Spatial pattern of localized disturbance along a Southeastern salt marsh tidal creek     

Janet M. Fischer  Tara Reed-Andersen  Jennifer L. Klug  Alice G. Chalmers 《Estuaries and Coasts》2000,23(4):565-571

Geomorphology may be an important predictor of vegetation pattern in systems where suceptibility to disturbance is unevenly distributed across the landscape. Salt marsh communities exhibit spatial pattern in vegetation at a variety of spatial scales. In coastal Georgia, the low marsh is a virtual monoculture ofSpartina alterniflora interspersed with patches of species that are more typical of the high marsh. These localized disturbances are most likely created by wrack mats, mats of dead vegetation which can compact and smother underlying vegetation creating bare patches for colonization by high marsh species. We investigated the spatial pattern of disturbed patches along a 2 km section of Dean Creek, a tidal creek at the southwestern end of Sapelo Island, Georgia, U.S. We used a discriminant model to explore the relationship between tidal creek morphology (e.g., the presence of drainage channels and creek bends) and the spatial distribution of disturbed patches. The model predicted vegetation pattern along the creek with relatively high accuracy (>70%). Areas where water movement is slowed or multidirectional (e.g., along creek bends and near drainage channels) were most susceptible to disturbance. Our findings suggest an important functional linkage between geomorphology and vegetation pattern in salt marsh communities.  相似文献   

An approach for shake table performance evaluation during repair and retrofit actions          下载免费PDF全文

Christopher Trautner  Yewei Zheng  John S. McCartney  Tara Hutchinson 《地震工程与结构动力学》2018,47(1):131-146

Large‐scale, servo‐hydraulic shake tables are a central fixture of many earthquake engineering and structural dynamics laboratories. Wear and component failure from frequent use may lead to control problems resulting in reduced motion fidelity, necessitating repairs and replacement of major components. This paper presents a methodology to evaluate shake table performance pre‐ and post‐repair, including the definition of important performance metrics. The strategy suggested is presented in the context of the rebuilding of a 4.9 × 3.1 m, 350‐kN‐capacity uniaxial shake table. In this case, the rebuild consisted of characterization of wear to table components, replacement of worn bearing surfaces, and replacement of hydraulic accumulators. To assess the effectiveness of the repair actions, sinusoidal and triangular waves, white noise, and earthquake histories were run on the table before and after the rebuild. The repair actions were successful in reducing the position and velocity dependence of friction, improving the ability of control algorithms to accurately reproduce earthquake motions. The maximum and average response spectral misfits in the period range of 0.1–2 seconds were reduced from approximately 50% to 15%, and from 5% to less than 2.5%, respectively.  相似文献   

Centurial Evolution of an Offshore Mud Deposition Area in the Changjiang (Yangtze) Estuary and Its Links to Environmental and Anthropogenic Activities     

Jingyi Cong  Gang Hu  Tara N. Jonell  Zhongpeng Yuan  Xianghuai Kong  Yong Zhang  Yonghong Wang 《中国海洋大学学报(英文版)》2020,19(4):790-800

Large amounts of sediments originating from the Changjiang(Yangtze) River are deposited in the subaqueous delta and in the adjacent muddy area off the mouth and on the inner shelf of the East China Sea. The terrestrial sediments deposited in these areas not only contain information about the composition and environment of the source area, but they also record changes in anthropogenic activities. A sediment piston core(CJ0702) was extracted from the Changjiang subaqueous depocenter(31.00°N, 122.67°E) in a water depth of 22.0 m. The core was subsampled at 1 – 2 cm intervals and analyzed for grain size, clay mineralogy, and major element geochemistry. Results indicate a relatively high sediment accumulation rate of approximately 3.11 cm yr~(-1). These parameters exhibited only minor cyclical fluctuations in the core, which resulted from many factors. During the past 120 years, the Changjiang River-derived sediment is the primary source of sediment in the offshore mud area without evidence for the Yellow River-derived sediment increasing. After the trunk stream shifted from the North Branch to South Branch, the variations of proxies are controlled by the periodic fluctuation possibly linked to El Ni?o-Southern Oscillation(ENSO) and the Pacific Decadal Oscillation(PDO). In addition, anthropogenic heavy metal concentrations can be divided into three stages, which coincide well with economic development and environmental protection policies.  相似文献   

