Non-linear magnetic diffusivity in mean-field electrodynamics

We consider non-linear transport and drift processes caused by an inhomogeneous magnetic field in a turbulent fluid. The coefficients of magnetic diffusivity and drift velocity are calculated by making use of the second-order correlation approximation. Transport processes in the presence of a sufficiently strong magnetic field become anisotropic with larger diffusion rate and turbulent electrical resistivity across the field than along the field. Non-linear effects also lead to a drift of the magnetic field away from the regions with a higher magnetic energy.     

Sediment‐water Fluxes of Nutrients and Dissolved Organic Carbon in Extensive Sea Cucumber Culture Ponds

Sediment samples were collected from three seawater aquaculture ponds, and soil characteristics, sediment oxygen consumption (SOC), dissolved organic carbon (DOC) and nutrient fluxes were measured using chamber incubations at laboratory. The three ponds were each representing a specific monoculture or polyculture model of sea cucumber. Total organic carbon (TOC) and total nitrogen (TN) contents in the dry sediment ranged from 0.14 to 0.26% and 0.022 to 0.037%, respectively. Total phosphorus (TP) contents in the sediment were more spatially and temporally variable. SOC ranged from 15.29 to 45.86 mmol m^–2 d^–1 and showed significant differences among the three ponds (p < 0.05). TOC, total carbon (TC) contents, and SOC of the sediment in the pond polycultured with jellyfish increased with culture time, indicating that jellyfish farming enhanced the accumulation of organic matter in the sediments to some extent. Sediment showed net nitrate and ammonium uptake in most ponds and months, and significant differences were found among months (p < 0.05). Dissolved inorganic phosphate (DIP) was released from the sediments in all ponds with low flux rates. DOC was released from the sediment in all ponds and ranged from 0.67 to 1.74 g DOC m^–2 d^–1. The results suggested that non‐artificial‐feeding sea cucumber culture ponds could not only yield valuable seafood products, but also effectively remove nutrients from the aquaculture systems and consequently alleviate nutrient loadings of the nearby coast.     

Flow and sediment processes in a cutoff meander of the Danube Delta during episodic flooding

This article analyzes the water and suspended solid fluxes through a straightened meander of the southern branch of the Danube Delta (the St. George branch) during episodic flooding. The Mahmudia study site corresponds to a vast natural meander which was cut off in 1984–1988 by an artificial canal opened to shipping. The meander correction accelerated fluxes through the artificial canal and dramatically enhanced deposition in the former meander. After his formation, the cutoff meander acted as sediment storage locations, essentially removing channel and point bar sediments from the active sediment budget of the main channel. Increases in slope and stream power in reaches upstream and downstream have also occurred, but to a lesser degree. During the one-hundred-year recurrent flood in April 2006, bathymetry, flow velocity and discharge data were acquired across several sections of both natural and artificial channels with an acoustic Doppler current profiler (aDcp Workhorse Sentinel 600 kHz, Teledyne RDI) in order to investigate the distribution of the flow and sediment and his impact on sedimentation in a channelized reach and its adjacent cutoff. The contrasting hydro-sedimentary processes at work in both channels and bifurcation/confluence nodal points are analyzed from the measured flux distribution, morphological profiles and velocity and concentration patterns. In the cutoff, a diminishing of the intensity of the flow velocity (c. 50%) and of the SSC was observed correlated with the aggradation of the river bed. In the bifurcation/confluence nodal points and in the artificial canal were observed the most intensive hydrodynamic activity (high flow velocity, SSC concentration, degradation of the river bad). Both the event-scale and long-term morphological trends of the alluvial system are discussed analyzing the boundary shear stress and SSC variability. Excess boundary shear stress in the sub-reaches directly affected by cutoffs resulted in scour that increased downstream bed material load. These high sediment loads play a key role in driving morphological adjustments towards equilibrium in the cutoff channel.The approach followed in this paper combines detailed episodic in-situ aDcp measurements and robust numerical 1D modeling in order to provide a practical comprehension of the relevant morphodynamical processes. The 1D model reproduces robustly the continuity of hydrodynamical variables along the streamwise axes of the two-channel network. The simulated are used in the paper for highlighting reach-scale morphological processes, at both event and long-term scales.     

