Coastal conglomerates around the Hadjer el Khamis inselbergs (western Chad,central Africa): new evidence for Lake Mega‐Chad episodes     

Mathieu Schuster  Philippe Duringer  Jean‐Francois Ghienne  Patrick Vignaud  Alain Beauvilain  Hassan Taisso Mackaye  Michel Brunet 《地球表面变化过程与地形》2003,28(10):1059-1069

This paper reports on a study dealing with the rhyolitic inselbergs of Hadjer el Khamis that formed palaeoislands during Lake Mega‐Chad events. Field observations have shown that: (1) conglomeratic patches of immature to mature clasts are preserved at the feet of the Hadjer el Khamis inselbergs; (2) in cross‐section, their pro?le reveals a well de?ned cliff–platform junction at a constant elevation (325 m). The monolithological clasts show all degrees of roundness, from angular cobbles to well rounded pebbles. This wide range of maturity suggests a coastal origin for these cobbles. The system was permanently fed with angular clasts, which were progressively worn by waves. Cobbles that were wave‐worked for the longest time are the best rounded. The cliff–platform junction is the result of erosion by waves, which attacked and undercut the inselberg cliffs during Lake Mega‐Chad events. Asymmetrical erosion pro?les moreover suggest a wind regime dominated by SW to NE oriented winds. These interpretations have two implications. The elevation of the cliff–platform junction is an indication of the highest water level of Lake Mega‐Chad at 320–325 m, which is in agreement with other observations elsewhere in the basin. The SW to NE oriented winds show that monsoon‐related winds were prevalent during Lake Mega‐Chad events, suggesting the Inter‐Tropical Convergence Zone was located higher in latitude than today. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Field measurement and numerical modelling of aeolian mass flux distributions on a sandy beach   总被引：6，自引：0，他引：6  

Steven L. Namikas 《Sedimentology》2003,50(2):303-326

ABSTRACT The vertical and horizontal distributions of aeolian mass flux were measured at Oceano Dunes, California, and these data were used to evaluate a numerical model of saltation. Grain‐size analyses showed that the distributions of the modal sediment size class corresponded closely to those of the total sediment population, and modelling thus focused on replicating the distributions of the mean grain size. Although much previous work has assumed that the mean launch speed of saltating particles varies in proportion to shear velocity, simulations using a constant mean launch speed were found to yield the closest approximations to the mass flux distributions observed in the field. Both exponential and gamma distributions of launch velocity produced realistic simulations, although the latter approach required the inclusion of an additional reptation component to achieve good results. A range of mean launch angles and an equivalent sphere correction were also found to generate comparable results, providing the other input parameters could be varied freely. All the modelling approaches overestimated the proportion of mass flux occurring at the bottom of the vertical distributions, and underestimated the proportion occurring at the upwind end of the horizontal distributions. No theoretical shortcoming that would account for these small, but systematic, discrepancies could be identified, and experimental error thus represents a more plausible explanation. The conclusion that mean grain launch speeds are essentially constant and independent of shear velocity suggests that the additional kinetic energy extracted by grains under more energetic wind conditions is largely transferred to the bed, and that increases in the transport rate are therefore driven primarily by the ejection of additional grains. It is suggested that the kinetic energy of rebounding grains is constrained by the ability of the bed to resist deformation, equivalent to a plastic limit. Hence, grains of larger mass (diameter) rebound from the bed at lower speeds, and follow shorter, lower trajectories, as has been widely observed previously.  相似文献   

东南沿海山地城市土地资源综合开发模式探讨--以温州乐清市为例   总被引：1，自引：0，他引：1  

郭丽英  胡业翠 《山地学报》2005,23(1):121-125

以温州乐清为例，在土地利用现状特点与问题分析的基础上，提出山地城市土地资源的利用潜力在于山地、平原和海域的综合资源优势。建议重视山地一平原一海涂土地资源开发的系统设计和分区规划，促进市域土地资源优化配置与持续高效利用，为统筹城乡协调发展开创良好的土地保障环境。  相似文献   

The impact of small-scale wood harvesting on neem Azadirachta indica A. Juss stands in the coastal savanna of Ghana     

Michael O'Neal Campbell 《Area》2005,37(1):8-16

Although the introduced neem tree Azadirachta indica A. Juss is a very important energy source in the coastal savanna of Ghana, the sustainability of small-scale wood harvesting (SSWH) has not been investigated with regard to this species. This paper assesses the sustainability of SSWH on neem stands in the coastal savanna, using a socio-economic and ecological survey. The results show that SSWH, incorporating coppicing, stand tolerance, balanced with total bole cutting and root removal, and based on socio-cultural supports, has created a modified yet sustainable system of SSWH. It is concluded that these systems may contribute to tree stand conservation.  相似文献   

Geomorphology of the Boao coastal system and potential effects of human activities - Hainan Island, South China     

ZHU Dakui  YIN Yong  I. Peter MARTINI 《地理学报》2005,15(2):187-198

The Boao coastal system along the eastern coast of Hainan Island is a dynamic delta-tidal inlet-barrier formed during the late Holocene. The delta developed inside a shallow lagoon barred by a sandy barrier with a narrow, shallow tidal inlet opening. Two major distributary channels separated by small islands characterize the delta. The lagoon is silting up receiving and trapping sediments from both the river and, in minor measure during storms, through the tidal inlet opening and barrier washovers. The barrier at the tidal inlet is highly dynamic and changes its form, accreting (migrating spit) against the inlet during fair-weather conditions and being eroded during storms and river floods. The delta has almost completely filled the lagoon and major concerns exist on the effect that ongoing large development plans may have on the environment. These concerns include the effect on floods and rate of siltation once banks of the islands have been stabilized and floodwater and sediment load are impeded from spreading over the lowlands, and the effect of increasing pollutant loads from the new facilities on the ecosystems of the increasingly restricting lagoon water and on the seashores.  相似文献   

