Erodibility study of sediment in a fast-flowing river

Determination of sediment stability in the field is challenging because bed shear stress (BSS), a determining factor of sediment erosion, can’t easily be directly measured. To tackle this challenge and reliably assess sediment erodibility in a fast flowing river, a standalone underwater camera system and a new insitu flume (ISF) were developed and applied in this study. The camera system was used to record sediment movement and the new ISF was used for measuring critical bottom shear stress (CBSS). The camera can be deployed alone in water to record videos or take pictures with light emitting diode (LED) lighting and flexible schedule settings. The ISF is based on the concept that the amount of force needed to erode the same particle under different flow conditions should be similar. Two high resolution Acoustic Doppler Current Profilers (ADCP) also were deployed in the field to collect velocity-depth profiles which are used by conventional methods to calculate BSS with the law of the wall. The sediment erodibility was then assessed based on the comparison between the obtained CBSS and BSS and then further verified with the recorded observations from the deployed camera. The results reveal that the widely used conventional method can produce large uncertainties and is not adequate to provide meaningful conclusion under these conditions.     

Effect of self-weight consolidation on a hydro-sedimentological model for the Río de la Plata estuary

The effect of the consolidation process on the morphodynamics and fine sediment dynamics of the Río de la Plata estuary is explored through a circulation-wave-sediment transport model. The consolidation model is calibrated based on settling column experimental data. Different simulations are done in order to initialize the mud layer distribution and to investigate the impact of different erosion parameter assumptions on the modeled sediment dynamics. Finally a two-year simulation is done with and without the consolidation process and realistic hydrodynamic forcings. Considering the consolidation process, the model correctly reproduces measured vertical density profiles in the Montevideo Bay access channel. The simulated suspended sediment dynamics behavior in Montevideo Bay with the consolidation process provides a more realistic deposition pattern in regard to the dredging activities.     

Tropical Cyclone in the North of the South China Sea as a Factor Affecting the Structure of the Vietnamese Current

Izvestiya, Atmospheric and Oceanic Physics - Tropical cyclones play a significant role in the formation of the hydrodynamic regime of the South China Sea, including its western part, which is...     

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Hydroelastic analysis of pontoon-type circular VLFS with an attached submerged plate

This paper is concerned with the hydroelastic analysis of a pontoon-type, circular, very large floating structure (VLFS) with a horizontal submerged annular plate attached around its perimeter. The coupled fluid–structure interaction problem may be solved by using the modal expansion method in the frequency domain. It involves, firstly, the decomposition of the deflection of a circular Mindlin plate with free edges into vibration modes that are obtained analytically. Then the hydrodynamic diffraction and radiation forces are evaluated by using the eigenfunction expansion matching method which can also be done in an exact manner. The hydroelastic equation of motion is solved by the Rayleigh–Ritz method for the modal amplitudes, and then the modal responses are summed up to obtain the total response. The effectiveness of the attached submerged annular plate in reducing the motion of VLFS has been confirmed by the analysis.     

NanoSIMS isotopic analysis of small presolar grains: Search for Si3N4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

We report isotopic ratio measurements of small SiC and Si₃N₄ grains, with special emphasis on presolar SiC grains of type Z, and new nucleosynthesis models for ²⁶Al/²⁷Al and the Ti isotopic ratios in asymptotic giant branch (AGB) stars. With the NanoSIMS we analyzed 310 SiC grains from Murchison (carbonaceous CM2 chondrite) separate KJB (diameters 0.25-0.45 μm) and 153 SiC grains from KJG (diameters 1.8-3.7 μm), 154 SiC and 23 Si₃N₄ grains from Indarch (enstatite EH4 chondrite) separate IH6 (diameters 0.25-0.65 μm) for their C and N isotopic compositions, 549 SiC and 142 Si₃N₄ grains from IH6 for their C and Si isotopic compositions, 13 SiC grains from Murchison and 66 from Indarch for their Al-Mg compositions, and eight SiC grains from Murchison and 10 from Indarch for their Ti isotopic compositions. One of the original objectives of this effort was to compare isotopic analyses with the NanoSIMS with analyses previously obtained with the Cameca IMS 3f ion microprobe. Many of the Si₃N₄ grains from Indarch have isotopic anomalies but most of these apparently originate from adjacent SiC grains. Only one Si₃N₄ grain, with ¹³C and ¹⁴N excesses, has a likely AGB origin. The C, N, and Si isotopic data show that the percentage of SiC grains of type Y and Z increase with decreasing grain size (from ∼1% for grains >2 μm to ∼5-7% for grains of 0.5 μm), providing an opportunity for isotopic analyses in these rare grains. Our measurements expand the number of Al-Mg analyses on SiC Z grains from 4 to 23 and the number of Ti analyses on Z grains from 2 to 11. Inferred²⁶Al/²⁷Al ratios of Z grains are in the range found in mainstream and Y grains and do not exceed those predicted by models of AGB nucleosynthesis. Cool bottom processing (CBP) has been invoked to explain the low ¹²C/¹³C ratios of Z grains, but this process apparently does not lead to increased ²⁶Al production in the parent stars of these grains. This finding is in contrast to presolar oxide grains where CBP is needed to explain their high ²⁶Al/²⁷Al ratios. The low ^46,47,49Ti/⁴⁸Ti ratios found in Z grains and their correlation with low ²⁹Si/²⁸Si ratios extend the trend seen in mainstream grains and confirm an origin in low-metallicity AGB stars. The relatively large excesses in ³⁰Si and ⁵⁰Ti in Z grains are predicted by our models to be the result of increased production of these isotopes by neutron-capture nucleosynthesis in low-metallicity AGB stars. However, the predicted excesses in ⁵⁰Ti (and ⁴⁹Ti) are much larger than those found. Even lowering the strength of the ¹³C pocket cannot solve this discrepancy in a consistent way.     

