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1.
James R. Zimbelman 《Geomorphology》2001,37(3-4)
The spatial resolution of image data tends to constrain the horizontal length scale of genetic hypotheses that are addressable by those data. No ‘simple’ formula exists when image resolution is sufficient to test a given geomorphic process, which is dependent on what characteristics are diagnostic of the particular process. Genetic hypotheses should be formulated along the lines of the “multiple working hypotheses” concept as described in a classic paper by Chamberlin [J. Geol. 5 (1897) 837]. An essential element of a viable working hypothesis is a clear indication of the characteristics predicted by, or a consequence of, the hypothesis. An untestable hypothesis is not an effective working hypothesis. The history of the study of lunar sinuous rilles is outlined as an illustration of the influence of image resolution and the formulation of genetic hypotheses on the subsequent advancement of understanding of the problem. Sinuous rilles on Venus and Mars, and controversial sinuous ridges on Mars are also reviewed. In the lunar case, the three-order-of-magnitude improvement in spatial resolution provided by Lunar Orbiter photographs over Earth-based telescopic photographs did not result in definitive examination and elimination of published hypotheses for the formation of sinuous rilles. Topographic data obtained from cartographically controlled Apollo orbital photographs, along with important observations and samples obtained by the astronauts on the lunar surface, did test and exclude several hypotheses. The formulation of a genetic hypothesis, including testable consequences of that hypothesis, is a greater determinant of its ultimate utility to the scientific community than is the image resolution available at any given time. 相似文献
2.
Great monumental places—Petra, Giza, Angkor, Stonehenge, Tikal, Macchu Picchu, Rapa Nui, to name a few—are links to our cultural past. They evoke a sense of wonderment for their aesthetic fascination if not for their seeming permanence over both cultural and physical landscapes. However, as with natural landforms, human constructs are subject to weathering and erosion. Indeed, many of our cultural resources suffer from serious deterioration, some natural, some enhanced by human impact. Groups from the United Nations to local civic and tourism assemblies are deeply interested in maintaining and preserving such cultural resources, from simple rock art to great temples. Geomorphologists trained in interacting systems, process and response to thresholds, rates of change over time, and spatial variation of weathering processes and effects are able to offer insight into how deterioration occurs and what can be done to ameliorate the impact.Review of recent literature and case studies presented here demonstrate methodological and theoretical advances that have resulted from the study of cultural stone weathering. Because the stone was carved at a known date to a “baseline” or zero-datum level, some of the simplest methods (e.g., assessing surface weathering features or measuring surface recession in the field) provide useful data on weathering rates and processes. Such data are difficult or impossible to obtain in “natural” settings. Cultural stone weathering studies demonstrate the importance of biotic and saline weathering agents and the significance of weathering factors such as exposure (microclimate) and human impact. More sophisticated methods confirm these observations, but also reveal discrepancies between field and laboratory studies. This brings up two important caveats for conservators and geomorphologists. For the conservator, are laboratory and natural setting studies really analogous and useful for assessing stone damage? For the geomorphologist, does cultural stone data have any real relevance to the natural environment? These are questions for future research and debate. In any event, cultural stone weathering studies have been productive for both geomorphologists and conservators. Continued collaboration and communication between the geomorphic, historic preservation, archaeological, and engineering research communities are encouraged. 相似文献
3.
不同土地利用类型对城市地表温度的影响——以广东东莞为例 总被引：31，自引：13，他引：18
利用Landsat TM6热红外遥感数据、土地利用专题图件、气象观测资料反演了快速城市化的典型地区东莞市的城市地表温度。构建了三个热效应评价指数:热效应贡献度指数、热像元权重指数和区域热像元权重指数,来定量分析不同土地利用类型对城市地表温度的贡献度,发现城乡工矿居民用地、林地和耕地的热效应贡献度较大,水域和草地的热效应贡献度较小,未利用地对区域热环境的贡献最小。此外,研究表明,随着平均温度的升高,东莞市水面和陆面1988~1998年的温度反差明显增大。 相似文献
4.
