ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880191

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTMovement of all sizes of sediment in the channel of a 4.03 km2drainage basin, Baisman Run, in the Piedmont of Maryland, USA, is episodic. Sediment yield to a forested basin from a headwater agriculture source area of 0.96 km2averaged 473 tonnes/km2/yr, with silt and clay predominant. This was transported in 20 to 30 events per year constituting 1 percent of the actual 4 year time span. Transient storage in 3.2 km of the second to fourth order channel represented 4–5 years of sediment inflow from the source area during years of relatively low runoff. Sand in patches and gravel (50 mm) moved roughly 5 to 7 times per year, and boulders (300- 600 mm) moved perhaps once every year or two. The proximity of large boulders (gneissic-schist containing much quartz) to bedrock outcrops suggests breakup rather than long distance transport. Headcut retreat at one site (18.2 m in 3 years) and translation of cobble bars en masse over a downstream distance of 4 to 6 channel widths indicates that channel processes are episodic in time and stepwise in space.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880192

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTLaboratory experiments, in which two different sized materials were continuously fed into a flume, revealed that a longitudinal sediment sorting produced a rhytmic fluctuation in the bedload transport rates. Bedload pulsation was immediately noticeable for conditions in which the finer size fraction was poor in the mixture. Three types of bed state—congested, transitional and smooth—repeated along the channel in an orderly manner; this is the longitudinal sediment sorting.Abundant sand particles thoroughly filled up the interstices of a gravel framework in the smooth state, which in turn invited mixing effects and gave a greater mobility. Sands and gravels were transported rapidly from reaches having the smooth state. An excess of sediment supply, however, was stored within the antecedent congested state, which created a bed surface closely packed by gravel particles. Gravels were intermittently in motion there under these conditions. The longitudinal sediment sorting was also accompanied by aggrading—degrading sequences and/or steep slope—gentle slope events. A descriptive model applicable to some problems in gravel-bed rivers was obtained.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880193

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTLaboratory experiments using luminescent tracers were conducted to analyze the effect of granulometry on the incipient motion of various grain-size fractions in sediment mixtures. The results demonstrate the basic mechanism of selective erosion, in which only a portion of the available bed material is transported. This phenomenon is essential for the development of bed armouring, a process which is important in numerical modelling of fluvial morphological processes. The laboratory findings are supported by an analysis of grain-size distribution of bed material from various rivers.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880194

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTMuddy sediments ranging from stiff clays and silts of early post-glacial age to fresh deposits of a buttery consistency are widely encountered in the intertidal zone of the estuary, a large macrotidal system swept by vigorous currents. Erosion of these muddy sediments is frequent, widespread and, in places, long-lasting. It is commonly expressed by the development of streamwise furrows and ridges on three distinct transverse wavelength scales (order 0.1 m, 1 m, 10 m). These occur on scoured surfaces ranging in extent from a few tens to many thousands of square metres. Coarse debris—the chief if not sole agent of erosion—is normally present in the bottoms of the furrows. The structures are discounted as obstacle marks because, with rare exceptions, they are in spacing and location independent of the distribution of bed irregularities. They appear instead to be related to secondary currents that arise due to the instability of the tidal shear flows, for which there is circumstantial field evidence. The furrows and riges of intermediate and large size probably depend on an instability mechanism constrained by flow depth. A second mechanism, taking a much smaller length scale, appears necessary to explain the small-scale structures, which in places appear to be active at the same time as those of intermediate dimensions.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880195

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTAlthough recent expansion of river monitoring programmes is generating a growing body of data on the magnitude of the suspended sediment yields of world rivers, much less is known about the particle-size characteristics of the sediment involved. Such information is, however, of increasing importance for a wide range of applications.Existing evidence indicates that there is considerable spatial variability in the particle-size characteristics of suspended sediment at the global scale and that rivers may also exhibit contrasting patterns of temporal variation. In response to a need for an improved understanding of the nature and controls of such variability at the local scale, this paper reports an investigation of the particle-size characteristics of suspended sediment transported by the River Exe and its tributaries in Devon, UK. Sediment samples were collected from eight measuring sites and the particle-size data obtained are used to emphasise the considerable contrasts in river behaviour that may exist at the local scale and to identify some of the important controlling factors.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880196

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTthe Colville River delta, located on the Arctic Ocean in northern Alaska, is in the zone of continuous permafrost. The delta's distributaries, which are ice covered for over eight months of the year, are subjected to high discharge rates for three to four weeks during the snow melt period in late May and June. At this time bank erosion is likely to occur along as much as 60% of the banks of the main channels. Although average erosion rates may range up to 4 m per year, variability is great both temporally and spatially.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880197

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTScroll topographic features, identical in the meander belts of both rivers, are indicators of ancient floods. The highest scrolls are dated dendrochronologically to around 1780 and the youngest to around 1840. A soil horizon buried beneath flood deposits gives radiocarbon dates consistent with the dendrochronological findings. Indirect indicators of floods include deflation surfaces and aeolian mounds.The significance of the floods is assessed in terms of climatic‘normality’, based on temperature and precipitation data for Leningrad, Kuusamo, Stockholm and Vardø. It is concluded that the considerably more positive water balance which prevailed during the Little Ice Age must have created conditions under which very much more extreme flooding could have taken place than has been normal during the present century. The timing of the peak floods to the 1780's appears to coincide with the exceptionally long and cold winters, which may have been the basic reason for the extremely high positive water balance at that time. The termination of the flooding by the middle of the 19th century proves to be highly compatible with general climatic trends.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880198

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTThe paper examines the geomorphological and sedimentological characteristics of the Rio Toro Amarillo alluvial fan located in the wet-tropical environment of the Atlantic watershed of Costa Rica. Alluvial fan development was found to be dominated by short periods of high sediment discharge to the fan, separated by longer, inactive periods characterized by weathering erosion, and reworking of fan sediments. Volcanic activity during 1963–65, and the resulting increase in sediment yield in the Toro Amarillo, produced major changes in fan characteristics over a ten year period. These changes are discussed. The length of the active period varies according to the magnitude, frequency, and type of sediment-producing event. It was estimated that the length of the inactive period for the Toro Amarillo may range from 100 to 500 years.The alluvial fan sediments are dominated by gravel that have a mean diameter about 30 to 40 cm. The sediment facies from the fan are described and their major characteristics outlined. Some comparisons are made between certain characteristics of the Toro Amarillo fan and alluvial fans in other climatic regions.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880199

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTThe formation of a large boulder delta complex found in the Båldakatj area, Swedish Lapland, is related to the catastrophic drainage of ice-dammed lakes. The interpretation of the sedimentological characteristics of these boulder deposits has resulted in the recognition of at least two phases in their development. An initial deposition of material by debris flows was subsequently followed by erosion and deposition by fluvial processes. The resulting reworking of the material was not uniform throughout the area. Two major lobes of boulder deposits can be recognized. These lobes have been exposed to different processes, as interpreted by differences in morphological and sedimentological characteristics, such as gradient, distal slope, channel pattern, scour marks, and the size, shape, roundness, imbrication and sorting of particles. Scour marks around large boulders occur frequently in the study area. They proved to be excellent flow direction indicators. The literature on similar boulder deposits, formed by flood flows or debris flows, in other regions or environments, has been reviewed and the results compiled for comparative purposes.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880200

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor