In the Bach Dang–Cam Estuary, northern Vietnam, mechanisms governing cohesive sediment aggregation were investigated in situ
in 2008–2009. As part of the Red River delta, this estuary exhibits a marked contrast in hydrological conditions between the
monsoon and dry seasons. The impact on flocculation processes was assessed by means of surveys of water discharge, suspended
particulate matter concentration and floc size distributions (FSDs) conducted during a tidal cycle at three selected sites
along the estuary. A method was developed for calculating the relative volume concentration for the modes of various size
classes from FSDs provided by the LISST 100X (Sequoia Scientific Inc.). It was found that all FSDs comprised four modes identified
as particles/flocculi, fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic
energy, their proportions varied but without significant modification of their median diameters. In particular, when the turbulence
level corresponded to a Kolmogorov microscale of less than ∼235 μm, a major breakup of flocs resulted in the formation of
particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater
discharge and by the tidal cycle. During the wet season, strong freshwater input induced a high turbulent energy level that
tended to generate sediment transfer from the coarser size classes (macroflocs, coarse microflocs) to finer ones (particles/flocculi
and fine microflocs), and to promote a transport of sediment seawards. During the dry season, the influence of tides predominated.
The turbulent energy level was then only episodically sufficiently high to generate transfer of sediment between floc size
classes. At low turbulent energy, modifications in the proportions of floc size classes were due to differential settling.
Tidal pumping produced a net upstream transport of sediment. Associated with the settling of sediment trapped in a near-bed
layer at low turbulent energy, this causes the silting up of the waterways leading to the harbour of Haiphong. 相似文献
INAA, ICP‐AES and ICP‐MS were used to elementally characterise four environmental reference materials – NIST SRM 1646a (Estuarine Sediment), NIST SRM 1400 (Bone Ash), IAEA‐395 (Urban Dust) and IAEA‐450 (Algae). An analytical scheme consisting of the three methods was first applied to NIST SRM 1646a to validate the methodology because it has been extensively analysed and has certified values for many elements. With repeated analyses of NIST SRM 1646a, the accuracy and measurement repeatability of the data obtained were evaluated based on two statistical calculations (zeta‐score and Horwitz ratio) and were observed to be good enough for the analytical scheme to be applied to similar sorts of environmental/geochemical samples. Applying the same approach to NIST SRM 1400, IAEA‐395 and IAEA‐450, enabled mass fractions of 29, 38 and 28 elements to be determined, respectively. Among these results, the data for rare earth elements are of particular interest, not only for IAEA‐450 but also for the other three reference samples. The data for Pr, Gd, Dy, Ho, Er and Tm in NIST SRM 1646a are newly reported in this study. By using small test portions (< 100 mg) for NIST SRM 1646a and IAEA‐395, and recommended minimum amounts for NIST SRM 1400 and IAEA‐450, sample homogeneity was evaluated. 相似文献
The Mei-yu front system occurring from 23 to 27 June 1999 consists of the Mei-yu front and the dewpoint front, which confine a warm core extending from the eastern flank of the Tibetan Plateau to the west of 145°E. To further understand the synopticscale physical mechanisms associated with the Mei-yu front system, the present study proposes another insight into the physical significance of the x-component relative vorticity (XRV) whose vertical circulation is expected to tilt isentropic surfaces. The XRV equation diagnoses exhibit that the twisting effect of the planetary vorticity (TEPV) is positive along the Mei-yu front and negative in the dewpoint front region, and tilts isentropic surfaces from south to north in the Mei-yu frontal zone. Conversely, the meridional gradient of the atmospheric buoyancy (MGAB) tilts isentropic surfaces in the opposite direction and maintains negative in the regions where the TEPV is positive and vice versa. Thus, the TEPV plays the role of the Mei-yu frontogenesis, whereas the MGAB demonstrates the Meiyu frontolysis factor. Both terms control the evolution of the cross-front circulation. The other terms show much minor contributions in this case study. The present simulations also indicate that the weakening of the upper-level jet evidently induces the weakening of the Mei-yu front and reduces the amplitude of the East Asia cold trough. Furthermore, the impact can also penetrate into the lower troposphere in terms of mesoscale disturbances and precipitation, proving that the upper-level jet imposes a noticeable top-down influence on the Mei-yu front system. 相似文献
A numerical model for the computation of the wind field,air temperature and humidity in the atmospheric boundary layer (ABL) including the urbancanopy was developed for urban climate simulation. The governing equations of the modelare derived by applying ensemble and spatial averages to the Navier–Stokes equation, continuityequation and equations for heat and water vapour transfer in the air. With the spatial averagingprocedure, effects of buildings and other urban structures in the urban canopy can be accounted for byintroducing an effective volume function, defined as the ratio between the volume of air in acomputational mesh over the total volume of the mesh. The improved k - model accounts for the anisotropyof the turbulence field under density stratification. In the improved k - model, the transportof momentum and heat in the vertical direction under density stratification is evaluated based onthe assumption of a near-equilibrium shear flow where transport effects on the stresses andheat fluxes are negligible. The heating processes at surfaces of buildings and ground are alsomodelled. The comparison of the computational results obtained with the present modeland existing observational data and numerical models shows that the present model is capableof predicting the structure of turbulence in the urban canopy layer under density stratification.Numerical experiments with the new model show that the flow behaviour of the air in the urbancanopy layer is strongly affected by the existence of buildings and density stratification. 相似文献
In non-resonant cases, a constant part coming from the perturbations can be easily separated from the observed mean mean motion, which can be called the perturbed part. An another part created by resonance must be separated from the observed mean motion in the case of the first three Galilean satellites. The determination of it gives better value of the semi-major axis.In this investigation, the analytical process is chosen to avoid a mixture of orders in successive expansions and integrations.The main terms entering in the computation of the libration are the great inequalities of the first three satellites. Each of them is introduced in the development by its eight components; while in Sampson's theory, only the great inequality of Satellite 2 is given by two components.The equations of motion used in this work are derived from Sagnier's theory.Proceedings of the Conference on Analytical Methods and Ephemerides: Theory and Observations of the Moon and Planets. Facultés universitaires Notre Dame de la Paix, Namur, Belgium, 28–31 July, 1980. 相似文献
We examine the effects of natural disasters on income and investment in China. Using macroeconomic province-level data and the provincial history of disaster exposure over the past two decades, we describe the relationship between disaster mortality and morbidity, disasters’ economic damages, government investment and regional economic activity, and infrastructure development. The Chinese government’s aggressive investment in post-disaster reconstruction is discussed, and the implications of this investment for post-disaster private sector economic activity are analyzed empirically. We further investigate the differential effects of natural disasters on economic activity in China’s diverse geographical regions.