青藏高原全新世环境变化的兹格塘错元素地球化学沉积记录 总被引：1，自引：0，他引：1
兹格塘错是位于藏北高原腹地的一个封闭型湖泊,流域内没有现代冰川和永久积雪存在,其湖泊的水量平衡主要是大气降水和蒸发之间的平衡,因此其湖泊的变化过程可直接反映区域的气候变化。为了探讨该区域全新世的气候与环境变化过程,在其湖泊中心水深30m处钻取了一支深727cm的沉积岩芯,对沉积岩芯进行了元素地球化学分析,利用主成分分析得出影响沉积物化学组成的三个主要因子,主要受控于流域侵蚀因子和径流因子,区域背景作用因子所占比重很小。结合Mg/Ca的变化,其结果反映了全新世期间湖泊水体退缩与扩张,水质的浓缩与淡化的过程,揭示了兹格塘错流域气候的干湿变化,呈现暖干／冷湿气候组合模式;元素地球化学所反映出的湖面波动与环境变化得到了其他资料较好的支持,同时又显示了区域的差异性。 相似文献
5.
Field surveys in the Belgian loess belt revealed the presence in many forested areas of large, permanent gully systems, most of which are currently inactive. In cultivated areas, such gullies can only be observed in cross-sectional soil profiles through hollows, as virtually all such large gullies are currently infilled with colluvium. Little is known about the spatial distribution, initiation and temporal evolution of these large, permanent gully systems on loess-derived soils. Therefore, the medium-term evolution of a gully initiated in a cultivated area on loess-derived soils southwest of Leuven (Belgium) in May–June 1986, was studied over 13 years. Two intense rainfall events created this (ephemeral) gully, which was not erased by subsequent tillage. Between June 1986 and the December 1999, eight field surveys were conducted to measure gully dimensions. During two surveys, topographic indices (e.g., slope and drainage area) were also measured. Daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully. Analysis of rainfall data showed that no extreme rainfall event was required to initiate such large (permanent) gullies, as observed in forested areas and through cross-sectional profiles in cultivated fields in the Belgian loess belt. Return periods of the event that caused the gully varied between <1 year and 25 years, depending on the assumptions used for defining event rain intensity. Once established, length, surface area and volume of the studied gully evolved with time, cumulative rainfall or cumulative runoff, following a negative exponential relation. This accords with observations reported for gullies in Australia and the USA. This study shows that a degressive increase of gully extension, can be largely explained by the evolution of a “slope–drainage area” factor (S×A, which is proportional to stream power) with time. While gully length and gully surface area asymptotically evolve towards a final value, gully volume decreased at a given point in time. From this, it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it. From this moment on, the combined effect of water and tillage erosion in the gully drainage area, will lead towards rapid infilling. This expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern Belgium. The permanent gullies observed under forest are attributed to the fact that after severe gully erosion, this area was reforested or abandoned. Therefore, the sediment source was cut off and the gully was not filled in by sediment deposition. 相似文献
6.