Geomorphology of the Boao coastal system and potential effects of human activities -Hainan Island, South China   总被引：2，自引：0，他引：2  

ZHUDakui YINYong PeterMARTINI 《地理学报(英文版)》2005,15(2):187-198

1 IntroductionThe objectives of this paper are to analyze the principal characteristics of the W anquan Riverdelta that has developed in a narrow , shallow lagoon, and to determ ine the effect that theever-increasing hum an activities of the area m ay hav…  相似文献   

Internal structure of a trough blowout, determined from migrated ground-penetrating radar profiles   总被引：2，自引：0，他引：2  

Adrian Neal  & Clive L. Roberts 《Sedimentology》2001,48(4):791-810

Ground‐penetrating radar (GPR) was used to investigate the relationship between the geomorphological development of a large aeolian trough blowout and the stratigraphy and internal sedimentary structure of its associated deposits. Although analogous, many of the data‐processing techniques routinely applied in seismic reflection are very rarely applied in GPR studies. In this study, a simple migration program was used that significantly enhanced the quality of GPR images from a large trough blowout at Raven Meols on the Sefton coast, northwest England. These improvements aided subsequent data interpretation, which was achieved through application of the principles of radar stratigraphy. GPR shows the pre‐blowout dunes to have a complex internal structure that suggests they were formed in the presence of at least a partial vegetation cover. Subsequent to stabilization of these dunes a thin soil developed. This dune soil forms an important radar sequence boundary and delineates a complex topography beneath the depositional lobe of the blowout. The internal structure of the depositional lobe of the blowout does not conform to a model of simple radial foreset deposition, as derived from contemporary process studies reported in the literature. Instead, the pattern of deposition has been extensively modified by the antecedent dune topography and by varying spatial and temporal exposure to important sand‐transporting winds that is partly controlled by interactions between the regional wind pattern and local dune morphology. Trough blowout deposits in coastal aeolian sedimentary sequences are likely to be recognized by the presence of laterally continuous packets of relatively high‐angle cross‐strata, which often display a spatially‐variable radial dip pattern that is only very poorly or partially developed. In addition, a soil, or other surface representing a significant hiatus in dune deposition, is likely to underlie the blowout deposits, the topography of which will show a clear relationship to the dip and orientation of the overlying cross‐strata.  相似文献   

Recent geomorphological evolution of the deltas of the rivers Seman and Vjosa,Albania     

Eric Fouache  Gjovalin Gruda  Skender Mucaj  Pal Nikolli 《地球表面变化过程与地形》2001,26(7):793-802

Rising in the Neogene hills of the Mallakaster, the rivers Seman and Vjosa have built up two large joint deltas on the Albanian Adriatic shore. This shoreline is characterized by a low sandy coast with bars and spits. Changes in the river courses and migration of the mouths of the deltas were rapid and numerous from the Holocene period until the beginning of drainage works in the 1950s. The drainage basins of the two rivers are developed in soft clastic rocks (flysch and molasse) in the proportion of 71·4 per cent for the Seman and 44·8 per cent for the Vjosa. Both rivers carry abundant sediment loads, amounting to 6·7 × 10⁶ tonnes per year for the Vjosa and 13·2 × 10⁶ tonnes per year for the Seman. This is the reason why the alluvial deposits of the Seman have built up two‐thirds of the alluvial plain. The use of a SPOT image dated 25 May 1995 (HRV 3 081‐268) enabled us to view the effects of coastal and fluvial dynamics, the role of neotectonics as well as the predominance of the plume of suspended sediment of the Seman river. Using this image, a geomorphological map was drawn, which identifies the palaeochannels of the Seman and the Vjosa. In order to date those palaeochannels we have made an archaeological inventory from oral and written published information. The location of the sites we studied was checked systematically in the field. The mediaeval and Ottoman archives kept in Tirana also provided substantial information, as well as the reconstitution of the evolution of the shoreline between 1870 and 1990, carried out using an inventory of topographic maps. This work allowed us to reconstitute the progression of the deltas of the Seman and the Vjosa since antiquity. We may then infer that from antiquity up to the Middle Ages, the deltas of the Seman and the Vjosa both progressed very moderately and in a comparable way. However, at the end of the 15th century the Seman underwent a major change in its course, through a southward migration of the river. The natural processes of alluviation and changes in the river courses seem to have been accelerated as agricultural exploitation of the Neogene hills that form most of the drainage basin of the Seman increased. This exploitation is linked with the massive exportation of cereal from the port of Skela e Pirgut, which started in the 14th century. It appears that the 20th century has been the period of the largest progression of the deltas during historical times. The speed of progression increased as early as the beginning of the century, as a result of the rapid growth of the rural population densities. Soil erosion from arable fields increased catchment sediment yields to promote rapid changes in the river courses. This resulted in abandonment of deltaic mouths, a phenomenon leading to a straightening of the coast. Thus to the south of the present mouth of the Seman the coast receded by 7 to 30 m per year between 1968 and 1990 as a result of the abandonment of a mouth. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

长江口岸滩近期演变及南水北调工程的可能影响   总被引：5，自引：0，他引：5  

杨世伦  赵庆英  朱骏 《上海国土资源》2001,(2):7-11

根据近几十年长江口前沿滩的地形观测资料和大通站输沙量资料，揭示了入海泥沙量变化对河口前沿岸滩冲淤的重要影响，初步估计：在年调水总量不超过450亿吨的背景下，入海泥沙的减少将＜0．2亿t/a，河口潮滩的冲淤动态不会发生实质性变，但同时指：当南水北调和流域的其它入人类活动叠加时，其影响不容忽视。  相似文献