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1.
Extracting palaeoflood data from coarse‐grained Pleistocene river terrace archives: an example from SE Spain 
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下载免费PDF全文 Field‐based palaeoflood event reconstruction has the potential to contribute to the development of our understanding of long‐term landscape evolution. However, the reconstruction of past flow event histories (magnitude and frequency) over long‐term (Quaternary) timescales is fraught with difficulties. Here we make a preliminary exploration of some of the practicalities of flood reconstruction from fluvial terrace archives using commonly available sedimentological and geomorphological observations from a field perspective. We utilize Manning and palaeostage indicators to reconstruct historic events that can be used as benchmarks for a lesser used competence based approach, which is applied to coarse‐grained strath terrace deposits. We evaluate the results against gauged records for extreme and catastrophic events that affected the same region in 1973 and 2012. The findings suggest that the competence approach is most effectively applied to terrace deposits if the channel geometry is taken into account when sampling both in cross‐section and in longitudinal section and calibrated against the sedimentology for palaeo‐flow depth. Problems can arise where constrictive channel geometries allow boulder jams to develop, acting as sediment traps for the coarsest material and leading to downstream ‘boulder starvation’. Useful sites to target for palaeoflood reconstruction, therefore, would be upstream of such constrictive reaches where the coarsest transportable bedload has been effectively trapped. Sites to avoid would be downflow, where the deposited material would poorly represent palaeoflood competence. Underestimation from maximum boulder preservation and limited section exposure issues would appear to outweigh possible overestimation concerns related to fluid density and unsteady flow characteristics such as instantaneous acceleration forces. Flood data derived from river terrace deposits suggests that basal terrace geometries and coarse boulder lags common to many terrace sequences are likely the result of extreme flow events which are subsequently filled by lesser magnitude flood events, in this environmental setting. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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Computer flow simulations using the HEC-2 step-backwater routine are used to demonstrate the effect of systematically varying river channel width, riffle spacing and channel roughness on the shear velocity, section-mean velocity and energy slope in fixed-bed pool-riffle sequences. Initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the River Severn. Subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored. The purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence ‘reversal’, such that pools scour at high flow whilst deposition occurs on riffles. It is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal. For prescribed conditions, the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width. Downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool-riffle topography. 相似文献
3.
In some cases the mechanical competence of chondrules in carbonaceous chondrites has been reduced by alteration of their mesostasis glass to friable phyllosilicate, providing a mechanism by which euhedral olivines can be separated from chondrules. Morphological features of isolated olivine grains found in carbonaceous chondrites are similar to those of olivine phenocrysts in chondrules. These observations suggest that the isolated olivine grains formed in chondrules, by crystallization from a liquid, rather than by condensation from a vapor. 相似文献
4.
Jenny Richards Sallie Burrough Giles Wiggs Tiger Hills David Thomas Molatlhegi Moseki 《地球表面变化过程与地形》2021,46(15):3078-3095
An association between salt pans or dry lake beds and distinctive crescentic lake-floor sand mounds (1–10 m high, tens to hundreds of metres wide) is commonplace in desert systems. In the Makgadikgadi Basin of northern Botswana, a debate about the formative processes of these landforms has persisted despite numerous morphometric, sedimentary and geochronological analyses, with mound landforms variously inferred to be aeolian dunes, subaqueous dunes, spring mounds or shoreline remnants. We propose a new formative mechanism which draws on the interaction between uneven moisture distribution on the pan surface and mobile aeolian sediments. We use a numerical model (ViSTA), which couples vegetation and aeolian sand transport dynamics, together with optically stimulated luminescence (OSL) dating of a mound in the Makgadikgadi Basin to investigate the feasibility of this ‘sticky mound hypothesis’. We find that under a range of modelled environmental conditions, uneven moisture distribution on the pan surface can lead to the development and stabilization of crescentic aeolian dunes, with these dunes growing upwind from the point of initial deposition, corresponding with the chronological data gained from OSL dating of a mound feature. On removal of this moisture, the modelled dunes erode and dissipate. These findings suggest that the formative mechanism of the mounds could be dependent on the interaction between differential drying of the pan surface and the competence of the aeolian sediment transport system across the pan floor. 相似文献
5.