The formation of superdense gaseous cores in the nuclei of disk galaxies: II     

B. Basu  Tara Bhattacharya 《Astrophysics and Space Science》1981,74(2):391-407

In a previous paper (hereafter referred to as Paper I) we have tried to show that superdense cores in the nuclei of disk galaxies can be formed by accretion of gas ejected by the evolved stars which populate the central bulge of these galaxies. Solving the equations for radial flow of a magnetized gas, we found that the accretion of an explodable mass at the core can be achieved over a time-scale ranging from a few times 10⁷ and a few times 10⁸ yr. It was shown, however, that the accretion process is seriously inhibited if the gas possesses sufficient rotational velocity but lacks any dissipative, mechanism within the system. Since rotational velocity is an observed parameter of the stars which shed the gas to be accreted, one must consider the existence of some dissipative force in it in order that the accretion process may be efficient. In the present paper, therefore, we have solved the problem of the flow of a rotating, viscous (variable), magnetized gas. With plausible assumptions regarding some of the parameters involved, the time-scale for the accretion of an explodable mass (10⁹ M ) at the core again turns out to be ranging between a few times 10⁷ and a few times 10⁸yr. Such time-scale has been proposed by several authors as that for repeated explosions in nuclei of these galaxies. It has also been proposed by many authors that the spiral arms are generated and destroyed in disk galaxies over the same time-scale. Our solution also yields a nearly linear rotational velocity law which is usually observed in the central regions of these galaxies.  相似文献   

Gravitational collapse of cosmic gas clouds and formation of star clusters     

B. Basu  Tara Bhattacharyya 《Astrophysics and Space Science》1984,105(1):85-97

In this paper the gravitational collapse of cosmic gas clouds and formation of star clusters has been considered. Hoyle's view of successive fragmentation has been taken as the basic mechanim in the present work. The initial masses of protostars have been estimated as the function of their distances from the centre of the cluster and the intensity of the magnetic field of the medium. It has been shown that the fragmentation process is greatly inhibited by the presence of a strong magnetic field. A model has been constructed showing how a protostar grows in mass by accretion from the surrounding medium, on the basis of the assumption that as the star moves at random in the cluster it picks up a fraction of the material through which it passes. It has been estimated that a protostar of initial mass of about 0.1M grows to one of 1.0M in a time period which ranges from a few multiples of 10⁵ to a few multiples of 10⁷ yr, depending on the parameters involved in the accretion process. The number of stars per unit mass range has also been estimated; it is found to be proportional tom ^–3.3,m being the mass of the star.  相似文献   

On the prognostics of growth profile based yield modelling     

R. P. Dubey  Tara Sharma 《Journal of the Indian Society of Remote Sensing》1993,21(4):191-197

In order to improve the prognostics of yield forecasts two approaches have been explored using NDVI-based growth profiles for wheat crop of 1987-88 and 1990-91 seasons for some districts of Punjab and Haryana. Correlation of yield with variables based on profile area segments and with product of profile segment area and time to peak occurrence of growth cycle have been investigated. While the correlations are low and inconsistent for area variables, the îndex time product moment (IIPM) variable shows consistent and significant correlations and advances the date of forecast by 45-50 days over other approaches.  相似文献   

Centrifuge modeling of load-deformation behavior of rocking shallow foundations   总被引：4，自引：0，他引：4  

Sivapalan Gajan  Bruce L. Kutter  Justin D. Phalen  Tara C. Hutchinson  Geoff R. Martin 《Soil Dynamics and Earthquake Engineering》2005,25(7-10):773-783

Shallow foundations supporting building structures might be loaded well into their nonlinear range during intense earthquake loading. The nonlinearity of the soil may act as an energy dissipation mechanism, potentially reducing shaking demands exerted on the building. This nonlinearity, however, may result in permanent deformations that also cause damage to the building. Five series of tests on a large centrifuge, including 40 models of shear wall footings, were performed to study the nonlinear load-deformation characteristics during cyclic and earthquake loading. Footing dimensions, depth of embedment, wall weight, initial static vertical factor of safety, soil density, and soil type (dry sand and saturated clay) were systematically varied. The moment capacity was not observed to degrade with cycling, but due to the deformed shape of the footing–soil interface and uplift associated with large rotations, stiffness degradation was observed. Permanent deformations beneath the footing continue to accumulate with the number of cycles of loading, though the rate of accumulation of settlement decreases as the footing embeds itself.  相似文献   