Lin Ma Philip J. Ashworth James L. Best Lionel Elliott Derek B. Ingham Leslie J. Whitcombe 《Geomorphology》2002,44(3-4)
This paper describes the application of a commercially available, three-dimensional computational fluid dynamic (CFD) model to simulate the flow structure in an upland river that is prone to flooding. Simulations use a rectangular channel geometry, smooth sidewalls and a bed topography obtained from the field site that contains a subdued pool–riffle sequence. The CFD model uses the RNG κ– turbulence closure scheme of Yakhot and Orszag (J. Sci. Comput. 1 (1986) 1), as implemented in FLUENT 4.4.4, with a free surface. Results are shown for numerical runs simulating a 1:100 year return interval flood. Output from the numerical model is compared to a physical model experiment that uses a 1:35 scale fibreglass mould of the field study reach and measures velocity using ultrasonic Doppler velocity profiling (UDVP). Results are presented from the numerical and flume models for the water surface and streamwise velocity pattern and for the secondary flows simulated in the numerical model. A good agreement is achieved between the CFD model output and the physical model results for the downstream velocities.Results suggest that the streamwise velocity is the main influence on the flow structure at the discharge and channel configuration studied. Secondary flows are, in general, very weak being below the resolution of measurement in the physical model and less than 10% of the streamwise velocity in the numerical model. Consequently, there is no evidence for a ‘velocity dip’. It is suggested that the subdued topography or inlet morphology may inhibit the development of secondary flows that have been recorded in previous flat-bed, rectangular open channel flows. A significant corollary of these results is that the morphological evolution of the pool–riffle sequence at high discharges may be controlled primarily by the downstream distribution of velocity and sediment transport with little role for lateral sorting and sediment routing by secondary flows. This paper also raises a number of issues that may be of use in future CFD modelling of three-dimensional flow in open channels within the geomorphological community. 相似文献
7.
基于改进层次分析法的干旱扬水灌区水土环境变迁响应评估 总被引：1，自引：0，他引：1
受大量的外调水资源和人工灌溉的影响,干旱扬水灌区的水土环境变迁过程具有独特性,其对区域环境要素的响应是长期、潜在而缓慢的,该过程可通过科学的评估与分析描述其发展的趋势。评价过程是否合理,评价结果是否准确是灌区可持续发展的关键。通过应用改进的层次分析法,对干旱扬水灌区水土环境变迁响应进行了评价。结果表明:干旱扬水灌区水土环境变迁响应因子都属于"微敏感"因子;地表水对水土环境的变迁响应相对较为强烈,其次为地下水、区域气候,而土地利用最为迟缓; 18个响应因子中最为敏感的是灌区提水量及灌溉用水量,次之为地下水水位及回归水水质。该评价模型的评价结果与当前灌区水土环境变化现状相吻合,验证了改进层次分析法在干旱扬水灌区水土环境变迁响应评价中的有效性和可操作性,可为灌区的建设和管理提供科学合理的参考依据。 相似文献
8.
宁夏北部自然环境对沙尘暴天气的影响分析 总被引：3，自引：2，他引：1
通过对宁夏北部1961—2000年沙尘暴和自然环境状况的调查分析,发现该区域地形、地貌和沙源地分布的不同是沙尘暴地域分布不均的主要原因,同时还发现当地沙尘暴的减少趋势与不同年代特有的气候因素有关外,还与人为活动有着直接关系。进入20世纪90年代 ,该区域通过加大植树造林和防护林带建设规模,有效地遏制了土地荒漠化,使沙地面积有所缩小,在大风次数正常偏多的情况下,沙尘暴却明显减少,起沙的风速阈值呈增大趋势, 说明人为改变下垫面的自然环境,使其良性发展,有效地增加地表粗糙度,对抑制区域性沙尘暴的出现是可行的,并由此提出了防治沙尘暴应着眼于环境治理和保护的对策建议。 相似文献
9.
Automated classifications of topography from DEMs by an unsupervised nested-means algorithm and a three-part geometric signature 总被引：7，自引：3，他引：7
An iterative procedure that implements the classification of continuous topography as a problem in digital image-processing automatically divides an area into categories of surface form; three taxonomic criteria–slope gradient, local convexity, and surface texture–are calculated from a square-grid digital elevation model (DEM). The sequence of programmed operations combines twofold-partitioned maps of the three variables converted to greyscale images, using the mean of each variable as the dividing threshold. To subdivide increasingly subtle topography, grid cells sloping at less than mean gradient of the input DEM are classified by designating mean values of successively lower-sloping subsets of the study area (nested means) as taxonomic thresholds, thereby increasing the number of output categories from the minimum 8 to 12 or 16. Program output is exemplified by 16 topographic types for the world at 1-km spatial resolution (SRTM30 data), the Japanese Islands at 270 m, and part of Hokkaido at 55 m. Because the procedure is unsupervised and reflects frequency distributions of the input variables rather than pre-set criteria, the resulting classes are undefined and must be calibrated empirically by subsequent analysis. Maps of the example classifications reflect physiographic regions, geological structure, and landform as well as slope materials and processes; fine-textured terrain categories tend to correlate with erosional topography or older surfaces, coarse-textured classes with areas of little dissection. In Japan the resulting classes approximate landform types mapped from airphoto analysis, while in the Americas they create map patterns resembling Hammond's terrain types or surface-form classes; SRTM30 output for the United States compares favorably with Fenneman's physical divisions. Experiments are suggested for further developing the method; the Arc/Info AML and the map of terrain classes for the world are available as online downloads. 相似文献
10.