Arsenic in groundwater of the Red River floodplain, Vietnam: Controlling geochemical processes and reactive transport modeling

The mobilization of arsenic (As) to the groundwater was studied in a shallow Holocene aquifer on the Red River flood plain near Hanoi, Vietnam. The groundwater chemistry was investigated in a transect of 100 piezometers. Results show an anoxic aquifer featuring organic carbon decomposition with redox zonation dominated by the reduction of Fe-oxides and methanogenesis. Enhanced P_CO2 pressure causes carbonate dissolution to take place but mainly in the soil and unsaturated zone. The concentration of As increases over depth to a concentration of up to 550 μg/L. Most As is present as As(III) but some As(V) is always found. Arsenic correlates well with NH₄, relating its release to organic matter decomposition and the source of As appears to be the Fe-oxides being reduced. Part of the produced Fe(II) is apparently reprecipitated as siderite containing less As. Results from sediment extraction indicate most As to be related to the Fe-oxide fractions. The measured amount of sorbed As is low. In agreement, speciation calculations for a Fe-oxide surface suggest As(III) to constitute only 3% of the surface sites while the remainder is occupied by carbonate and silica species. The evolution in water chemistry over depth is homogeneous and a reactive transport model was constructed to quantify the geochemical processes along the vertical groundwater flow component. A redox zonation model was constructed using the partial equilibrium approach with organic carbon degradation in the sediment as the only rate controlling parameter. Apart from the upper meter a constant degradation rate of 0.15 C mmol/L/yr could explain the redox zonation throughout the aquifer. Modeling also indicates that the Fe-oxide being reduced is of a stable type like goethite or hematite. Arsenic is contained in the Fe-oxides and is first released during their dissolution. Our model further suggests that part of the released As is adsorbed on the surface of the remaining Fe-oxides and in this way may be retarded.     

Comparison of Calibration Techniques for Ground-Based C-Band Radiometers

We quantify the performance of three commonly used techniques to calibrate ground-based microwave radiometers for soil moisture studies, external (EC), tipping-curve (TC), and internal (IC). We describe two ground-based C-band radiometer systems with similar design and the calibration experiments conducted in Florida and Alaska using these two systems. We compare the consistency of the calibration curves during the experiments among the three techniques and evaluate our calibration by comparing the measured brightness temperatures (T_Bs) to those estimated from a lake emission model (LEM). The mean absolute difference among the T_Bs calibrated using the three techniques over the observed range of output voltages during the experiments was 1.14 K. Even though IC produced the most consistent calibration curves, the differences among the three calibration techniques were not significant. The mean absolute errors (MAEs) between the observed and LEM T_B s were about 2-4 K. As expected, the utility of TC at C-band was significantly reduced due to transparency of the atmosphere at these frequencies. Because IC was found to have a MAE of about 2 K that is suitable for soil moisture applications and was consistent during our experiments under different environmental conditions, it could augment less frequent calibrations obtained using the EC or TC techniques     

基于遗传算法的多光谱影像非监督训练分类系统

本文将遗传算法（GA）应用于非监督训练,提高了遥感数据的分类精度。遗传竞争学习算法（GA-CL）综合了遗传算法和简单的竞争学习算法,可用于改进非监督训练的结果。遗传算法在典型样本聚类的过程中可以避免得到局部最优值。Jeffries-Matusita（J-M）距离法是通过统计测量两个训练类别之间的分离度,可用于评价这种算法。将此算法应用于TM数据的结果显示,遗传算法改进了简单的竞争学习算法,与其他非监督训练算法相比,其提供了K-均值,GA-K-均值和简单的竞争学习算法。