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1.
John Pitts 《地球表面变化过程与地形》1983,8(6):589-603
A chronology of landsliding is presented, including suggestions as to a date for initiation. Periods of activity known from historical sources are correlated with known periods of climatic deterioration. The current morphology of the landslide slopes is closely related to the geological succession and structure. These permit the landslides and their development to be differentiated on the basis of whole slope and toe morphology, and much of the whole-slope activity can be related to conditions at the toe. Examples of geomorphological maps, slope categories maps, and cross sections are presented along with an example of the ‘evolutionary’ maps which may be derived from Ordnance Survey plans and aerial photographs. These generally indicate that weakening of materials by weathering, seepage erosion at the toe, and marine erosion result in frequent mass movement events of a low magnitude. These events ultimately influence the stability of larger slipped blocks behind, which fail less frequently. It is suggested that whole-slope failures in this region have a maximum frequency of once in 120 to 150 years, and that problems of interpretation of historical accounts may mean that it is very much less frequent than this. 相似文献
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We present observational data for two long-period and three dynamically new comets observed at heliocentric distances between 5.8 to 14.0 AU. All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant. We have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated below the temperature of the amorphous-crystalline ice phase transition (T∼137 K). We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System. Experimental data show that large grains can be ejected at low velocity during annealing and that the rate of brightening of the comet should decrease as the heliocentric distance decreases. These results are consistent with both historical observations of distant comet activity and with the data presented here. If observations of the onset of activity in a dynamically new comet are ever made, the distance at which this occurs would be a sensitive indicator of the temperature at which the comet had formed or represents the maximum temperature that it has experienced. New surveys such as Pan STARRS, may be able to detect these comets while they are still inactive. 相似文献
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Sedimentary furrows in fine-grained sediments have been observed in a variety of settings ranging from the deep ocean and
deep lake bottoms to shallow estuaries and are commonly described as persistent, long-term features of the seabed. A series
of 12 sidescan sonar surveys over the course of three years reveal that transient, longitudinal sedimentary furrows regularly
form and then occasionally dissipate within the middle portion of the York River. Varying furrow morphologies were observed
depending on current conditions, ranging from large regularly space (0.7–7 m) linear furrows during low current conditions
to large patches of meandering furrows as the mean current increases or no bed forms during the higher current conditions.
Based on210Pb and137Cs profiles of kasten cores, differences in physical mixing depths of ∼25 cm between cores collected <2 m apart indicate a
high degree of small-scale spatial heterogeneity within the seabed. By documenting the position of kasten cores using a digital
sidescan sonar system, we showed that a core taken within a furrow had a mixing depth 15 cm shallower than an adjacent core
taken between furrows. A time-series of mixing depths over the 35 mo of the study reveals that, along with the ∼25 cm scale
differences in mixing depths due to the formation and destruction of furrows, there is a longer temporal signal of mixing
producing 100-cm-scale changes in mixing depths on the annual to interannual time frame. Although the formation and destruction
of the furrows appear to be a significant process contributing to decimeter-scale seabed mixing, there is a longer-term unknown
process which is controlling the meter-scale seabed mixing. 相似文献
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Disruption of the natural patterns of freshwater flow into estuarine ecosystems occurred in many locations around the world beginning in the twentieth century. To effectively restore these systems, establishing a pre-alteration perspective allows managers to develop science-based restoration targets for salinity and hydrology. This paper describes a process to develop targets based on natural hydrologic functions by coupling paleoecology and regression models using the subtropical Greater Everglades Ecosystem as an example. Paleoecological investigations characterize the circa 1900 CE (pre-alteration) salinity regime in Florida Bay based on molluscan remains in sediment cores. These paleosalinity estimates are converted into time series estimates of paleo-based salinity, stage, and flow using numeric and statistical models. Model outputs are weighted using the mean square error statistic and then combined. Results indicate that, in the absence of water management, salinity in Florida Bay would be about 3 to 9 salinity units lower than current conditions. To achieve this target, upstream freshwater levels must be about 0.25 m higher than indicated by recent observed data, with increased flow inputs to Florida Bay between 2.1 and 3.7 times existing flows. This flow deficit is comparable to the average volume of water currently being diverted from the Everglades ecosystem by water management. The products (paleo-based Florida Bay salinity and upstream hydrology) provide estimates of pre-alteration hydrology and salinity that represent target restoration conditions. This method can be applied to any estuarine ecosystem with available paleoecologic data and empirical and/or model-based hydrologic data. 相似文献
7.
David E. Pitts Michael R. Helfert Kamlesh P. Lulla Mary Fae McKay Victor S. Whitehead David L. Amsbury 《国际地球制图》2013,28(4):69-80
Abstract The impact of animals upon the landscape, through such actions as trampling, feeding, burrowing, mounding, and dam building, is a topic of interest in university geography classrooms. However, the relatively fine‐scale nature of most such impacts precludes its examination via remote sensing. Nevertheless, remote sensing of many types of animal impact is possible, and in this paper I illustrate several such examples, primarily drawn from free‐access sites on the World Wide Web, using a variety of remote sensing platforms. Instructors of physical geography or resource/land use management courses can thus introduce a variety of remote sensing concepts and imagery types in their classrooms while at the same time examining the role of animals on the landscape. 相似文献
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Forrest R. Pitts 《The Professional geographer》2013,65(3):422-423
This study examines the spatial distribution and impact of nonroutine accidental releases of hazardous materials relative to the demographic composition of residents in nearby communities. First, atmospheric dispersion modeling methods are used to delineate the impact zones of worst-case accidents in two New York counties over the last ten years. Next, using accidental reports for 1997, GIS and statistical operations are used at the census tract level of the two counties to determine whether these incidents disproportionately affected disadvantaged neighborhoods. The results suggest that the areas of high-impact from accidental releases of hazardous materials are best characterized by a large proportion of families below the poverty line, Hispanics, and other minorities. 相似文献
10.
Restoration of Florida’s Everglades requires scientifically supportable hydrologic targets. This study establishes a restoration
baseline by developing a method to simulate hydrologic and salinity conditions prior to anthropogenic changes. The method
couples paleoecologic data on long-term historic ecosystem conditions with statistical models derived from observed meteorologic
and hydrologic data that provide seasonal and annual variation. Results indicate that pre-drainage freshwater levels and hydroperiods
in major sloughs of the Everglades were about 0.15 m higher and two to four times greater, respectively, on average compared
to today’s values. Pre-drainage freshwater delivered to the wetlands and estuaries is estimated to be 2.5 to four times greater
than the modern-day flow, and the largest deficit is during the dry season. In Florida Bay, salinity has increased between
5.3 and 20.1 with the largest differences in the areas near freshwater outflow points. These results suggest that additional
freshwater flows to the Everglades are needed for restoration of the freshwater marshes of the Everglades and estuarine environment
of Florida Bay, particularly near the end of the dry season. 相似文献