Kay Helfricht Michael Lehning Rudolf Sailer Michael Kuhn 《Geografiska Annaler: Series A, Physical Geography》2015,97(4):721-736
Snow deposition and redistribution are major drivers of snow cover dynamics in mountainous terrain and contribute to the mass balance of alpine glaciers. The quantitative understanding of inhomogeneous snow distribution in mountains has recently benefited from advances in measuring technologies, such as airborne laser scanning (ALS). This contribution further advances the quantitative understanding of snow distribution by analysing the areas of maximum surface elevation changes in a mountain catchment with large and small glaciers. Using multi‐annual ALS observations, we found extreme surface elevation changes on rather thin borders along the glacier margins. While snow depth distribution patterns in less extreme terrain have presented high inter‐annual persistence, there is little persistence of those extreme glacier accumulations between winters. We therefore interpret the lack of persistence as the result of a predominance of gravity‐driven redistribution, which has an inherently higher random component because it does not occur with all conditions in all winters. In highly crevassed zones, the lidar‐derived surface elevation changes are caused by a complex interaction of ice flux divergence, the propagation of crevasses and snow accumulation. In general, the relative contribution of gravitational mass transport to glacier snow cover volume was found to decrease for glaciers larger than 5 km2 in the investigated region. We therefore suggest that extreme accumulations caused by gravitational snow transport play a significant role in the glacier mass balance of small to medium‐size glaciers and that they may be successfully parameterized by simple mass redistribution algorithms, which have been presented in the literature. 相似文献
11.
Geomorphology as science: the role of theory 总被引：1，自引：0，他引：1
Because geomorphology is a science, it is permeated by theory. Overt recognition of this actuality is frequently resisted by geomorphologists. Earth history does not represent an alternative to earth science, it is an essential component of earth science. In its broadest sense science seeks to discover new knowledge through a two-stage activity involving the creation and justification of ideas (theory). Deduction is generally regarded as the only logically-consistent method of justifying ideas. The creation of ideas is a much more controversial topic. Some methodologists deem it beyond logic; certainly deductive arguments, which are nothing more than formal, logical expressions of theory, play no role in the conception of new ideas. Many earth scientists generate possible explanations of observed phenomena based on abductive reasoning. Others advocate reliance on purported forms of “pure” induction, such as serendipity and intuition, in which observations assume primacy over theory. Besides lacking consistency and educability, the latter posture is flawed because it mistakenly implies that becoming well-versed in theory is irrelevant to or impedes scientific discovery. Irrational or subjective factors play a role in the creation of ideas, but it is erroneous to claim that these factors are divorced from theory. Science is first and foremost a cognitive activity; thus, the primacy of observations in science is a myth. All observations are theory-laden in the sense that the act of observation inherently involves interpretation and classification, both of which can only occur within the context of theoretical preconceptions. Even discoveries based on unexpected observations require the fortunate investigator to recognize the theoretical importance of what is seen or measured.The most useful view of geomorphology as a science is one in which theory is seen as central, but fragile, and in which theory and observation are viewed symbiotically with theory providing the generative force and observation providing a vital policing role. Much of the current debate in geomorphology centers around differences in characteristics of theory, type of scientific arguments, and metaphysical perspectives among investigators working at different temporal scales. Full recognition and understanding of these differences are essential for developing a unified approach to the science of geomorphology. 相似文献
12.
