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31.
Hydraulic roughness over simple subaqueous dunes 总被引:2,自引:1,他引:1
Jesper Bartholdy Burg W. Flemming Verner B. Ernstsen Christian Winter Alex Bartholom ä 《Geo-Marine Letters》2010,30(1):63-76
Detailed studies of flow over subaqueous dunes in laboratory flumes were used to suggest a virtual near-bed layer of twice
the dune height in which the mean velocity is accelerated towards the crest by contraction. The mean flow velocity in this
layer above the crest, transformed into friction velocity by means of the surface skin roughness, is shown to give values
consistent with measured values. The resulting dimensionless shear stress due to skin friction is depth-independent, in contrast
to that derived by means of often cited traditional methods. As a result of the relationship between dune height and the thickness
of the near-bed layer, an expression for the expansion loss behind dunes was formulated and used to relate form resistance
directly to dune height. 相似文献
32.
This paper analyses the extent to which specialisation gains can be achieved by liberalising access to fishing quotas within the European Union (EU). Fishing quotas are today exchanged between EU member states at a rate of 4% of total turnover in EU fisheries. Germany, Belgium, Denmark and the Netherlands are the most active. Only one fourth of these exchanges are permanent. With the management systems in EU fisheries differing among countries, comparative advantages in fisheries exist in member states with the best management practices. Hence, although positive but small specialisation gains exist in EU fisheries today, these gains might potentially be increased by liberalising access to fishing quotas and allowing transferability of quotas between individuals from different countries on a permanent basis. Increasing the gains might, however, affect relative stability. 相似文献
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Poul Degnbol Henrik Gislason Susan Hanna Svein Jentoft Jesper Raakjær Nielsen Sten Sverdrup-Jensen Douglas Clyde Wilson 《Marine Policy》2006
Fisheries management benefits from the contribution of several academic disciplines, each with their own perspectives, concerns and solutions. In this essay we argue that the contribution of biology, economics, sociology and other relevant disciplines to fisheries would be improved if they originated from broader, more integrated analytical perspectives that are attuned to the empirical realities of fisheries management. Today, disciplinary boundaries narrow the perspectives of fisheries management, creating tunnel vision and standardized technical fixes to complex and diverse management problems. Having worked separately and together for a number of years in fisheries research and consultancy in many parts of the world we, as a group of biologists, economists and sociologists, feel that the time to rid ourselves from disciplinary dogmatism is long overdue. We claim that improvements in fisheries management will be realized not through the promotion of technical fixes but instead by embracing and responding to the complexity of the management problem. 相似文献
35.
This policy brief is a summary product of seven case studies examining the integration of local ecological knowledge in fisheries management. Each case began with a series of in-depth interviews with local fishers, after which their answers were examined using both social and biological approaches to assess the possibility of using the information as the basis of simple, valid and locally acceptable indictors for fisheries management. We found that allocation and knowledge issues are closely interlinked and must be addressed in concert, and that the negotiation of shared understandings between multiple sources of knowledge must be a continuous process within an adaptive framework rather than a question of identifying a fixed set of indicators. 相似文献
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Tracer Tests and Image Analysis of Biological Clogging in a Two-Dimensional Sandbox Experiment 总被引:1,自引:0,他引:1
A two-dimensional flow experiment on biological clogging was carried out by biostimulating a sandbox packed with sand inoculated with bacteria. Biostimulation consisted of continuously injecting nutrients (acetate and nitrate). Clogging was visualized by frequently carrying out colored tracer experiments using Brilliant Blue. The tracer experiments were recorded with a digital camera and converted to concentration maps using an image-analysis method that revealed in detail the complex spreading pattern surrounding clogged areas. Clogging resulted in a finger-like spreading of the tracer around the main clogged area. Fingers were asymmetric and their dominant direction changed over time. Although the flow field was complex around the main clogged area, the effect on the bulk hydraulic conductivity at the scale of the sandbox was very small. 相似文献
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Overview of eutrophication indicators to assess environmental status within the European Marine Strategy Framework Directive 总被引:1,自引:0,他引:1