Evolution and inheritance of a rock coast: western Galicia,northwestern Spain     

R. Blanco Chao  M. Costa Casais  A. Martínez Cortizas  A. Prez Alberti  A. S. Trenhaile 《地球表面变化过程与地形》2003,28(7):757-775

There is increasing evidence that shore platforms and other elements of rock coasts may be inherited, at least in part, from interglacial stages when sea level was similar to today's. Most of this evidence, which includes ancient beaches and datable terrestrial deposits, has been obtained from areas of resistant, slowly eroding rock, where the platforms often appear to be much too wide to have developed since the sea reached its present level. It is much more dif?cult to demonstrate that inheritance has occurred in areas of weaker rock, which generally lack any datable material. The coast of western Galicia in northwestern Spain has shore platforms in igneous and metamorphic rocks that were deeply weathered during the Tertiary. These platforms are closely associated with ancient beaches from the last interglacial stage, and associated periglacial and ?uvio‐nival deposits that covered and fossilized most of the Eemian platforms and cliffs during the late middle and late Weichselian glacial stage. The sedimentary processes and the thickness and facies of the sediments were determined by the height, aspect and gradient of the coastal mountains, and their distance from the coast. Radiocarbon dating, sedimentary analysis and platform morphology indicate that the shore platforms of Galicia have been inherited from at least the last interglacial stage. They were fossilized in places beneath thick Weichselian deposits and then exhumed during the Holocene transgression. The abundant evidence for inheritance in Galicia has important implications for other coasts in fairly weak rocks where such evidence is generally lacking. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Weathering control over geomorphology of supermature Proterozoic Delhi quartzites of India     

Jayant K. Tripathi  V. Rajamani 《地球表面变化过程与地形》2003,28(13):1379-1387

Quartz and quartzite are thought to be resistant as a mineral and a rock respectively; however, we have shown that the presence of small amounts of pyrite in the quartzites makes them vulnerable to weathering. We observe that weathering of Proterozoic quartzite in the semi‐arid conditions around Delhi proceeded from fractures towards the inside and produced weathering rinds. The chemical index of alteration (CIA), which is actually a measure of weathering of aluminosilicate minerals, increases from the core outwards, through the rinds. Although aluminosilicate minerals occur only as minor phases (<2 per cent), their weathering indicates a movement of the weathering front from the periphery towards the core. We have suggested a coupled mechanism in which the dissolution of pyrites by moving water produced a sulphate‐bearing acidic solution and ferrous iron, which reacted with aluminosilicate minerals and quartz, respectively. This initially makes the Delhi quartzite porous and subsequently friable. The total disintegration of grain to grain contacts imparted friability to this quartzite to produce silica sand. Subsequent physical erosion of loose sand, produced during rind development in the outermost zones, has given rise to features like tors, spheroids, gullies, cavities and small‐scale caves on these quartzites. Thus, the terrain has acquired ruggedness in semi‐arid conditions. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Mid‐Holocene sub‐blanket‐peat alluvia and sediment sources in the upper Liffey valley,Co. Wicklow,Ireland     

Martin Thorp  Peter Glanville 《地球表面变化过程与地形》2003,28(9):1013-1024

A recently exposed section across a ?rst‐order valley buried beneath the regional blanket peat on hillside slopes in the upper Liffey valley, Co. Wicklow, is described. The section shows two alluvia within a shallow valley form underlain by an extensive boulder and stone line over regional till and weathered granite. ¹⁴C dates from wood in the alluvia indicate the older alluvium to have formed between 4324 ± 53 BP and 4126 ± 45 BP and the younger between 3217 ± 53 BP and 2975 ± 53 BP . The basal layer of the overlying peat yielded a date of 2208 ± 61 BP . The younger alluvium shows the effects of soil paludi?cation prior to the peat expansion. Dated pollen analyses elsewhere in the upper catchment con?rm the spread of blanket peat over most areas above 350 m after 4000–3600 BP . The buried valley was contributing sediments to the mid‐Holocene ?oodplains in the upper Liffey valley prior to the extension of blanket peat over the catchment after which sediment yields from it and the other catchment slopes declined. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel     

Ian J. Walker  William G. Nickling 《地球表面变化过程与地形》2003,28(10):1111-1124

Topographic interactions generate multidirectional and unsteady air?ow that limits the application of velocity pro?le approaches for estimating sediment transport over dunes. Results are presented from a series of wind tunnel simulations using Irwin‐type surface‐mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp‐crested model dunes. Findings complement existing theories on secondary air?ow effects on stoss transport dynamics and provide new information on the in?uence of lee‐side air?ow patterns on dune morphodynamics. For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a signi?cant drop in time‐averaged measurements of streamwise shear stress at this location. This effect is believed suf?cient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and ?ow acceleration cause an increase in ?ow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60–70 per cent greater than at ?ow reattachment on the lower stoss of closely spaced dunes. Streamwise ?ow accelerations, rather than turbulence, have greater in?uence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed ?ow within the separation cell generates signi?cant surface shear (30–40 per cent of maximum values) for both spacings. This supports ?eld studies that suggest reversed ?ow is competent enough to return sediment to the dune directly or in a de?ected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time‐averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and ?ow steadiness increase within 6 h (h = dune height) of reattachment and approach upwind values by 25 h. A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to ?uvial estimates. The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost‐effective design shows promise for validating these ?ndings in natural dune settings. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Optimal distribution of linear control intensity over the frequency range     