浅谈城市发展与防震减灾 总被引：1，自引：0，他引：1
通过对加强防震减灾规划,加强地震安全性评价工作,加强地震应急救援能力以及增强公众防震减灾意识等4个方面的分析,揭示了城市发展与防震减灾的关系,强调了建设地震安全社区的重要性。 相似文献
6.
Determination of the suspension competence of debris flows based on particle size analysis 总被引：1，自引：0，他引：1
The determination of the critical particle size between solid and fluid phases, i.e., the suspension competence, is fundamental for debris flow. A method for determining suspension competence based on particle size analysis is presented in this paper. Suspension competence of static experimental water-debris mixtures prepared with the sediment of Jiangjia Gully is -0.025 mm if the bulk density is less than 1,800 kg m-3 and it increases with bulk density of more concentrated mixtures. Suspension competence of natural debris flows in Jiangjia Gully increases exponentially with the bulk density. These two data sets are compared in order to understand the suspension mechanism. It is concluded that turbulence may play a leading role in particle suspension in non-viscous and sub-viscous debris flows, while in viscous debris flows both matrix strength and excess pore water pressure play important roles. 相似文献
7.
Aerosol particles over land mainly come from man- made source such as biomass burning, industrial de-bris and natural source such as soil dust, sea salt parti-cles, etc. More and more research results show that, aerosols impact global and regional energy radiative budget; aerosol particles also modify the cloud mi-crophysics, as a result, aerosol particles may change the cloud radiative properties. Aerosol particles also play an important role in many biogeochemical cycles. All the above-menti… 相似文献
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Peter R. Wilcock 《地球表面变化过程与地形》1992,17(3):289-298
The largest grains found in samples of transported sediment are commonly used to estimate flow competence. With samples from a range of flows, a relationship between the flow and the largest mobile grain can be derived and used to estimate the critical shear stress for incipient motion of the different grain sizes in the bed sediment or, inversely, to estimate the magnitude of the flow from the largest grain found in a transport sample. Because these estimates are based on an extreme value of the transport grain-size distribution, however, they are subject to large errors and are sensitive to the effect of sample size, which tends to vary widely in sediment transport samples from natural flows. Furthermore, estimates of the critical shear stress based on the largest sampled moving grain cannot be scaled in a manner that permits reasonable comparison between fractions. The degree to which sample size and scaling problems make largest-grain estimates of fractional critical shear stress deviate from a true relationship cannot be predicted exactly, although the direction of such a deviation can be demonstrated. The large errors and unknown bias suggest that the largest sampled mobile grain is not a reliable predictor of either critical shear stress or flow magnitude. It is possible to define a single flow competence for the entire mixture, based on a central value of the transport grain-size distribution. Such a measure is relatively stable, does not require between-fraction scaling, and appears to be well supported by observation. 相似文献
10.