Whanganui Inlet is a low mesotidal environment where wave energy at the shoreline is limited due to a small fetch, a narrow entrance and tidal flat accretion to intertidal elevations. Wave energy is therefore only an erosive force at high tide and under storm conditions. Despite this low-energy environment extensive shore platforms occur within the inlet. They are sub-horizontal and range in width from 4.1 to 185.2 m with an average of 44.9 m. All the platforms are formed in sandstone of low resistance (mean N-type Schmidt Hammer rebound value of 17 ± 8) and have their seaward edges buried by intertidal sediment flats. The majority of platforms occur at around MHWN level, corresponding to the elevation of those flats. Where wave energy is highest, opposite the inlet's entrance and at those sites with the largest fetch, platforms develop to 0.5–1.0 m below MSL. A higher platform level is also found at MHWS elevations, however it appears to be relict with active erosion of its seaward edge occurring and therefore is most likely related to a higher mid-Holocene sea level. Apart from the location of the lowest platforms little correspondence is found between platform morphology and wave energy. Platform evolution appears to be intrinsically linked to the intertidal sediment flats which determine the degree of surface saturation of the bedrock and, hence, the number of wetting and drying cycles the platforms may undergo. As the seaward edge is buried platform development is primarily through retreat of the landward cliff. This process can, however, be complicated by the migration of intertidal water channels onto the seaward edge of the platforms or relative sea level fall which may rejuvenate landward retreat of the low-tide cliff. 相似文献
13.
New field observations of beach berm growth resulting in the sub-aerial closure of an intermittently closed and open lake or lagoon are presented. These perched estuarine systems frequently exhibit ephemeral entrances that respond to process forcing almost instantaneously on a geomorphic timescale, with closure by a supra-tidal berm of depositional origin. The observations were made following a mechanical opening of the entrance and show very rapid vertical growth of an initially low beach berm as a result of swash overtopping and sediment overwash. Very little simultaneous seaward progradation was observed. Sediment overwash volumes were obtained from survey data over a period of 12 days either side of spring tide and the observations also provide an accurate measure of swash sediment transport at the berm crest. A process-based parametric model is developed through robust parameterisations of wave run-up, wave run-up distributions and sediment transport and is applied to predict the total overwash transport into and infilling the estuary entrance. The model is tested against the field data and compares well with the observations. While the field data are site-specific, the proposed modelling framework represents a first step in modelling the complex growth and recovery of natural beach berms in broad-scale morphodynamic models. 相似文献
14.
北极冰面融池对于研究北极海冰质量平衡、海洋混合层热收支和盐量收支等具有重要意义。为了获得准确的融池覆盖率,本研究提出了一种利用无人机进行北极海冰融池及冰面粗糙度信息提取的方法。在第7次中国北极科学考察期间,利用无人机获取加拿大海盆周边浮冰区冰面航拍影像,针对海冰航拍图像特殊性改进了基于暗原色先验的图像去雾算法,对拼接后的航拍图像进行融池识别,计算得到航拍区域的融池覆盖率。同时利用航拍影像三维建模得到海冰表面相对高程和冰面粗糙度,继而对融池覆盖率和海冰表面粗糙度分布规律进行研究。结果表明,在本次航拍区域,海冰粗糙度大的区域具有更多小面积的融池,而融透的、面积大的融池多出现在粗糙度小的平整冰区。 相似文献
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16.
