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31.
Jérôme Jourde Christine Dupuy Hien T. Nguyen David Mizrahi Nyls de Pracontal Pierrick Bocher 《Estuaries and Coasts》2017,40(4):1159-1170
In tropical South America, the mudflats of the Amazonian coast are unique because of their large size and unrivaled migration dynamics. On Guiana’s coast, macrofaunal communities are believed to be well-adapted to these dynamic conditions. In this study, the benthic macrofauna was sampled in April 2012 in the Awala-Yalimapo region of western French Guiana and at two sites in Suriname: Warappa Kreek and Bigi Pan. These sites are found 800, 920, and 1140 km from the Amazon delta, respectively. The richness, diversity, and densities of the macrofaunal communities in these mudflats are here described for the first time. Only 38 operational taxonomic units (OTUs) were recorded, among which two species were common and widely distributed: the tanaid crustacean Halmyrapseudes spaansi and the polychaete Sigambra grubii; the former represented 84% of all individuals collected, with densities reaching up to 73,000 individuals m?2. Most of the OTUs consisted of relatively small individuals (<10 mm in length). The very low richness and diversity and the small sizes of the organisms are likely linked to the instability and softness of the substrate on these mudflats. This study suggests that the differences in macrofaunal community composition among sites could be due to the migration stage of banks rather than the distance from the Amazon delta and associated effects of river discharge. 相似文献
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The paper presents observations made in CPTs performed under controlled laboratory conditions in normally consolidated clay–sand mixtures at various penetrations. These results are combined with data from field tests to develop a relationship for CPT data in young normally consolidated soils that combines end resistance and friction sleeve data. This relationship can be used to assist assessment of soil type and CPT parameters for un-aged normally consolidated soils as well as allowing judgements to be made in relation to likely levels of under-consolidation, structure and over-consolidation in any given soil deposit.
相似文献34.
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AbstractA nonlinear mathematical model for estimating the water content dependent undrained shear strength of clayey soils was developed. Three types of clay mixtures (kaolinite, bentonite, and kaolinite-bentonite) were considered. The shear strength of the given soil samples was determined via torvane tests. Experimental results were compared with three numerical results: (i) the analytical function fit, (ii) modeling without the water content effect, and (iii) modeling with the water content effect using the Mohr-Coulomb (M-C) model. There was good agreement among the experimental, analytical, and numerical results with and without the water content effect in the fully softening zone. However, there was a large difference between the numerical results obtained from the developed model with and without the water content effect in the flow zone with a high liquidity index, because the shear strength may decrease significantly to low value in the case of an abrupt increase of the water content. The greatest advantage of the developed model is that it can simulate the reduction of the shear strength and shear band development under the high water content condition, which may trigger a large mobile mass movement. 相似文献
36.
Ping Yin Van Phach Phung Dinh Lan Tran Huy Cuong Do Van Vuong Bui Xiaoyong Duan 《中国海洋大学学报(英文版)》2018,17(6):1269-1271
Both China and Vietnam confront the challenges of natural geohazards and environmental changes in their deltas and coastal zones due to rapid urbanization, economic development, and the impacts of global climate change. China and Vietnam initiated a comparative study of the Holocene sedimentary evolution of the Yangtze River Delta(YRD) and Red River Delta(RRD) for the period 2015–2018 in order to improve the understanding of the two delta evolution histories in the Holocene. Previous investigative data of the two rivers, onshore delta plains, and offshore subaqueous deltas have been explored and reinterpreted. New data gleaned from boreholes, piston cores, shallow seismic and hydrodynamic sources have been collected from the offshore YRD and the East China Sea inner shelf, and surface sediments and short cores have been collected from the RRD near-shore areas. Six focal areas of the joint project have been defined for comparative studies of the two deltas, including morphological development, sequential stratigraphy, coastline shifting, sedimentary characteristics, sedimentary dynamics, and correlation with anthropogenic global climate change. The results of these study areas are presented herein. The joint project also includes cooperative capacity building; exchanges of young scientists have been organized during the project period, and hands-on training in laboratory geochemical analysis, numerical modeling, and seismic data processing and interpretation have been provided by China and its Vietnamese geoscientist partners. Joint field excursions were organized to the upstream of the Yangtze and Red Rivers in Yunan Province, China, all the way downstream along the Vietnamese portion of the Red River. These joint studies have, over the past three years, improved understanding of the evolutionary history of these two major rivers and their mechanisms of source to sink. Joint project results of these two major deltas are not limited to the geosciences; the cooperative mechanical and operational experiences have been helpful for future cooperation in the field of marine geoscience between China and Vietnam, as well for cooperative activities with other ASEAN member countries. 相似文献
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The relationships between meteorological conditions (temperature, wind-speed and direction, relative humidity, surface-inversion depth and strength, and stability) and PM2.5 concentrations in Fairbanks, Alaska were investigated using ten years of observational data. The results show that during wintertime (November through February) PM2.5 concentrations exceeding the 24 h National Air Quality Standard (35 μg/m3) occurred under calm wind, extremely low temperature (≤20 °C) and moisture (water-vapor pressure < 2 hPa) multiday surface-inversion conditions that trap the pollutants in the breathing level and inhibit transport of polluted air out of Fairbanks. PM2.5 concentrations tend to be higher under stable than other conditions, but are not sensitive to the degree of stability. The presence of a surface inversion and calm wind are necessary, but in combination with low temperatures and humidity, the conditions are sufficient for high PM2.5 concentrations. The low temperatures are required because they lead to increased emission rates from domestic heating and power production. During multiday inversions with temperatures above ? 20 °C, high relative humidity (> 75%) partly caused by water-vapor emission reduces PM2.5 concentrations. 相似文献
38.