A field‐based project was initiated in order to characterize velocities and sediment entrainment in a forced‐pool and riffle sequence. Three‐dimensional velocities and turbulence intensities were measured with an acoustic Doppler velocimeter at 222 different points at three similar flows that averaged approximately 4·35 m3 s−1 within a large pool–riffle unit on North Saint Vrain Creek, Colorado. Sediment‐sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location. Tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow, which reached a maximum level of 14·8 m3 s−1. Velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools. Patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle. Flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing, and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply. Jet flow, flow separation, vortex scour and turbulence generation enhance scour in the centre of pools. Ultimately, multiple processes appear to play some role in maintenance of this forced pool and the associated riffle. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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In Europe, when small (> 0.01 km2) lakes are considered, approximately 500,000 natural lakes occur. This considerable number points to the fact that lakeshore habitats and ecosystems are of significant importance to the total biodiversity of European landscapes, not only because of their expanse, but also because they are ecotones between land and water, which attract many kinds of wildlife, economical, cultural and recreational uses and human settlement. These very considerable stretches of shore are not currently registered, mapped or evaluated anywhere.Apart from providing habitat, the littoral biocoenosis performs a string of additional functions in the ecosystem, of which several are also of great importance to people, such as self purification, buffer zone, erosion protection and recreation scenery. In particular, the recreational function provides a main economic base for many lake areas and even whole countries in Europe. However, human interests have resulted in lake shore deterioration, such that many European lakes are now bare natural shores or retain only relics of them. Apart from the loss of the ecological functions, this also leads to a substantial loss of economic benefits. This precipitates the need for a responsible management, which can be done only on the base of a sound assessment method and a continuous monitoring of the status of lake shore areas. On a European scale, the European Water Framework Directive (WFD, 2000/60/EC) and the Habitat Directive (92/43/EEC) provide the frame for the assessment and monitoring of the status of littoral habitats of lakes. The WFD focuses on entire surface water bodies, including their associated wetlands under influence of their natural water-level fluctuations. However, the provided set of quality elements has to be adapted for an approach specific to lake shores. Apart from quality elements indicating the natural spatial pre-requisites, biocoenotic diversity and integrity, and ecosystem function and dynamics, also quality elements representing the human pressure of land use and the social and economic value of the lake shore zone should be included in an integrated lakeshore assessment and monitoring concept.However, the application of such an integrated quality assessment scheme requires an integrated administrative counterpart, just as the WFD requires for water management aspects on a catchment scale. Only by overcoming the splitting of competence among a variety of authorities and planning corporations can an integrated approach to sustainable shore development be translated into action. 相似文献
13.
The seismic future of cities 总被引：1，自引：0，他引：1
Roger Bilham 《Bulletin of Earthquake Engineering》2009,7(4):839-887
The final projected doubling in Earth’s population in the next half century, requires an additional 1 billion housing units,
more dwellings constructed in a single generation than at any time in Earth’s history. Earth’s tenfold increase in population
has occurred during a time that is short compared to the return time of damaging earthquakes. In the next century, therefore,
earthquakes that had little impact on villages and towns, will be shaking urban agglomerations housing upwards of 12 million
people. An epicentral hit on a megacity has the potential to cause 1 million fatalities. The incorporation of earthquake resistant
structures in the current global building boom, despite successes in the developed nations, has been neglected in the developing
nations where historically earthquake damage has been high. The reasons for this neglect are attributed to indifference, ignorance
and corrupt practices, not due to an absence of engineering competence. Never has a generation of earthquake engineers been
faced with such a grave responsibility to exercise their skills, both political and technical, as now.
The eye is bewildered by “a city become an heap”. Robert Mallet (1862).相似文献
14.
太湖地区环境敏感地空间分布及对策——以苏州市为例 总被引：1，自引：0，他引：1
随着工业化和城市化进程的迅速推进,区域空间发展向坡地、沿湖地区等环境敏感地区伸展,加剧了环境敏感地区的环境压力和生态胁迫,影响了区域可持续发展。针对苏州市自然地域特点,对环境敏感地进行分类,划出区域未来开发与保护的空间范围与布局,对实现区域健康发展具有重要的现实意义。本文将生态保护需求、水源保障、污染影响、农田生态保护、灾害防范等作为环境敏感评价的基本因子,将苏州市环境敏感地划分为生态功能敏感地、地表水源敏感地、污染影响敏感地、优质农田敏感地以及潜在灾害敏感地五类,在明确其空间范围的基础上,对环境敏感地进行综合评价,进而有针对性的提出减缓敏感响应、保护生态及发展区域经济的对策。 相似文献
15.
An in-channel surficial depositional feature (surficial fine-grained laminae) composed of loosely bound fine sediment deposited during low flow conditions has often been observed in river systems in south-western Ontario. The physical characteristics of this feature have been determined by a direct observation image analysis system. This sediment consists primarily of flocculated fine-grained material. The size distributions of surficial fine-grained laminae and suspended sediment were not significantly different. Each distribution is bimodal in nature and shows a characteristic grain size deficiency in the 4-5 μm size range. This observation suggests that flocculation and not low discharge or low competence is the dominant mechanism for the formation of surficial fine-grained laminae under conditions of low flow in fluvial systems of south-western Ontario. A quantitative assessment of this feature shows its potential importance as a source of fine-grained sediment and associated contaminants for downstream transport. 相似文献
16.
