LOAD   总被引：14，自引：0，他引：14  

G. WILLIAMS 《Sedimentology》1964,3(4):257-287

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INTEGRATION OF THE MODELS OF ANNAGNPS AND REMM TO ASSESS RIPARIAN BUFFER SYSTEM FOR SEDIMENT REDUCTION     

Yongping YUAN Ronald BINGNER Randall WILLIAMS Richard LOWRANCE David BOSCH Joe SHERIDAN 《国际泥沙研究》2007,22(1):60-69

The United States Department of Agriculture (USDA) Annualized Agricultural Non-Point Source Pollution model (AnnAGNPS) is used to help evaluate a watershed response to agricultural management practices to control water quality. However, AnnAGNPS version 3.5 does not contain features to estimate the effect of a riparian buffer (RB) system on water quality. The Riparian Ecosystem Management Model (REMM) is used to simulate the impact of riparian buffer systems on water quality. However, frequently the lack of measured upland loadings that are required by REMM simulation limits the application of REMM. To address this data gap, a study was conducted to integrate AnnAGNPS with REMM for RB system simulation. AnnAGNPS was used to simulate water and sediment loadings from an upland field into a three-zone RB system at the Gibbs Farm located in the Georgia coastal plain. These AnnAGNPS outputs were used as the inputs to REMM. REMM was used to simulate water and sediment movement along the riparian buffers. The AnnAGNPS simulated amount of annual runoff at the edge of the field was close to observed amounts (Nash-Sutcliffe efficiency of 0.92). It is believed that a substantial portion of sand was removed from the runoff one meter into the grass buffer where the samplers were located; therefore, sand was excluded from the AnnAGNPS simulation for comparison with observed sediment. Excluding sand, the AnnAGNPS predicted amount of annual sediment matches the observed amount fairly well (Nash-Sutcliffe efficiency of 0.46). In addition, based on evaluating the percent reduction of sediment at each zonal interface, the AnnAGNPS/REMM model well simulated the function of the RB system to reduce sediment.  相似文献   

“THE END OF MODERN HISTORY”?*     

MICHAEL WILLIAMS 《Geographical review》1998,88(2):275-300

ABSTRACT. The first half of the twentieth century marked both an end to confidence in human control of the earth and a prelude to the contemporary era of intense environmental concern after 1950. This conspectus of the environmental history/historical geography of the transitional years between 1900 and 1950 focuses on rising concerns over resource supplies and exploitation, particularly land settlement, timber supplies, soil erosion, and river-basin control. Conservation and the pervasiveness of ecological thinking and analogy form a strong underlying theme.  相似文献   

Mapping seabird nesting habitats in Franz Josef Land, Russian High Arctic, using digital Landsat Thematic Mapper imagery     

MEREDITH WILLIAMS  JULIAN A. DOWDESWELL 《Polar research》1998,17(1):15-30

Supervised classification of digital Landsat satellite images was used to locate seabird nesting habitats in the Russian High Arctic archipelago of Franz Josef Land, a region where the avifauna is poorly known and ecologically vulnerable. Major seabird nesting colonies are readily identifiable in Landsat Thematic Mapper (TM) imagery of the region due primarily to the distinctive spectral signature of vegetation on ornithogenically altered soils below bird cliffs. Supervised image classification was used to pinpoint areas displaying spectral characteristics typical of documented seabird nesting habitats. A total of 101 seabird nesting colony locations identified in Russian and Western literature from 1898 to 1996 was used as training sites to develop spectral signatures from a summer TM image mosaic for use in a supervised maximum likelihood classification. The classified image was thresholded and compared to a map of documented nesting locations. Of the 101 field-documented nesting sites, 96 were clearly identified in the classified image. An inventory was produced of all undocumented seabird habitats suggested by the classification, totalling over 300 sites. The methodology used may be applicable to other arctic regions and is intended as a first step when planning ecological protection zones in remote and inaccessible arctic regions.  相似文献