基于SHMS的润扬悬索桥桥址区北风特性的研究

气象研究表明,作用在润扬悬索桥上的强风荷载主要是夏季台风和冬季偏北大风。运营近三年来,润扬悬索桥结构健康监测系统（SHMS）中的风速仪记录了经过该桥址区的大量强风样本。本文通过对北京时间2007-03-10 T13：00～2007-03-10 T16：00期间的实测强北风风速风向数据的分析,得到了该时段的风速和风向、紊流强度、紊流积分尺度、紊流功率谱密度函数等强风特性。结果表明,桥址区实测北风风速风向较稳定;顺风向紊流功率谱密度函数与Kaimal谱吻合较好;紊流强度较规范偏高。     

On the locality of the kinetic energy transfer in a wave space

For the model of thermal turbulence in the Boussinesq approximation with heating from below a change in the structure of the nonlinear interaction of the harmonics of the velocity field with the appearance of rotation is examined. The parameters of convection with rotation are chosen in such a way to correspond to the typical regimes in the models of the planetary dynamo. For such regimes, the structure of the triadic mechanism of the kinetic energy transfer through the spectrum is investigated.     

不同下垫面湍流通量计算方法的比较研究

本文利用1990~1994年在中国内蒙古奈曼市半干旱地区沙丘和植被区下垫面观测的微气象数据,采用变分法、波文比能量平衡方法和空气动力学方法计算了7种下垫面（沙丘、草原、四种放牧强度的草地、玉米田）的湍流通量并进行了比较.结果表明：除沙丘和重度放牧草地外,三种方法计算的湍流通量在大多数时刻是比较一致的,相关性较高,能量闭合程度也较好.在用于计算植被相对茂盛下垫面的湍流通量时,变分法得到的结果更好一些.随着放牧强度的增大,地表生物量、覆盖率和植被高度相应减少,潜热和动量通量相应减小,而感热通量增大.草地等植被茂盛的下垫面能够增加垂直方向的动量输送,增加空气动力学粗糙度,减小风速,阻止地表沙粒的运动和沙丘的起伏,对防止沙漠化能起到较大作用.     

土壤含水量空间尺度分布非均匀对中尺度脉动量和通量的影响研究

本文采用RAMS（Regional Atmospheric Modeling System）模式模拟研究了没有背景风的情况下,土壤湿度非均匀分布的长度尺度分别为40km逐渐减小到2km时,地表通量的分布和大气边界层的响应.运用二维傅里叶变换,分析了地表通量、中尺度脉动量和中尺度通量的二维幅度谱分布,初步探讨大尺度模式中非均匀地表条件下的边界层参数化问题.分析结果显示各试验的地表水、热通量和中尺度脉动量的幅度谱的极大值都出现在与各自非均匀尺度相对应的波数处,当有不同尺度的非均匀斑块共存时,最大的非均匀尺度占主导.但是中尺度水、热通量的结果有所不同,除了在与各自非均匀尺度相对应的波数处有峰值之外,在其他波数还有多个峰值.这些结果表明地表水、热通量的空间分布尺度与非均匀尺度之间存在较好的对应关系,而中尺度水、热通量与非均匀尺度的关系并不明显,说明地表水、热通量的网格平均值的代表性较好,但是不能反映次网格脉动的影响,而中尺度通量的网格平均值的代表性较差.     

A new method for evaluating errors in high‐frequency ADV measurements

The acoustic Doppler velocimeter (ADV) measures three‐dimensional velocities in a small, remote sampling volume at high frequencies, however, these measurements incorporate errors that are intrinsic to the measurement technique. This paper demonstrates a new method for calculating the total measurement errors, including sampling errors, Doppler noise and errors due to velocity shear in the sampling volume associated with single‐point ADV measurements. This procedure incorporates both the effects of instrument configuration and the distribution of errors between velocity components for any probe orientation. It is shown that the ADV can characterize turbulent velocity fluctuations at frequencies up to the maximum sampling rate and that Reynolds shear stress errors are very small. Copyright © 2000 John Wiley & Sons, Ltd.     