Ottavia Corbi 《地震工程与结构动力学》2003,32(2):173-185

In this paper, a control strategy for structural systems is proposed and developed in the frequency domain. The algorithm is substantially based on a linear derivative feedback and a convolution of the control parameter, whose distribution in the frequency field is chosen in such a manner as to comply with the requirements of an ad hoc formulated constrained optimum problem, with the response data monitored until the instant of control action application. Some numerical testing is carried out by referring to given recorded accelerograms, showing a good performance of the proposed approach. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Sliding fragility of block‐type non‐structural components. Part 1: Unrestrained components     

D. Lopez Garcia  T. T. Soong 《地震工程与结构动力学》2003,32(1):111-129

Motivated by the development of performance‐based design guidelines with emphasis on both structural and non‐structural systems, this paper focuses on seismic vulnerability assessment of block‐type unrestrained non‐structural components under sliding response on the basis of seismic inputs specified by current seismic codes. Two sliding‐related failure modes are considered: excessive relative displacement and excessive absolute acceleration. It is shown that an upper bound for the absolute acceleration response can be assessed deterministically, for which a simple yet completely general equation is proposed. In contrast, fragility curves are proposed as an appropriate tool to evaluate the excessive relative displacement failure mode. Sample fragility curves developed through Monte‐Carlo simulations show that fragility estimates obtained without taking into account vertical base accelerations can be significantly unconservative, especially for relatively large values of the coefficient of friction. It is also found that reasonable estimates of relative displacement response at stories other than the ground in multistorey buildings cannot in general be obtained by simply scaling the ground acceleration to the peak acceleration at the corresponding storey. Failure modes considered in this study are found to be essentially independent of each other, a property that greatly simplifies assessment of conditional limit states. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

A procedure for evaluating seismic energy demand of framed structures     

Chung‐Che Chou  Chia‐Ming Uang 《地震工程与结构动力学》2003,32(2):229-244

Energy serves as an alternative index to response quantities like force or displacement to include the duration‐related seismic damage effect. A procedure to evaluate the absorbed energy in a multistorey frame from energy spectra was developed. For low‐ to medium‐rise frames, it required a static pushover analysis of the structure to determine the modal yield force and ductility factor of an equivalent single‐degree‐of‐freedom system for the first two modes. The energy spectra were then used to determine the energy contribution of each mode. A procedure was also developed to distribute the energy along the frame height based on energy shapes. This study showed that the second‐mode response in some cases needs to be considered to reflect the energy (or damage) concentration in the upper floors. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Observing artificially induced strain: implications for subglacial deformation   总被引：1，自引：0，他引：1  

John F. Hiemstra  Kenneth F. Rijsdijk 《第四纪科学杂志》2003,18(5):373-383

By analysing a series of four successive thin‐sections from a ceramic clay that was subjected to uniaxial compression, we were able to monitor the development of microstructures in a fine‐grained sediment. The artificially induced microstructures, such as unidirectional clay reorientations and linear and circular grain arrangements, are identical to features that have been observed in thin‐sections of subglacially deformed tills, and therefore may be used as representative analogues. We argue that the structures, reflecting slip, planar shear displacements as well as rotational movements, can be explained by assuming a Coulomb‐plastic response to imposed shear. We conclude that sediments subjected to subglacial deformation behave as Coulomb materials, at least during the final stages of the deformation. The present study bridges the gap between field studies, experimental studies and theoretical modelling. The microscopic observations assist in visualising inferred subglacial processes and facilitate up‐ and downscaling between diverse methodological approaches. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

水稻低温冷害遥感监测技术与方法进展   总被引：7，自引：0，他引：7  

王连喜  秦其明  张晓煜 《气象》2003,29(10):3-7

根据国内外水稻低温冷害遥感监测和遥感灾损评估研究进展情况，分析了水稻低温冷害遥感监测的可行性和各种方法的优缺点，提出了低温冷害遥感监测方面需要开展的相关研究领域和需要解决的几个问题。  相似文献   

日光温室低温寡照灾害指标   总被引：27，自引：4，他引：27       下载免费PDF全文

魏瑞江 《气象科技》2003,31(1):50-53

文章根据1965～2000年河北省日光温室试验观测资料及各地历年低温寡照对蔬菜的影响程度，确定了低温寡照灾害等级指标，分析了低温寡照的时空分布规律，20世纪80年代以后，河北省各市低温寡照发生次数比60、70年代明显增多；随着纬度增高，河北省低温寡照发生次数明显减少，危害程度明显减轻。  相似文献