River channel adjustments downstream from channelization works in England and Wales   总被引：1，自引：0，他引：1  

Andrew Brookes 《地球表面变化过程与地形》1987,12(4):337-351

Relatively little attention has been given to river channel adjustments that occur downstream from channelization works. This study is concerned with the nature of channel adjustments downstream from a total of 46 channelization works located in low and high energy environments in England and Wales. Channel changes are identified principally by the method of field survey and by reconstructing the original positions of eroded beds and banks. Use is also made of maps, aerial photographs, and engineering drawings of different dates and the technique of space-for-time substitution is applied. Enlargement of channel cross-sections through erosion had occurred downstream from a variety of types, sizes, and dates of channelization works. The maximum increase of channel size was 153 per cent. Out of a total of 14 sites with enlarged channel cross-sections, seven had undergone a change of width only, at a further three width increased rather than depth, and at the remaining four sites depth increases were dominant. These sites all have relatively high stream powers. Factors causing spatial variation of erosion included tree roots locally binding bank sediments and the occurrence of bends. Planform change had taken place at only one site. A further three high stream power sites had downstream reaches incised into bedrock and therefore did not exhibit adjustment. Channel enlargement is explained in terms of increased flood flows downstream from channelization works causing higher stream velocities, which in turn cause erosion, thereby increasing channel width and/or depth. Examination of flow records for 35 stations revealed flood events which would formerly have spread overbank but are now confined by the channelization works and are therefore likely to alter downstream flows. At sites with downstream change it is proposed that the energy of increased flows was sufficient to exceed a threshold required for erosion of perimeter sediments. By contrast the absence of change at a majority of sites in low energy lowland areas could be a reflection of both the incompetence of increased flows to erode and resistance provided by perimeter sediments. Sites with erosion features appear not to have yet attained new equilibrium conditions.  相似文献   

From pastoralism to tourism: The historical impact of changing land use practices in Namaqualand   总被引：1，自引：2，他引：1  

M.T. Hoffman  R.F. Rohde   《Journal of Arid Environments》2007,70(4):641

We use the concept of ecological revolutions to explain the environmental history of Namaqualand, from the advent of pastoralism 2000 years BP, to colonial settlement in the 18th century and finally to the recent trend of de-agrarianization from the middle of the 20th century. Early traveller's records and census data are used to assess changes in the human population of the region and how this affected wildlife and agricultural practices. Pre-colonial indigenous hunter-gatherer (Bushmen) and pastoralist (Khoekhoen) populations in Namaqualand consisted of probably no more than a few thousand individuals. Over the next three centuries, the general population rose steadily to more than 65,000 people but has fallen in recent years. Wildlife appears not to have been abundant in Namaqualand's pre-colonial landscapes and large springbok ‘treks’ were probably a rare event. The number of domestic livestock in Namaqualand peaked in 1957 largely as a result of an increase in the number of sheep which have fallen steadily since this time. Crop production was absent from Namaqualand's pre-colonial landscapes but increased to cover nearly 30,000 ha in the early 1970s. The area under cultivation has declined by nearly two thirds since this time largely as a result of the large-scale abandonment of wheat farming in marginal environments. We touch on differences between the communal areas and private farms, particularly in terms of their human populations and agricultural impact on the land. Repeat landscape photographs support our main findings which suggest that both rocky, upland habitats and rivers have not been transformed substantially by land use practices in Namaqualand. Instead, sandy pediments have borne the brunt of human impacts in the region. Finally, we highlight the beginning of a new ecological revolution in Namaqualand due to changes in the global and national political economy.  相似文献   

Hillslope gullying in the Solway Firth — Morecambe Bay region, Great Britain: Responses to human impact and/or climatic deterioration?     

