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Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean.Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler(ADCP) to monitor bed shear stress,applying a prescribed boundary layer model,previously used for discharge estimation.The model parameters include the local roughness length and a dip correction factor to account for sidewall effects.Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data.We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP.To validate the results,co-located coupled ADCPs were used to infer bed shear stress,both from Reynolds stress profiles and from mean velocity profiles. From HADCP data collected over a period of 1.5 years,a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River,East Kalimantan,Indonesia.A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs.The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow,which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP.Bed shear stresses from the coupled ADCP are representative of a much more localized flow,while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales.Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods,and highlight the robustness of the method to uncertainty in the estimates of the roughness length. 相似文献
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华北北部古生代大陆地壳增生过程中的岩浆作用与成矿效应 总被引：13，自引：7，他引：6
包括前寒武纪克拉通与显生宙造山带两大构造单元的华北北部,古生代时期板块构造体制下古亚洲洋的裂解-扩展-消亡与汇聚大陆边缘的俯冲-碰撞-伸展循环构成了其大陆地壳增生与再造演化的基本图景,并形成了一系列记录这些因果演变连续过程的岩浆侵入与喷发事件。这些物质记录无疑是研究汇聚大陆边缘壳-幔作用和物质交换、大陆地壳演化与大规模成矿作用耦合关系的绝佳窗口。本文在系统总结最近几年积累的年代学和岩石地球化学资料的基础上,以一些带有特定过程印记的标志性岩浆事件为纲领,表征这些古生代岩浆作用形成过程中的地球动力学演变,揭示其所记录的华北北部大陆地壳的多阶段增生与再造演化机制,并初步探讨了有利地球动力学过程制约下的岩浆行为引起的成矿物质聚集效应。 相似文献
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We describe the spatial and temporal dynamics of macrozooplankton on the southern North West Shelf (NWS) of Australia over two consecutive summers (1997/1998 and 1998/1999). Sampling was conducted using submersible light traps, deployed at the surface and at depth, along one cross-shelf transect in 1997/1998, and along two cross-shelf transects and one long-shore transect in 1998/1999. Our results revealed large inter-annual changes in macrozooplankton assemblages during the two summers. An upwelling regime associated with El Niño conditions prevailed during the first summer, resulting in high chlorophyll concentrations and an abundance of both meso- and macrozooplankton. During this time, there were distinct inshore and offshore macrozooplankton assemblages that reflected an abrupt transition from upwelling-enriched shelf waters to oligotrophic oceanic waters. In contrast, the second summer was characterised by a downwelling regime associated with La Niña conditions that resulted in low chlorophyll concentrations and decreased abundances of meso- and macrozooplankton. The lack of upwelling activity weakened horizontal gradients in macrozooplankton assemblages so that differences between surface and deep assemblages were more pronounced. Catches in light traps were dominated by hyperiid amphipods during the first summer and by the neritic euphausiid Pseudeuphausia latifrons during the second. There was little within-season change in macrozooplankton assemblages during both summers, and only weak relationships were found between the environmental parameters and the distribution of individual macrozooplankton taxa. 相似文献
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T Sancelme J Goetze S Jaquemet MG Meekan A Flam AM Watts 《African Journal of Marine Science》2020,42(1):95-107
Coastal populations of maritime countries in eastern Africa rely on fish as a primary source of protein, but baseline information on the abundance of fish communities on these coastlines is often lacking. We used baited remote underwater video stations to compare the abundance and diversity of reef fishes targeted by fishing at two sites in southern Mozambique, one at Lighthouse Reef within the Bazaruto Archipelago National Park and the other to the south at San Sebastian Reef on the San Sebastian Peninsula. Fish that are known targets of fisheries (mostly small-scale and artisanal) had an abundance that was almost three-times greater at San Sebastian Reef (80.22 ind. h–1 [SE 18.00]) than at Lighthouse Reef (29.70 ind. h–1 [SE 8.91]). Similarly, there was greater mean species richness at San Sebastian Reef (38.74 species h–1 [SE 2.79]) than at Lighthouse Reef (25.37 species h–1 [SE 3.66]). The main drivers of targeted fish abundance were habitat and depth, with shallow (<15 m) and mixed reef areas having the greatest abundance and richness. More sampling was done over sand habitat at Lighthouse Reef, which likely led to the lower abundance and species richness observed at this site; however, that finding could also be attributable to the fact that protection is provided to only a section of available coral reef habitat in a small area. Nevertheless, fish community structure was comparable between the sites, with similar proportions of carnivores (78–81%), herbivores (12–14%) and omnivores (7–8%). Our findings highlight the variation in species abundance and assemblages of coral-reef fish targeted by fishing in Mozambique and emphasise the importance of localised environmental variables as a driver of these patterns. To ensure maximum protection of Lighthouse Reef fish communities, we recommend an extension of the no-take zone to include the entire reef complex. 相似文献
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