Velocity characteristics along a small step–pool channel

This paper summarizes measurements of velocity along three reaches of a small mountain channel with step–pool bedforms. A one‐dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph. Measurement cross‐sections were located upstream from a bed‐step, at the step lip, downstream from the step, and in a uniform‐gradient run. Data analyses focused on characteristics of velocity profiles, and on correlations between velocity characteristics and the potential control variables bedform type, reach gradient and flow depth. To test the hypothesis that velocity characteristics are related to channel bedform types, ANOVA and ANCOVA tests were performed for the average velocity and coefficient of variation of point velocity data. Results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge. Velocity became more variable as stage increased, particularly at low‐gradient reaches with less variable bed roughness. Velocity profiles suggest that locations immediately downstream from bed‐steps are dominated by wake turbulence from mid‐profile shear layers. Locations immediately upstream from steps, at step lips, and in runs are dominated by bed‐generated turbulence. Adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation, whereas favourable pressure gradients at steps are suppressing turbulence. The bed‐generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake‐generated turbulence and form drag of step–pool bedforms. Copyright © 2000 John Wiley & Sons, Ltd.     

Biogeochemical transport in the Loxahatchee River estuary, Florida: The role of submarine groundwater discharge

The distributions of dissolved organic carbon (DOC), Ba, U, and a suite of naturally occurring radionuclides in the U/Th decay series (²²²Rn, ^{223,224,226,228}Ra) were studied during high- and low-discharge conditions in the Loxahatchee River estuary, Florida to examine the role of submarine groundwater discharge in estuarine transport. The fresh water endmember of this still relatively pristine estuary may reflect not only river-borne constituents, but also those advected during active groundwater/surface water (hyporheic) exchange. During both discharge conditions, Ba concentrations indicated slight non-conservative mixing. Such Ba excesses could be attributed either to submarine groundwater discharge or particle desorption processes. Estuarine dissolved organic carbon concentrations were highest at salinities closest to zero. Uranium distributions were lowest in the fresh water sites and mixed mostly conservatively with an increase in salinity. Suspended particulate matter (SPM) concentrations were generally lowest (< 5 mg L^− 1) close to zero salinity and increased several-fold ( 18 mg L^− 1; low discharge) toward the seaward endmember, which may be attributed to dynamic resuspension of bottom sediments within Jupiter Inlet.Surface water-column ²²²Rn activities were most elevated (> 28 dpm L^− 1) at the freshwater endmember of the estuary and appear to identify regions of the river most influenced by the discharge of fresh groundwater. Activities of four naturally occurring isotopes of Ra (^{223,224,226,228}Ra) in this estuary and select adjacent shallow groundwater wells yield mean estuarine water-mass transit times of less than 1 day; these values are in close agreement to those calculated by tidal prism and tidal frequency. Submarine groundwater discharge rates to the Loxahatchee River estuary were calculated using a tidal prism approach, an excess ²²⁶Ra mass balance, and an electromagnetic seepage meter. Average SGD rates ranged from 1.0 to 3.8 × 10⁵ m³ d^− 1 (20–74 L m^− 2 d^− 1), depending on river-discharge stage. Such calculated SGD estimates, which must include both a recirculated as well as fresh water component, are in close agreement with results obtained from a first-order watershed mass balance. Average submarine groundwater discharge rates yield NH₄⁺ and PO₄^− 3 flux estimates to the Loxahatchee River estuary that range from 62.7 to 1063.1 and 69.2 to 378.5 μmol m^− 2 d^− 1, respectively, depending on river stage. SGD-derived nutrient flux rates are compared to yearly computed riverine total N and total P load estimates.