The leeway of shipping containers at different immersion levels     

Øyvind Breivik  Arthur A. Allen  Christophe Maisondieu  Jens-Christian Roth  Bertrand Forest 《Ocean Dynamics》2012,62(5):741-752

The leeway of 20-ft containers in typical distress conditions is established through field experiments in a Norwegian fjord and in open-ocean conditions off the coast of France with a wind speed ranging from calm to 14 m s⁻¹. The experimental setup is described in detail, and certain recommendations were given for experiments on objects of this size. The results are compared with the leeway of a scaled-down container before the full set of measured leeway characteristics are compared with a semianalytical model of immersed containers. Our results are broadly consistent with the semianalytical model, but the model is found to be sensitive to choice of drag coefficient and makes no estimate of the crosswind leeway of containers. We extend the results from the semianalytical immersion model by extrapolating the observed leeway divergence and estimates of the experimental uncertainty to various realistic immersion levels. The sensitivity of these leeway estimates at different immersion levels are tested using a stochastic trajectory model. Search areas are found to be sensitive to the exact immersion levels, the choice of drag coefficient, and somewhat less sensitive to the inclusion of leeway divergence. We further compare the search areas, thus, found with a range of trajectories estimated using the semianalytical model with only perturbations to the immersion level. We find that the search areas calculated without estimates of crosswind leeway and its uncertainty will grossly underestimate the rate of expansion of the search areas. We recommend that stochastic trajectory models of container drift should account for these uncertainties by generating search areas for different immersion levels and with the uncertainties in crosswind and downwind leeway reported from our field experiments.  相似文献   

Human-driven topographic effects on the distribution of forest in a flat,lowland agricul-tural region简   总被引：1，自引：0，他引：1  

Mette V. ODGAARD  Peder K. B CHER  Tommy DALGAARD  Jesper E. MOESLUND  Jens-Christian SVENNING 《地理学报(英文版)》2014,24(1):76-92

Complex topography buffers forests against deforestation in mountainous regions. However, it is unknown if terrain also shapes forest distribution in lowlands where human impacts are likely to be less constrained by terrain. In such regions, if important at all, to- pographic effects will depend on cultural-historical factors and thus be human-driven （an- thropogenic） rather than natural, except in regions where the general climate or extreme soils limit the occurrence of forests. We used spatial regression modeling to assess the extent to which topographic factors explain forest distribution （presence-absence at a 48x48 m resolu- tion） in a lowland agricultural region （Denmark, 43,075 km2） at regional and landscape scales （whole study area and 10x10 km grid cells, respectively）, how landscape-scale for- est-topography relationships vary geographically, and which potential drivers （topographic heterogeneity, forest cover, clay content, coastal/inland location） determine this geographic heterogeneity. Given a moist temperate climate and non-extreme soils all landscapes in Denmark would naturally be largely forest covered, and any topographic relationships will be totally or primarily human-driven. At regional scale, topographic predictors explained only 5% of the distribution of forest. In contrast, the explanatory power of topography varied from 0%-61% at landscape scale, with clear geographic patterning. Explanatory power of topog- raphy at landscape scale was moderately dependent on the potential drivers, with topog- raphic control being strongest in areas with high topographic heterogeneity and little forest cover. However, these conditioning effects were themselves geographically variable. Our findings show that topography by shaping human land-use can affect forest distribution even in flat, lowland regions, but especially via localized, geographically variable effects.  相似文献