Academics usually teach about geomorphology in the classroom, where the audience is enthusiastic, but generally small. Less traditional settings offer opportunities to reach a wider audience, one that is equally enthusiastic, given its love of geomorphic features in the National Parks, but one which has little knowledge of the science behind what they are seeing. I have “taught” geomorphology in four non-traditional settings: at a summer camp, a state wildlife refuge, on community field trips, and at meetings for clubs and government boards. This paper discusses my experiences and offers suggestions to others who may wish to follow this less-traveled educational path.As Head of Nature Programs at Camp Pemigewassett in New Hampshire, I have worked, over the last 33 years, with thousands of campers ranging from 8 to 15 years old. Our setting, in a glaciated valley on a small lake, exhibits a wide range of geomorphic features and offers many opportunities for direct learning through field investigations. I have found that even 8-year olds can do real science, if we avoid the jargon. Once “taught” they carry their knowledge about landforms and processes with them and eagerly share it with their friends and family on outings and trips, thus reaching an even wider public.Parks, wildlife refuges, nature preserves, and other similar areas generally have nature trails, often with educational information about the environment. Generally, interpretive signs are prepared by biologists and the content ignores the site's physical features, as well as the connections between ecological communities and the underlying geology and geomorphology. My students and I have addressed this situation at two places in Connecticut, one a state wildlife management area, also used for training teachers to teach Environmental Education, and the other, a town recreation area. We catalogued the geomorphic features, looked at relationships of the community level ecology to those features, and prepared interpretive signs that added this perspective to the trails. The public response has been extremely favorable.Geomorphology can also be taught by leading field trips for community organizations. I have done this twice, once for the Manchester (NH) Historical Society and once for a small watershed association. The attendance and interest surprised me. We finally had to limit the Manchester trip to one full busload (45) and the watershed trip, which was part of a “trails day,” drew over 90 people.Finally, I have found that organizations such as Sierra Club chapters and town conservation boards are frequently looking for speakers for their periodic meetings. Why not a geomorphologist? After all, much of what conservationists do is related to what geomorphologists do. I have given several of these presentations and the receptions have always been enthusiastic.While the work involved in preparing to teach in one of these non-traditional settings is frequently substantial, the rewards are equally large. It is a way to reach masses of people who know little about the science of geomorphology and to demonstrate its importance to them. Taking our message directly to the public in these settings is an effective way to put geomorphology in the public eye. 相似文献
17.
This paper provides data on the landforms, soils, and sediments within a unique northern Michigan landscape known as the Grayling Fingers, and evaluates these data to develop various scenarios for the geomorphic development of this region. Composed of several large, flat-topped ridges that trend N–S, the physiography of the “Fingers” resembles a hand. Previously interpreted as “remnant moraines”, the Grayling Fingers are actually a Pleistocene constructional landscape that was later deeply incised by glacial meltwater. The sediments that comprise the Fingers form a generally planar assemblage, with thick (>100 m), sandy glacial outwash forming the lowest unit. Above the outwash are several meters of till that is remarkably similar in texture to the outwash below; thus, the region is best described as an incised ground moraine. Finally, a thin silty “cap” is preserved on the flattest, most stable uplands. This sediment package and the physiography of the Fingers are suggestive of geomorphic processes not previously envisioned for Michigan.Although precise dates are lacking, we nonetheless present possible sequences of geomorphic/sedimentologic processes for the Fingers. This area was probably a topographic high prior to the advance of marine isotope stage 2 (Woodfordian) ice. Much of the glacial outwash in the Fingers is probably associated with a stagnant, early Woodfordian ice margin, implying that this interlobate area remained ice-free and ice-marginal for long periods during stage 2. Woodfordian ice eventually covered the region and deposited 5–10 m of sandy basal till over the proglacial outwash plain. Small stream valleys on the outwash surface were palimpsested onto the till surface as the ice retreated, as kettle chains and as dry, upland valleys. The larger of these valleys were so deeply incised by meltwater that they formed the large, through-flowing Finger valleys. The silt cap that occupies stable uplands was probably imported into the region, while still glaciated. The Fingers region, a col on the ice surface, could have acted as a collection basin for silts brought in as loess or in superglacial meltwater. This sediment was let down as the ice melted and preserved only on certain geomorphically stable and fluvially isolated locations. This study demonstrates that the impact of Woodfordian ice in this region was mostly erosional, and suggests that Mississippi Valley loess may have indirectly impacted this region. 相似文献
18.