R.C. Chiverrell  A.M. Harvey  G.C. Foster 《Geomorphology》2007,84(3-4):317

In the Solway Firth — Morecambe Bay region of Great Britain there is evidence for heightened hillslope instability during the late Holocene (after 3000 cal. BP). Little or no hillslope geomorphic activity has been identified occurring during the early Holocene, but there is abundant evidence for late Holocene hillslope erosion (gullying) and associated alluvial fan and valley floor deposition. Interpretation of the regional radiocarbon chronology available from organic matter buried beneath alluvial fan units suggests much of this geomorphic activity can be attributed to four phases of more extensive gullying identified after 2500–2200, 1300–1000, 1000–800 and 500 cal. BP. Both climate and human impact models can be evoked to explain the crossing of geomorphic thresholds: and palaeoecological data on climatic change (bog surface wetness) and human impact (pollen), together with archaeological and documentary evidence of landscape history, provide a context for addressing the causes of late Holocene geomorphic instability. High magnitude storm events are the primary agent responsible for gully incision, but neither such events nor cooler/wetter climatic episodes appear to have produced gully systems in the region before 3000 cal. BP. Increased gullying after 2500–2200 cal. BP coincides with population expansion during Iron Age and Romano-British times. The widespread and extensive gullying after 1300–1000 cal. BP and after 1000–800 cal. BP coincides with periods of population expansion and a growing rural economy identified during Norse times, 9–10th centuries AD, and during the Medieval Period, 12–13th centuries AD. These periods were separated by a downturn associated with the ‘harrying of the north’ AD 1069 to 1070. The gullying episode after 500 cal. BP also coincides with increased anthropogenic pressure on the uplands, with population growth and agricultural expansion after AD 1500 following 150 years of malaise caused by livestock and human (the Black Death) plagues, poor harvests and conflicts on the Scottish/English border. The increased susceptibility to erosion of gullies is a response to increased anthropogenic pressure on upland hillslopes during the late Holocene, and the role of this pressure appears crucial in priming hillslopes before subsequent major storm events. In particular, the cycles of expansion and contraction in both population and agriculture appear to have affected the susceptibility of the upland landscape to erosion, and the hillslope gullying record in the region, therefore, contributes to understanding of the timing and spatial pattern of human exploitation of the upland landscape.  相似文献   

广东大亚湾石珊瑚群落的分布及动态变化   总被引：2，自引：0，他引：2  

陈天然  余克服  施祺  李淑  王嵘  赵美霞 《热带地理》2007,27(6):493-498

2006年6月对位于南海北部亚热带的大亚湾和大鹏湾石珊瑚群落的分布、种类和覆盖率进行了调查,结果显示大亚湾共有石珊瑚24种,以秘密角蜂巢珊瑚(Favites abdita)为优势种,总覆盖率为20%,大鹏湾有石珊瑚21种,以精巧扁脑珊瑚(Platygyra daedalea)为优势种,总覆盖率为7.9%。大亚湾石珊瑚群落健康状况良好,但生长稀疏,群落密度低,空间结构简单,生长带窄;与1964年、1983年、1984年和1991年的调查结果相比,覆盖率从76.6%下降到20%,优势种从枝状的霜鹿角珊瑚(Acropora Pruinosa)转变成块状或皮壳状的秘密角蜂巢珊瑚,群落生态退化严重,主要原因可能是人为破坏活动对珊瑚本身以及生长环境造成的压力所致,包括过度捕捞、盗采珊瑚、沿岸施工和水体污染等,建议采取相应的保护措施。  相似文献   

Holocene development and anthropogenic disturbance of a shallow lake system in Central Ireland recorded by diatoms     

K. A. Selby  A. G. Brown 《Journal of Paleolimnology》2007,38(3):419-440

Three cores from two connected lakes in Central Ireland (Lough Kinale and Derragh Lough) were investigated using diatom analysis to establish the Holocene development of the lacustrine system, any local variations within the lakes and any anthropogenic influences. The study area was situated in a lowland location and the lakes were shallow, unstratified and interconnected. Litho-and bio-stratigraphical analyses of the lake cores and deposits beneath a mire separating the two lakes showed the changing spatial configuration of the lake system in the early Holocene and the separation of the initial lake into three basins (cf. lacustrine cells) and finally into two interlinked lakes. The evolution of the lake system is conceptualised as the development of distinct lacustrine cells, and its sediments have recorded changes in the physical (geography, depth and sedimentation) and chemical (water chemistry) properties of the lakes inferred through diatom analyses. The longest sequence, from the early Holocene, records fluctuating lake levels and these are correlated with geomorphological mapping and surveying of palaeoshorelines. The diatom assemblages of the upper 2 m of the three cores, covering approximately the last 2000–3000 radiocarbon years show considerable difference in trophic status and life-form categories. This is related to the location of the cores in the lake and also the distance from human settlement with particular reference to proximity to crannog (artificial island) construction and use. The most central core from the deepest part of Lough Kinale has the least representation of the human settlement and agricultural activity in the catchment and on the fringes of the lake, whereas the core taken from the edge of a crannog is able to identify when construction and use of the crannog occurred. The local nature of the palaeoecological response to human activity due to incomplete water mixing has the advantage of allowing the lake sediment cores to be used to determine spatially discrete settlement patterns.  相似文献   

