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31.
Spring blooms of bottom ice algae are a common feature of landfast congelation ice in polar regions. Because ice algae are usually associated with a substrate, their population dynamics can be followed with considerable confidence. Although ice algal dynamics are closely related to irradiance, their dynamics and distributions are influenced by other abiotic and biotic factors. Ice algal abundance varies horizontally over all scales examined. Factors such as grazing and nutrient availability may contribute to local and geographic differences. Loss terms for most sea ice assemblages are largely unquantified. Ice algal biomass is most concentrated near the ice-water interface in spring.Environmental factors affecting ice algal abundance and productivity are considered here, emphasizing recent results from several well-studied sites. Biomass accumulation, growth rates and productivity have been documented for spring blooms of bottom interstitial and sub-ice assemblages. On an areal basis biomass accumulation in bottom ice assemblages can be comparable with planktonic systems. At low ambient temperatures and irradiances average specific growth rates (≤ 0.25 d−1) and production rates (≤ 1.0 mg C mg Chl−1 h−1) for ice algae are low. Current methods of measuring productivity are compared. Results are consistently low but variable with little systematic difference among them. At present, apparent differences in productivity between bottom ice assemblages in the Arctic and Antarctic, or among different antarctic assemblages, are so confounded by methodological and other sources of variability that no firm differences can be detected. 相似文献
32.
Martin Smith 《现代隧道技术》2004,(Z2):54-61
The paper describes some of the key issues that need to be addressed in the design and execution of underground structures to ensure their long term suitability as watertight units and outlines through practical examples of structures that have not achieved their design requirements which situations need to be considered. Waterproofing is not an exact science but a combination of theoretical knowledge and practical experience based on the five key factors: Environment, Design, Materials, Application and Use of the Structure.In conclusion the paper outlines the advances in crack control to date that can provide the necessary long term guarantees.The adoption of re - injectable crack inducers permitted the Client to save over 10% on construction costs and reduced the construction time by six months. 相似文献
33.
Katrina Smith Korfmacher 《Coastal management》2013,41(3):191-192
Ecosystem management has been deemed a promising approach to environmental management in many settings. The ecosystem approach promises improved use of scientific information, better management coordination, and more meaningful public participation in environmental decision making. Yet, there is little evidence for how well ecosystem management efforts actually achieve their goals. This article presents an in‐depth analysis of one ecosystem management effort, the Albemarle‐Pamlico Estuarine Study (APES), which was part of the National Estuary Program. The APES failed to meet the objectives of ecosystem management in several visible ways. However, this case study reveals that the APES also had many positive results that were less visible to participants and observers. The performance of the APES is explained with reference to seven paradoxes inherent in the concept of ecosystem management. These findings have implications for appropriate expectations and evaluation of other ecosystem management efforts. 相似文献
34.
面向凹凸结构曲面的喷漆机器人轨迹优化研究 总被引:1,自引:0,他引:1
针对表面有凹凸结构的复杂曲面,采用匀速喷涂方法,曲面上曲率的变化造成涂层厚度差较大.根据实验数据推导出平面上的漆膜累积速率二次函数后,采用微分几何的面积放大定理推导出自由曲面上的漆膜累积速率二次函数.为提高曲面上涂层厚度的一致性,采用轨迹优化方法对表面有凹凸结构的复杂曲面的涂层厚度差进行补偿.最后通过计算机仿真验证了数学模型和优化算法的有效性和可行性. 相似文献
35.
Jonathan Levine Moira Zellner María Arquero de Alarcón Yoram Shiftan Dean Massey 《运输规划与技术》2018,41(5):463-480
This article reports on an integrated modeling exercise, conducted on behalf of the US Federal Highway Administration, on the potential for frequent automated transit shuttles (‘community transit’), in conjunction with improvements to the walking and cycling environment, to overcome the last-mile problem of regional rail transit and thereby divert travelers away from car use. A set of interlocking investigations was undertaken, including development of urban visualizations, distribution of a home-based survey supporting a stated-preference model of mode choice, development of an agent-based model, and alignment of the mode-choice and agent-based models. The investigations were designed to produce best-case estimates of the impact of community transit and ancillary improvements in reducing car use. The models in combination suggested significant potential to divert drivers, especially in areas that were relatively transit-poor to begin with. 相似文献
36.
M.J. Smith 《Transportation Research Part B: Methodological》1981,15(6):443-451
We consider a network with interactions and capacity constraints at each junction. We give conditions on the interactions and constraints which, if satisfied at each separate junction, ensure that any feasible assignment problem has an equilibrium solution. Two illustrative examples are provided; the first arises naturally and does not satisfy our conditions, while the second does satisfy our conditions but is somewhat unnatural. 相似文献
37.
Abstract The term “mitigation”; has been used in connection with habitat losses attributable to federal water projects as early as the Rivers and Harbors Act of 1899 and the Fish and Wildlife Coordination Act of 1934. Most of the current requirements to mitigate habitat losses associated with water development projects apply to wetlands, as they are biologically productive and have been destroyed at an increasing rate during recent years by developers seeking prime land in the coastal zone. A case study of the Port of Coos Bay and the City of North Bend, Oregon, provides an actual experience of estuarine mitigation in a dredging project and an airport fill. The case was developed as part of a larger curriculum research study for seaport management. This article is primarily written as a case with a brief introduction and discussion of mitigation requirements, followed by the text of the case and ending with a series of hypotheses for further research. 相似文献
38.
Dean A. JonesJulie L. Farkas Orr BernsteinChad E. Davis Adam TurkMark A. Turnquist Linda K. NozickBrian Levine Carmen G. RawlsScott D. Ostrowski William Sawaya 《Research in Transportation Economics》2011,32(1):3-14
International containerized freight movement is a vital part of the supply chain for many companies, and a critical element of moving consumer goods to points of retail sale within the U.S. Containerized imports also present a clear security concern (e.g., terrorists attempting to ship “dirty bombs,” chemical, biological or even nuclear weapons, into the U.S. in a shipping container). The goal of the research presented here is to create a modeling tool for analyzing flows of U.S. imports and exports of containerized freight, and the potential changes in those flows under a variety of conditions (e.g., port disruptions, extensive security-related delays, etc.). Our focus is on movements through maritime container ports, and not overland movements between the U.S. and Canada or Mexico.The network model, referred to as the System for Import/Export Routing and Recovery Analysis (SIERRA), represents container movements between the U.S. and 46 other countries that account for the vast majority of U.S. imports and exports. The SIERRA model is a network equilibrium model that predicts flows between foreign countries and North American ports, the total volumes handled (import and export) by each port, the modal volumes (truck and rail) moving domestically into and out of each port, and volumes between each port and a set of transportation analysis zones within the U.S. 相似文献
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