Chuan-Chou Shen Kuei-Shu Li Danny Natawidjaja Xianfeng Wang Doan Dinh Lam Tung-Yung Fan Fred W. Taylor Hong-Wei Chiang 《Geochimica et cosmochimica acta》2008,72(17):4201-4223
One hundred eighty U-Th data, including 23 isochrons on 24 pristine modern and Holocene corals and 33 seawater samples, were analyzed using sector-field mass spectrometry to understand the variability of initial 230Th/232Th (230Th/232Th0). This dataset allows us to further assess the accuracy and precision of coral 230Th dating method. By applying quality control, including careful sampling and subsampling protocols and the use of contamination-free storage and workbench spaces, the resulting low procedural blanks give an equivalent uncertainty in age of only ±0.2-0.3 yr for 1-2 g of coral sample. Using site-specific 230Th/232Th0 values or isochron techniques, our study demonstrates that corals with an age less than 100 yrs can be 230Th-dated with precisions of ±1 yr. Six living subtidal coral samples were collected from two continental shelf sites, Nanwan off southern Taiwan in the western Pacific and Son Tra off central Vietnam in the South China Sea; one coral core was drilled from an open-ocean site, Santo Island, Vanuatu, in the western tropical Pacific; and modern and fossil intertidal coral slabs, 17 in total, were cut from six sites around the islands of Simeulue, Lago, North Pagai and South Pagai of Sumatra in the eastern Indian Ocean. The results indicate that the main source of thorium is the dissolved phase of seawater, with variation of 230Th/232Th0 depending on local hydrology. With intense input of terrestrial material, low 230Th/232Th0 atomic ratios of 4.9 × 10−6 and 3.2 × 10−6 with a 10% variation are observed in Nanwan and Son Tra, respectively. At the Santo site, we find a value of 5.6 × 10−6 at 4 horizons and one high value of 24 × 10−6 in a sample from AD 1974.6 ± 0.5, likely due to the upwelling of cold water during a La Niña event between AD 1973 and 1976. The natural dynamics of 230Th/232Th0 recorded in the intertidal corals at sites in the Sumatran islands are complicated so that this value varies significantly from 3.0 to 9.4 × 10−6. Three of the 141 modern coral 230Th ages differ from their true ages by −23 to +4, indicating the presence of detrital material with anomalous 230Th/232Th values. Duplicate measurement of coeval subsamples is therefore recommended to verify the age accuracy. This improved high precision coral 230Th dating method raises the prospects of refining the age models for band-counted and tracer-tuned chronologies and of advancing coral paleoclimate research. 相似文献
39.
Khang Dang Kyoji Sassa Hiroshi Fukuoka Naoki Sakai Yuji Sato Kaoru Takara Lam Huu Quang Doan Huy Loi Pham Van Tien Nguyen Duc Ha 《Landslides》2016,13(6):1525-1534
Around hundred landslides were triggered by the Kumamoto earthquakes in April 2016, causing fatalities and serious damage to properties in Minamiaso village, Kumamoto Prefecture, Japan. The landslides included many rapid and long-runout landslides which were responsible for much of the damage. To understand the mechanism of these earthquake-triggered landslides, we carried out field investigations with an unmanned aerial vehicle to obtain DSM and took samples from two major landslides (Takanodai landslide and Aso-ohashi landslide) to measure parameters of the initiation and the motion of landslides. A series of ring-shear tests and computer simulations were conducted using a measured Kumamoto earthquake acceleration record from KNet station KMM005, 10 km west of Aso-ohashi landslide. The research results supported our assumed mechanism of sliding-surface liquefaction for the rapid and long-runout motion of these landslides. 相似文献
40.