Alternate bars in a sandy gravel bed river: generation,migration and interactions with superimposed dunes

A field study was carried out to investigate the development of alternate bars in a secondary channel of the Loire River (France) as a function of discharge variations. We combined frequent bathymetric surveys, scour chains and stratigraphical analysis of deposits with measurements and modelling of flow dynamics. The channel exhibited migrating bars, non‐migrating bars and superimposed dunes. Possible mechanisms of bar initiation were found to be chutes associated with changes of bank direction and instability resulting from interactions between existing bars during the fall in water level after floods. We propose that the reworking of bar sediments during low flows (high width‐to‐depth ratio β), reinforced by high values of the Shields mobility parameter, can explain the formation or re‐generation of new alternate migrating bars during a subsequent flood. The migration pattern of the bars was found to be cyclic and to depend mainly on (i) channel layout and (ii) the dynamics of superimposed dunes with heights and lengths depending on location and discharge value. For instance, the hysteresis affecting the steepness of dunes influences the flow resistance of the dunes as well as the celerity of migrating bars during flood events. We compare the findings from the field with results from theoretical studies on alternate bars. This gives insight in the phenomena occurring in the complex setting of real rivers, but it also sheds light on the extent to which bar theories based on idealized cases can predict those phenomena. Copyright © 2014 John Wiley & Sons, Ltd.     

Spatial variations of tidal water level and their impact on the exposure patterns of tidal land on the central Jiangsu coast

The exposed area of intertidal zone varies with tidal water level changes. If intercomparisons of satellite images are adopted as a method to determine geomorphological changes of the intertidal zone in response to accretion or erosion processes, then the e?ect of water level variations must be evaluated. In this study, two Landsat TM images overpassing the central Jiangsu coastal waters on 2 January and 7 March 2002, respectively, were treated by the changing detection analysis using Image Di?erencing and Post-classiˉcation Comparison. The simultaneous tide level data from four tide gauge stations along the coast were used for displaying the spatial variations of water levels and determining the elevations of waterlines. The results show that the spatial variations of water levels are highly signiˉcant in the central Jiangsu coastal waters. The huge di?erences of tidal land exposure patterns between the two imaging times are related mainly to the spatial variations of tidal water levels, which are controlled by the di?erences in tidal phases for di?erent imaging times and the spatial variations of water level over the study area at each imaging time. Under complex tidal conditions, e.g., those of the central Jiangsu coastal waters, the tide-surge model should be used to eliminate e?ectively the e?ects of water level variations on remote sensing interpretation of geomorphological changes in the intertidal zone.     

Spatial and temporal variations of Synechococcus and picoeukaryotes in the Taiwan Strait,China

The size-fractionated phytoplankton biomass, and the spatial and temporal variations in abundance of Synechococcus (SYN) and picoeukaryotes (PEUK) were measured in the Taiwan Strait during three cruises (August 1997, February-March 1998, and August 1998). The results show that picophytoplankton and nanophytoplankton dominate the phytoplankton biomass, in average of 38% and 40%, respectively. SYN and PEUK varied over time in abundance and carbon biomass, greater in summer than in winter, in range of (7.70–20...     

Spatial and temporal variations of <Emphasis Type="Italic">Synechococcus</Emphasis> and picoeukaryotes in the Taiwan Strait,China

The size-fractionated phytoplankton biomass, and the spatial and temporal variations in abundance of Synechococcus (SYN) and picoeukaryotes (PEUK) were measured in the Taiwan Strait during three cruises (August 1997, February–March 1998, and August 1998). The results show that picophytoplankton and nanophytoplankton dominate the phytoplankton biomass, in average of 38% and 40%, respectively. SYN and PEUK varied over time in abundance and carbon biomass, greater in summer than in winter, in range of (7.70–209.2)×10⁶ and (0.75–15.4)×10⁶ cells/cm² in the abundance, and 1.93–52.3 and 1.57–32.4 μgC/cm² in the carbon biomass, for SYN and PEUK, respectively. The horizontal distributions of both groups were diurnal but heterogeneous in abundance, depending on the groups and layer of depths. Temperature is the key controlling factor for picophytoplankton distribution (especially in winter) in the Strait. Supported by Natural Science Foundation of China (No.40730846; 40521003)     

GRACE’s implication of temporal inertia moment and length of day

This paper aims to study the Earth’s temporal inertia moment and its influence on length of day (LOD). First, the GRACE data are processed by wavelet analysis to remove abnormal jumps and noises. Then the theoretical impacts of the second-order potential coefficients on the inertia moment and LOD are studied. Finally, the processed GRACE data are applied and results show that mass redistribution has led to decreasing tendencies of inertia moment and LOD as well as some unexpected seasonal oscillations in the recent 6 years. Supported by the Open Fund of Key Laboratory of Geospace Environment and Geodesy, Ministry of Education, China (No.08-02-02), the Special Project Fund of State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing (China), the National 863 Program of China (No.2006AA12Z211).     