Paleolimnological assessment of human impacts on an urban South African lake     

F. García-Rodríguez  C. R. Anderson  J. B. Adams 《Journal of Paleolimnology》2007,38(3):297-308

North End Lake is a polluted and eutrophic freshwater system located in Port Elizabeth, South Africa. Since the lake is expected to be used for recreational/tourist purposes by 2010, a rehabilitation program will have to be designed. For this reason, we retrieved a sediment core from the central region of the lake to decipher the effect of historical human impacts on the water body. Pre-disturbance paleolimnological inferences indicate that the lake was likely mesotrophic. After ∼1831, when sheep farming activities were undertaken in the catchment, increases in trophic state and changes in sediment composition were observed. After ∼1937, increases in trace metal levels, organic matter, spheroidal carbonaceous particles (SCP) and changes in sediment composition were recorded. The system became eutrophic as indicated by the dominance of the diatom Actinocyclus normanii, a cosmopolitan species often observed in systems where water quality has been dramatically degraded. The conditions worsened after 1986 because of the construction of a storm-water retention system, which intentionally channeled storm-water runoff into the lake. Because of this, extremely high values of fecal coliforms (i.e. 2 × 10⁶ every 100 ml) have been measured in the water column. The paleolimnological information identified the sharp increase in organic content in the uppermost section of the core, and this could be correlated to the operation of the storm-water retention system. Therefore, as an immediate management measure, we suggest that the storm-water retention system should either no longer be utilized, or the storm-water runoff should be treated before disposal into the lake. In addition, an effective sewage system has to be constructed.  相似文献   

人类起源研究回顾与中国古人类学展望   总被引：4，自引：1，他引：4  

吴新智 《地球科学进展》2001,16(5):629-633

根据对人类起源研究的回顾和对我国人类进化概况的认识，将我国古人类学研究推向更高的境界的首要任务是找到新的比较完整的化石和有关材料，关键之举是高水平的创新研究，发现和综合考量蕴藏在其中的各种信息，因此在新的世纪，必须大力进行野外调查，发现更多人类化石和旧石器以及有关的材料，进行深入的形态学和考古学研究，还要寻求人类古生物学与分子生物学在人类起源与进化研究方面的协调，构建更加坚实的我国古人类年龄框架，适当地进行我国古环境与人类关系的研究。  相似文献   

泥河湾层的研究现状和展望   总被引：11，自引：0，他引：11       下载免费PDF全文

夏正楷 《第四纪研究》2001,21(3):262-269

泥河湾层是我国北方著名的第四纪河湖相地层,它以巨厚的沉积、丰富的哺乳动物化石和人类遗存为世人所瞩目.经过几代人近80年的努力,泥河湾层在地层学、古生物学、沉积学、古地理学和古人类学等方面的研究上都取得了丰硕的成果,被誉为我国研究程度最高的第四纪地层之一.近年来,随着第四纪学科的发展和全球变化研究的深入,加大泥河湾层研究力度,提高"泥河湾层”研究水平的呼声日趋高涨.预计在新的世纪,泥河湾层研究将成为我国第四纪工作的热点,它将对第四纪科学和全球变化研究做出重大的贡献.  相似文献   

城市边缘区景观变化与人为影响的空间分异研究   总被引：38，自引：5，他引：38  

陈浮  葛小平  陈刚  彭补拙 《地理科学》2001,21(3):210-216

利用1988、1993、1998年的遥感影像，应用最大似然法和概率松弛法对马山景观组分进行了测量，在此基础上，对分类结果进行比较求得研究区景观变化的转移短也阵，分析了人为改造活动与景观组分之间的对应关系和空间变化过程，马山区已经从1988年的传统农业景观转化为1998年的城郊混合景观，人口、经济、产业结构均发生了巨大变化，借助人为影响指数，利用网格采样和地统计学分析，研究了马山1988-1998年间景观的人为开发利用活动，揭示人为活动的空间变异强度分布及景观转变过程的社会驱动力。  相似文献   

长江三峡大宁河流域3000年来的环境演变与人类活动   总被引：7，自引：6，他引：7  

张芸  朱诚  于世永 《地理科学》2001,21(3):267-271

长江三峡大宁河流域张家湾东周-汉代古遗址的孢粉学和沉积学等分析结果表明3000年来该区环境演变与人类活动的互动影响，连续的孢粉和粒度记录证实，该区古人类活动初期处于暖湿的气候，东周-汉代时期转变为温凉略干的温带气候，东周-汉代后，该区经历了一次规模较大的大宁河洪水泛滥期，导致汉代文化层中断。此后不久，由于人类活动剧烈，自然植被严重破坏，水土流失比较严重，山洪灾害频繁，导致汉代之后该区没有连续的文化层堆积，而由于人类活动剧烈，自然植被严重破坏，水土流失比较严重，山洪灾害频繁，导致汉代之后该区没有连续的文化层堆积，而由于短暂洪水暴发引起大面积坡面片流，导致坡积物的大量堆积，汉代之后该区频繁的兴涝灾害和山洪灾害体现了人地关系的复杂性。  相似文献