Procedures for computation of saturation radiances: A case study for proposed sensors onboard Indian remote sensing satellite-1C     

Tara Sharma  T. T. Medhavy 《Journal of the Indian Society of Remote Sensing》1994,22(1):9-20

Saturation radiance (SR) is one of the important parameters required while designing a sensor and it depends on application requirements as well as sensor requirements. This study describes the procedures for computation of SR values using theoretical calculations of earthatmosphere radiances as well as radiances observed by existing satellites such as IRS-1A/1B LISS-I, Landsat TM and SPOT sensors. A possible set of saturation radiances for IRS-1C LISS-III, WIFS and PAN sensors has been suggested.  相似文献   

Assessing municipal vulnerability to predicted sea level rise: City of Satellite Beach, Florida     

Randall W. Parkinson  Tara McCue 《Climatic change》2011,107(1-2):203-223

In the fall of 2009 the City of Satellite Beach (City), Florida, authorized a study designed to assess municipal vulnerability to rising sea level and facilitate discussion of potential adaptation strategies. The project is one of the first in Florida to seriously address the potential consequences of global sea level rise, now forecast to rise a meter or more by the year 2100. Results suggest the tipping point between relatively benign impacts and those that disrupt important elements of the municipal landscape is +?2 ft (0.6 m) above present. Seasonal flooding to an elevation of +?2 ft is forecast to begin around 2050 and thus the City has about 40 years to formulate and implement an adaptation plan. As an initial step, the Comprehensive Planning Advisory Board, a volunteer citizen committee serving as the City??s local planning authority, has recommended a series of updates and revisions to the City??s Comprehensive Plan. If approved by the City Council and Florida??s Department of Community Affairs, the amendments will provide a legal basis for implementing specific policies designed to reduce the City??s vulnerability to sea level rise.  相似文献   

The importance of health co-benefits in macroeconomic assessments of UK Greenhouse Gas emission reduction strategies     

Henning Tarp Jensen  Marcus R. Keogh-Brown  Richard D. Smith  Zaid Chalabi  Alan D. Dangour  Mike Davies  Phil Edwards  Tara Garnett  Moshe Givoni  Ulla Griffiths  Ian Hamilton  James Jarrett  Ian Roberts  Paul Wilkinson  James Woodcock  Andy Haines 《Climatic change》2013,119(2):223-237

We employ a single-country dynamically-recursive Computable General Equilibrium model to make health-focussed macroeconomic assessments of three contingent UK Greenhouse Gas (GHG) mitigation strategies, designed to achieve 2030 emission targets as suggested by the UK Committee on Climate Change. In contrast to previous assessment studies, our main focus is on health co-benefits additional to those from reduced local air pollution. We employ a conservative cost-effectiveness methodology with a zero net cost threshold. Our urban transport strategy (with cleaner vehicles and increased active travel) brings important health co-benefits and is likely to be strongly cost-effective; our food and agriculture strategy (based on abatement technologies and reduction in livestock production) brings worthwhile health co-benefits, but is unlikely to eliminate net costs unless new technological measures are included; our household energy efficiency strategy is likely to breakeven only over the long term after the investment programme has ceased (beyond our 20 year time horizon). We conclude that UK policy makers will, most likely, have to adopt elements which involve initial net societal costs in order to achieve future emission targets and longer-term benefits from GHG reduction. Cost-effectiveness of GHG strategies is likely to require technological mitigation interventions and/or demand-constraining interventions with important health co-benefits and other efficiency-enhancing policies that promote internalization of externalities. Health co-benefits can play a crucial role in bringing down net costs, but our results also suggest the need for adopting holistic assessment methodologies which give proper consideration to welfare-improving health co-benefits with potentially negative economic repercussions (such as increased longevity).  相似文献