南海不同海区叶绿素a和海水荧光值的垂向变化

本文根据1985—1990年的实测资料,探讨南海某些海区叶绿素a和海水荧光值的垂向变化特征及其与某些环境因子的关系。结果表明,南海南部、东沙群岛附近(夏季)、巴士海峡西部和台湾海峡南部海区的叶绿素a和荧光值的垂向变化曲线以单峰型为主,次表层出现较高值,认为与温跃层、营养盐和溶解氧的垂直梯度有关。采用最小二乘拟合法,得出南海南部和东沙群岛附近海区叶绿素a和荧光值的垂向变化计算公式。     

南海地区海—气热交换的年变化和年际变化

本文利用ＥＯＦ方法，对南海地区（０°－２０°Ｎ，１０４°－１３６°Ｅ）感热通量和潜热通量的时空变化进行了分析。结果表明：年变化中前三个特征向量具有明显的物理意义，模态一表征了南海地区感热和潜热通量的气候变化特征；模态二反映了南海地区感热和潜热通量变化与南海ＩＴＣＺ变化的联系；模态三反映了南海地区感热和潜热通量西北－东南向的半年周期振荡。在热通量的年际变化中，夏季（６－８月）的第一特征向量含有较强的     

塘沽海洋站水位变化及其极值与海面风的相关关系研究

在海洋工程设计中,水位极值是一个重要的设计参数。对塘沽海洋站54a每小时的水位观测资料(1950—2003)和38a的海面风观测资料(1965—2003)进行了分析,探讨了该站水位的变化特征及海面风与水位极值的相关关系。分析结果表明,在水位时间序列中M2分潮占总能量的60%以上,六个主要分潮M2,S2,N2,K2,K1和O1占总能量的93%以上。年平均海平面的变化并不是简单的线性变化,而是存在3~5a的周期性波动,这可能与厄尔尼诺、太阳黑子活动和交点周期等周期性和准周期性变化过程有关。由54a实测资料计算得的年平均海平面的年变率为1.64mm/a。54a中最高水位为5.81m,发生在1992年9月;最低水位为-0.99m,发生在1968年11月。100a一遇的最高水位为6.02m,由偏东向风引起,而西向风引起塘沽海洋站减水。对于100a一遇的各向最大风速,北向偏东风的100a一遇的风速为最大,其值为26.78m/s,其可能引起的最大增水为2.06m。西风的100a一遇最大风速为20.5m/s,其可能引起的减水水位为2.28m。     

Vertical structure of larval fish assemblages during diel cycles in summer and winter in the southern part of Bahía de La Paz, México

The effect of environmental variables on the vertical structure of larval fish assemblages in a tropical coastal lagoon was analyzed. Ichthyoplankton samples were collected from the near-bottom and surface strata near the mouth of a subtropical lagoon during contrasting seasonal conditions of temperature, photoperiod, light intensity, and tidal heights. During summer, larval fish assemblages had high species richness (R) and were dominated by tropical species. During winter, assemblages had lower R values and were dominated by subtropical and temperate species. Vertical distribution patterns of the taxa were determined by the interaction of environmental variables and behavior of each species to maintain their position in a stratum in the water column, or to achieve vertical migrations induced by environmental stimuli that, in this case, were thermal gradient, column water stratification, and intensity of light. Depth position and vertical migration of fish larvae, coupled with the flood and ebb tide conditions, played an important role in their retention and displacement toward the lagoon. Fish larvae with distribution restricted to the inner part of the inlet, such as Achirus mazatlanus, Etropus sp., and several gobies, were more abundant in the near-bottom stratum during the ebb tide, allowing them to avoid exportation, whereas those that could spawn outside, but depended on the inlet as a nursery area, were more abundant near the surface during flood tide, such as Abudefduf troschelii and Stegastes rectifraenum.     

Spatial variability of submarine groundwater discharge,Ubatuba, Brazil

Submarine groundwater discharge (SGD) is delivered into the oceans at various hydrogeological settings. Where coastal aquifers have spatial heterogeneous properties such as preferential flowpaths, SGD is also commonly heterogeneously distributed along the coast. SGD from a heterogeneous, fractured rock aquifer on the Ubatuba coast, Brazil has been investigated by depth profiling of ground conductivity. On spatial scales of <10 m, a significant variation in sub-surface conductivity and associated discharge rate is documented. These geophysical observations were used to identify preferential flowpaths, and to explain significant differences in average discharge rates observed in seepage meter records placed only a few metres apart from each other. This paper illustrates the utility of sub-surface conductivity measurements for accurate investigations of SGD fluxes.