摘要:

ABSTRACTPalmer's Drought Severity Index (PDSI) is used to represent moisture conditions in the southeastern United States. Temporal trends in annual PDSI for 53 climate divisions in the region are examined for the period 1895 to 1988. Spearman's rank correlation reveals that 19 climate divisions have increasing trends in annual PDSI at the 0.05 significance level, changing towards a wetter condition during the study period. There is no division with a significant decreasing trend. Such temporal changes also display a well-defined spatial pattern. All but one of the divisions with significant increasing trends are located in the southwestern part of the region, centered at Alabama. Climate divisions in Alabama have experienced the most consistent change towards a wetter condition. The increasing rates in the annual PDSI during the study period range from 0.021/year to 0.037/year. Further analysis indicates that the increases in annual PDSI are mostly the result of increasing annual precipitation.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880378

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTLandslides are triggered by factors such as heavy rainfall, seismic activity, and construction on hillslopes. The leading cause of landslides in Puerto Rico is intense and/or prolonged rainfall. A rainfall threshold for rainfall-triggered landsliding is delimited by 256 storms that occurred between 1959 and 1991 in the central mountains of Puerto Rico, where mean annual rainfall is close to or in excess of 2,000 mm. Forty one of the 256 storms produced intense and/or prolonged rainfall that resulted in tens to hundreds of landslides. A threshold fitted to the lower boundary of the field defined by landslide-triggering storms is expressed asI = 91.46 D-0.82where I is rainfall intensity in millimeters per hour, and D is duration in hours. Landslide-producing storms occurred at an average rate of 1.2 per year. In general the landslides triggered by short-duration, high-intensity rainfall events were mainly shallow soil slips and debris flows, while the long-duration, low-intensity rainfall produced larger, deeper debris avalanches and slumps. For storms that had durations of up to 10 h, landsliding did not occur until rainfall intensity was as much as three times as high as the rainfall intensity reported as sufficient to trigger landsliding in temperate regions. As storm durations approach 100 h, the rainfall conditions necessary to initiate landsliding in Puerto Rico converge with those defined for temperate regions. A comparison of the Puerto Rico threshold with rainfall data from other humid-tropical regions suggests that the threshold developed for Puerto Rico may be applicable to other similar environments throughout the world.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880379

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTExperiments were performed using a tumbling’mill containing gravel-sized ceramic spheres and crushed Brazilian quartz sand to simulate comminution in high energy, mixed sediment fluvial environments. Results show that the sand is quickly reduced to silt (16 hours at 155 rpm) by a sequence of edge chipping, partial cracking, splitting and crushing. Comminution is most rapid at higher rotation speeds but significantly occurs even at the lowest speed of 55 rpm. Approximately 50% of the angular silt produced lies in the size range 20–63μm characteristic of loess-sized silt. Under suitable conditions‘fluvial crushing’could provide a viable mechanism for some silt production in high energy environments such as meltwater streams and the turbulent bores of flash floods.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880380

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTA new conceptual diagram is presented that aids in visualizing glacial processes, their effects and, hopefully through its use, may help us improve our understanding of glacial systems. Subglacial processes and products are emphasized in this paper. Ice, water, and till-forming processes including meltout, flow, deformation, and lodgement occupy the six corners of the octahedron. Symbols plotted within the octahedron represent the relative importance of the various processes/products involved. A bent arrow within the diagram traces the history of processes/effects at a site or in an area. Thus, the octahedron emphasizes glacial processes/products as transitional, interrelated phenomena within a continuum rather than as end-members. Two examples demonstrate contrasting uses of the octahedron. In the first, a triple patterned arrow traces the subglacial history of two grounded, competing lobe complexes along the south-central margin of the Laurentide Ice Sheet. In the second, a double patterned arrow depicts changing depositional settings during cyclic marginal grounding of a tidewater glacier. The octahedron is flexible and can be modified to emphasize other glacial systems, depending on the interests of the investigator.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880381

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTMeasurements of surface velocities, subglacial topography, mass balance and discharge of meltwater were made on a subpolar valley glacier Erikbreen (79°40'N, 12°30'E), Svalbard, in 1990 and 1991. Comparative studies were made on Hannabreen, located ca. 5 km from Erikbreen. Maximum measured surface velocity on Erikbreen (45 m/a) was considerably higher than on Hannabreen (8 m/a) and other investigated glaciers on Svalbard of similar size. The highest velocities on Erikbreen, observed early in the melting seasons, are due to high basal sliding rates, which induces a high potential for glacial erosion. The calculated ice flux near the equilibrium line altitude equals the mean net mass balance of the accumulation area in 1990 and 1991, so Erikbreen is probably near a state of dynamic equilibrium. This study shows that small valley glaciers with constantly high ice fluxes, similar to valley glaciers in the Scandinavian mountains, exist as far as 80°N on Svalbard.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880382

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTModelled future, green-house gas induced, climate change is likely to affect the frequency and intensity of geomorphic processes. As the projected warming is more pronounced at higher latitudes the potential impact on arctic/alpine environments may be considerable. Previous comprehensive process and sediment budget studies can provide“base-line”information for comparisons with current and future process rates. Future studies need to be sustained over longer time, ideally within official environmental monitoring programs. Methodologically, future studies ought to be standardized, preferably on an international level, as to allow between site comparisons.From a Swedish perspective, Rapp's (1960) work in Kärkevagge, northern Sweden, provide an obvious focus for this type of study. We summarize current research with the aim to: replicate older experiments, establish new representative experiments, gain a better understanding (through automatic meteorological and hydrological recordings) of how local climate conditions in the valley affect various processes and, finally, to make a new sediment budget for the catchment. A new photogrammetric topographic base-map (attached) has been produced (Kärkevagge Catchment Map 1993).

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880383

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880384

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor