Mesoscale distribution of fish larvae in relation to an upwelling filament off Northwest Africa     

《Deep Sea Research Part I: Oceanographic Research Papers》1999,46(11):1969-1984

Fish eggs and larvae were studied in relation to an upwelling filament off Northwest Africa, near the Canary Islands, during August 1993. The filament was entrained around a quasi-permanent cyclonic eddy of diameter 100 km. The sampled ichthyoplanktonic community was dominated by neritic larvae, 94.2% of which were captured in the oceanic area. Horizontal distributions of neritic and oceanic larvae were strongly influenced by the oceanographic features characteristic of the coastal margin. Neritic larvae were associated with upwelling and filament structures, while oceanic larvae were strongly excluded from these features. The results suggest that coastal upwelling and filaments are mechanisms of transport for neritic ichthyoplankton into oceanic waters, that sardine larvae are good tracers of offshore movements of upwelled waters, and that the cyclonic eddy functions as a larval nursery ground for neritic fish species.  相似文献   

Designing a protected area network for conservation planning in <Emphasis Type="Italic">Jhum</Emphasis> landscapes of Garo Hills,Meghalaya     

A. Kumar  B. G. Marcot  G. Talukdar 《Journal of the Indian Society of Remote Sensing》2010,38(3):501-512

We studied vegetation and land cover characteristics within the existing array of protected areas (PAs) in South Garo Hills of Meghalaya, northeast India and introduce the concept of protected area network (PAN) and methods to determine linkages of forests among existing PAs. We describe and analyse potential elements of a PAN, including PAs, reserved forests, surrounding buffers as zones of influence, and connecting forest corridors, which collectively can provide old-forest habitat for wildlife species linked across a landscape dominated by jhum (shifting cultivation) agriculture. ANOVA and Chisquare analyses of patch characteristics and forest tree diversity suggested the presence of equally species-rich and diverse old forest cover (tropical evergreen, semi-evergreen and deciduous forest types) in portions of unprotected private and community owned land, which could be designated as additions to, and network linkages among, existing PAs. Such additions and linkages would help provide for conservation of elephants and existing native forest biodiversity and would constitute a PAN in the region. Most (80%) of the total forest cover of the region belongs to private or community owned land. Therefore, such additions could be formally recognized under the aegis of the 2003 amendments of the Wildlife (Protection) Act 1972, which include provisions to designate selected forest patches within private lands as Community Reserves.  相似文献   

Distinguishing between debris flows and hyperconcentrated flows: an example from the eastern Swiss Alps          下载免费PDF全文

Nancy C. Calhoun  John J. Clague 《地球表面变化过程与地形》2018,43(6):1280-1294

Much research has been done on water‐rich mass flows, but the distinction between hyperconcentrated flows and debris flows, and whether the two are indeed different processes, continue to be debated. Here, we contribute to the ongoing discussion of these phenomena by describing and interpreting the deposit of a large landslide‐induced mass flow in the eastern Swiss Alps. About 9400 years ago, 10‐12 km³ of limestone detached from the wall of the Vorderrhein River valley and rapidly fragmented while sliding towards the valley bottom. The rock mass struck the valley floor with enormous force and liquefied at least 1 km³ of valley‐fill sediments. A slurry of liquefied sediment – the ‘Bonaduz gravel’ – traveled tens of kilometres down the Vorderrhein valley from the impact site, carrying huge fragments of rockslide debris that became stranded on the valley floor, forming hills termed ‘tomas’. Part of the flow was deflected by a cross‐valley barrier and traveled 14 km up a tributary of the Vorderrhein valley. The Bonaduz gravel is >65 m thick and fines upward from massive sandy cobble gravel at its base to silty sand at its top. Sedimentologic and geomorphic evidence indicates that Bonaduz gravel was transported as a hyperconcentated flow, likely above a basal carpet of coarse diamictic sediment that behaved as a debris flow. The large amount of water involved in the flow indicates that at least part of the Flims rockslide entered a lake. The Bonaduz deposit shares many properties with sediments left by hyperconcentrated flows generated in flumes, including normal grading and elutriation pipes produced by the rapid escape of fluids when the flow comes to rest. These properties are characteristic of non‐Newtonian laminar flows with high sediment concentrations. Our study reinforces laboratory and theoretical studies showing that debris flows and hyperconcentrated flows are different processes. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

Large wood inhibits debris flow runout in forested southeast Alaska     

Adam M. Booth  Christian Sifford  Bryce Vascik  Cora Siebert  Brian Buma 《地球表面变化过程与地形》2020,45(7):1555-1568

Due to their potentially long runout, debris flows are a major hazard and an important geomorphic process in mountainous environments. Understanding runout is therefore essential to minimize risk in the near-term and interpret the pace and pattern of debris flow erosion and deposition over geomorphic timescales. Many debris flows occur in forested landscapes where they mobilize large volumes of large woody debris (LWD) in addition to sediment, but few studies have quantitatively documented the effects of LWD on runout. Here, we analyze recent and historic debris flows in southeast Alaska, a mountainous, forested system with minimal human alteration. Sixteen debris flows near Sitka triggered on August 18, 2015 or more recently had volumes of 80 to 25 000 m³ and limited mobility compared to a global compilation of similarly-sized debris flows. Their deposits inundated 31% of the planimetric area, and their runout lengths were 48% of that predicted by the global dataset. Depositional slopes were 6°–26°, and mobility index, defined as the ratio of horizontal runout to vertical elevation change, ranged from 1.2 to 3, further indicating low mobility. In the broader southeast Alaskan region consisting of Chichagof and Baranof Islands, remote sensing-based analysis of 1061 historic debris flows showed that mobility index decreased from 2.3–2.5 to 1.4–1.8 as average forest age increased from 0 to 416 years. We therefore interpret that the presence of LWD within a debris flow and standing trees, stumps, and logs in the deposition zone inhibit runout, primarily through granular phenomena such as jamming due to force chains. Calibration of debris flow runout models should therefore incorporate the ecologic as well as geologic setting, and feedbacks between debris flows and vegetation likely control the transport of sediment and organic material through steep, forested catchments over geomorphic time. © 2020 John Wiley & Sons, Ltd.  相似文献