Biologically induced differences in erodibility and aggregation of subtidal and intertidal sediments: a possible cause for seasonal changes in sediment deposition

This study was carried out to describe the difference in erodibility and aggregation in a tidal basin including both subtidal and intertidal study sites and to use these results to explain the shifting erosion/deposition cycles at the sites. Erosion thresholds, erosion rates and settling velocities of the eroded material were measured at a mudflat transect and at sediment cores taken from a nearby tidal channel during surveys made in May 2000 and March 2002. Surface samples were analysed for grain-size, chl. a content, faecal pellet content, dry bulk density and organic content. Additionally, surface samples were taken at eight occasions in the period January 2002 to May 2003 from shallow tidal channels in the area. These samples were analysed for mud content and showed that major shifts in sediment distribution occurred in the period. The erodibility of the mudflat was generally high due to pelletization by the mudsnail Hydrobia ulvae but close to the salt marsh much lower erodibility was found, probably due to stabilisation by microphytobenthos. In contrast, the erodibility of the channel bed seemed to be very little influenced by biological activity and the relatively low erodibility found here was caused by physical characteristics of the sediment. The sediment eroded from the mudflat was generally strongly pelletized and showed high settling velocities whereas less aggregation and lower settling velocities were found for the channel bed sediments. Temporal variations of the mudflat stability and hydrodynamics resulted in temporal variations of deposition and erosion and the changing stability at the mudflat is likely to be one of the main reasons for a general transport of fine-grained sediment from the mudflat to the channel in the cold seasons and vice versa during the rest of the year.     

Wakes from Large High-Speed Ferries in Confined Coastal Waters: Management Approaches with Examples from New Zealand and Denmark

Large high-speed craft carrying passengers and vehicles produce wake waves that are different from both conventional vessels and smaller fast vessels. Wakes from these high-speed craft can cause environmental problems (such as beach change, ecological disturbance, and damage to structures and archaeological sites) and safety problems (for navigation and for users of the beach and nearshore) in confined waters. As a consequence of the higher speed, the vessel wakes also have a longer period than wakes caused by conventional ships and may lead to substantial wave action in shallow water environments. In both New Zealand and Denmark, issues relating to high-speed craft wakes were not addressed until after the vessels had begun operation, and complex coastal management issues with possibly broader application have had to be addressed. Emerging management strategies have involved regulation using speed and wave height criteria.     

丹麦城市桥梁建设一瞥

该文系统地介绍了前几年笔者考察北欧国家——丹麦期间,对哥本哈根、马尔默、斯卡恩等城市桥梁建设的总体印象和分类考察的情况、相关启示和建议,为国内同行们提供有借鉴意义的资料和启示,以期促进我国城市桥梁建设的健康发展。     

自行车交通系统政策和规划及实施探讨——以丹麦欧登塞市为例

分析、研究了丹麦欧登塞市发展自行车的政策和规划,包括提高自行车出行比例、安全和质量目标,自行车道路规划、环境控制区、自行车设施和技术措施组成,研究认为,欧登塞市的自行车政策和规划是多项政策与规划的协调集成.政策与规划的制定与实施采用动态循环的政策质量监督管理程序,用行动计划推动实施,通过评估给出实施效果.文章认为,长期战略,综合、集成的政策和规划,完善的政策制度、集成与逐步创新发展自行车政策和规划将给予中国城市最直接的启示.     

Formation of Denmark Strait overflow water and its hydro-chemical composition

The dense overflow across the Denmark Strait is investigated with hydrographic and hydro-chemical data and the water mass composition of the Denmark Strait Overflow Water (DSOW) is determined by multivariate analysis. Hydrographical properties, the transient tracers CFC-11 and CFC-12, oxygen and nutrients are utilized for the water mass definitions. Distribution and characteristics of water masses north of Denmark Strait are described, the important water masses at the sill and the variability on weekly time-scales are discussed, and the entrainment and mixing of water into the overflow plume in the northern Irminger Basin is calculated. The analysis indicates that water masses both from the Nordic Seas and the Arctic Ocean are important for the formation of DSOW. It is found that water masses transported with the East Greenland Current make up about 75% of the overflow at the sill. The overflow at, and shortly south of, the sill is inhomogeneous with a low-salinity component dominated by Polar Intermediate Water. The high-salinity component of the overflow is mainly of Arctic origin. The water mass composition, and the short-term variability for 7 repeats of sections close to the sill are described, and these illustrate that the overflow is in fact a composite of a number of water masses with different formation and transport histories. This indicate that the overflow is a robust feature, but that it responds to variations in the circulation or atmospheric forcing that influences the formation of intermediate and deep water masses within the Arctic Mediterranean and the North Atlantic. At a section about 400 km south of the sill the overflow is well mixed and modified by entrainment of, mainly, Iceland–Scotland Overflow Water and Labrador Sea Water, together constituting 30% of the overflow plume. The entrainment of Middle Irminger Water dominates shortly downstream of the sill, before the overflow plume reaches too deep but the entrainment seems to be intermittent in time.