随着发达地区经济社会的快速发展,建设用地急剧扩张,改变地表下垫面水文条件,成为导致区域水环境不断恶化的重要因素,并引发一系列的资源环境问题.为了促进资源环境与经济社会均衡发展,近年来地理学界开展了大量空间开发功能分区和适宜性分区的研究,为区域空间管理和建设用地调控提供科学依据,但是这些研究大多以单一行政区为对象,缺少流域等跨行政区的自然地理单元的分区研究,忽视了流域经济社会发展与资源环境保护的整体性.为此,本文在借鉴相关分区研究的基础上,以太湖流域为例,探讨了流域单元空间开发适宜性分区指标选择与综合评价等技术方法,将太湖流域划分为高适宜区、中等适宜区、低适宜区和不适宜区等四种类型,并采用数据包络分析方法,探讨了建设用地配置规模与空间开发适宜性的对应关系和均衡程度,据此判断流域空间开发的失衡状态. 相似文献
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Jeffrey R. Moore Johnny W. Sanders William E. Dietrich Steven D. Glaser 《地球表面变化过程与地形》2009,34(10):1339-1352
Collapse of cliff faces by rockfall is a primary mode of bedrock erosion in alpine environments and exerts a first‐order control on the morphologic development of these landscapes. In this work we investigate the influence of rock mass strength on the retreat rate of alpine cliffs. To quantify rockwall competence we employed the Slope Mass Rating (SMR) geomechanical strength index, a metric that combines numerous factors contributing to the strength of a rock mass. The magnitude of cliff retreat was calculated by estimating the volume of talus at the toe of each rockwall and projecting that material back on to the cliff face, while accounting for the loss of production area as talus buries the base of the wall. Selecting sites within basins swept clean by advancing Last Glacial Maximum (LGM) glaciers allowed us to estimate the time period over which talus accumulation occurred (i.e. the production time). Dividing the magnitude of normal cliff retreat by the production time, we calculated recession rates for each site. Our study area included a portion of the Sierra Nevada between Yosemite National Park and Lake Tahoe. Rockwall recession rates determined for 40 alpine cliffs in this region range from 0·02 to 1·22 mm/year, with an average value of 0·28 mm/year. We found good correlation between rockwall recession rate and SMR which is best characterized by an exponential decrease in erosion rate with increasing rock mass strength. Analysis of the individual components of the SMR reveals that joint orientation (with respect to the cliff face) is the most important parameter affecting the rockwall erosion rate. The complete SMR score, however, best synthesizes the lithologic variables that contribute to the strength and erodibility of these rock slopes. Our data reveal no strong independent correlations between rockwall retreat rate and topographic attributes such as elevation, aspect, or slope angle. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
19.
Abstract The runoff response of a small drainage basin in southern Ontario has been monitored over a 5 year period, during which 25 per cent of its surface has been modified by suburban construction activity. Direct runoff response to summer and autumn rainstorms has not been noticeably affected by the development, but response to spring snowmelt has increased by three to four times. Summer and autumn storm runoff still appears to be generated mainly by direct precipitation on a natural, saturated, contributing zone alongside the channel, which has been little affected by the construction activity. The construction surfaces have had their infiltration capacities greatly reduced by devegetation and compaction, but runoff from them does not contribute to the storm hydrograph unless ephemeral channels and swales which link them to the main stream are saturated and capable of conducting flow. This happens rarely during rainstorms, but consistently during snowmelt, when ground conditions are extremely wet. The seasonal contrasts in the effect of partial suburban development on direct runoff response, are therefore caused by seasonal variations in the extent to which the construction surfaces are integrated into the active runoff contributing zone of the drainage basin. 相似文献