Gregory A. Pope 《自然地理学》2013,34(4):315-338
While most research on quartz weathering has focused primarily on surface textures and morphologies, very little is known about the internal weathering of quartz. This study demonstrates that internal weathering is ubiquitous in quartz. Internal weathering is measured in terms of porosity, which represents mass loss from the quartz grain, hence silica lost through dissolution. Mass loss calculated from porosity suggests higher-than-expected rates of quartz dissolution in the terrestrial environment. Internal weathering occurs through various grain defects, and is classified into several forms (in decreasing order of frequency): fractures, enlarged grain boundaries, holes, and networks. These features may be distinguished from occasional artifact voids left by laboratory procedures. The most intensely weathered grains exhibit large fractures and extensive networks, and occasionally contain secondary weathering products within the void areas. The presence of internal weathering in quartz supports field and laboratory observations of particle comminution in sediment transport systems, and can account for at least part of the production of silt- and clay-sized quartz. Given the potentially large surface area afforded by these internal defects, internal weathering plays an important role in the generation of quartz particles and dissolved silica, and presents a new avenue of study for the generation of secondary porosity in detrital sediments. [Key words: geomorphology, quartz, silica, weathering.] 相似文献
19.
There is an increasing awareness of the influence of surface moisture on aeolian entrainment and transport of sediment. Existing wind tunnel studies have shown the impact of a limited range of moisture contents on entrainment thresholds but similar investigations are lacking in the field. The research reported here investigated the influence of changes in surface moisture content on sand entrainment and transport on a meso-tidal beach in Anglesey, North Wales.High frequency (1 Hz) wind velocities measured with hot-wire anemometers were combined with grain impact data from a Sensit monitor and mass flux measurements from a standard sand trap. Surface and near-surface moisture contents were assessed gravimetrically from surface sand scrapes and also directly by using a ThetaProbe. Critical threshold values for entrainment were specified using a modified form of the time fraction equivalence method (Stout, J.E., Zobeck, T.M., 1996a. Establishing the threshold condition for soil movement in wind-eroding fields. Proceedings of the International Conference on Air Pollution from Agricultural Operations. MWPS C-3, Kansas City, 7–9 February 1996, pp. 65–71).Results indicate a time-dependent change in dominant control of the sand transport system from moisture to wind speed, dependent upon the moisture content of the surface sediment. This interchange between controlling parameters on both entrainment and transport was very sensitive to prevailing moisture conditions and took place over a period of minutes to hours. Under conditions experienced in the experiments presented here, the critical moisture threshold for sediment entrainment was determined to be between 4% and 6%, higher than the 1–4% specified in previous wind tunnel experiments. Furthermore, a moisture content of nearly 2% (where moisture was adhered to transported sediment) appeared to have little or no impact on the rate of sand flux. 相似文献
20.
Probability of occurrence, hazard intensity and encounter probability are key parameters in the quantitative risk analysis (QRA) of landslides. All are strongly dependent on magnitude of the landslides. As a result, magnitude–frequency analysis should be a part of QRA. Deriving representative magnitude–frequency relationships for debris avalanches and debris flows, however, is difficult. One key problem is illustrated with the example of a unique database from the coastal region of British Columbia, Canada, which was compiled entirely from detailed ground investigations. The magnitude of debris avalanches and debris flows is not an independent statistical quantity, but a function of the scale of a given slope, as characterized by the slope length. Thus, attempting to derive probability and magnitude for a given location or sub-region from a regionally-derived magnitude–frequency curve may lead to incorrect predictions. The same problem is pertinent to the application of the same approach to any type of landslide in which the largest combined dimension of the source volume (including entrainment) is of the same order as the length of the slope. It is recommended that greater emphasis be placed on site-specific geological observations, at the expense of generalized statistics. 相似文献