Prediction of Siltation in the Downstream Approach Channel of Tidal Locks

Silt deposition occurs in the downstream approach,channel of the tidal lock as in a closed channelor excavated dock basin.It is often difficult to calculate or predict siltation because of complex flow and sedi-ment conditions and many other affecting factors.In this paper,the characteristics of flow movement in theapproach channel(including its mouth)of the tidal lock are analyzed,the basic laws of sediment movementand siltation mechanism are investigated,the conditions for three types of siltation(circumfluence siltation,density flow siltation and slow flow siltation)are discussed,and corresponding calculating formulas are pro-posed.A practical example shows that the difference between measured and calculated results is small,indi-cating that the present calculating methods could be used in design and management of practical engineeringprojects.     

Compositional modes in the ash fraction of some modern pyroclastic deposits: their determination and significance

 This paper illustrates some problems involved in the quantitative compositional study of pyroclastic deposits and proposes criteria for selecting the main petrographic and textural classes for modal analysis. The relative proportions of the different classes are obtained using a point-counting procedure applied to medium-coarse ash samples that reduces the dependence of the modal composition on grain size and avoids tedious counting of different grain-size fractions. The major purposes of a quantified measure of component distributions are to: (a) document the nature of the fragmenting magma; (b) define the eruptive dynamics of the eruptions on a detailed scale; and (c) ensure accuracy in classifying pyroclastic deposits. Compositional modes of the ash fraction of pyroclastic deposits vary systematically, and their graphical representation defines the compositional and textural characteristics of pyroclastic fragments associated with different eruptive styles. Textural features of the glass component can be very helpful for inferring aspects of eruptive dynamics. Four major parameters can be used to represent the component composition of pyroclastic ash deposits: (a) juvenile index (JI); (b) crystallinity index (CrI); (c) juvenile vesicularity index (JVI); and (d) free crystal index (FCrI). The FCrI is defined as the ratio between single and total crystal fragments in the juvenile component (single crystals+crystals in juvenile glass). This parameter may provide an effective estimate of the mechanical energy of eruptions. Variations in FCrI vs JVI discriminate among pyroclastic deposits of different origin and define compositional fields that represent ash derived from different fragmentation styles. Received: 15 January 1998 / Accepted: 8 February 1999     

A spatial assessment of hydrologic alteration caused by dam construction in the middle and lower Yellow River,China

The ‘range of variability approach’ (RVA) and mapping technique are used to investigate the spatial variability of hydrologic alterations (HA) due to dam construction along the middle and lower Yellow River, China, over the past five decades. The impacts of climate variability on hydrological process have been removed during wet and dry periods and the focus is on the impacts of human activities, such as dam construction, on hydrological processes. Results indicate the following: (1) The impacts of the Sanmenxia reservoir on the hydrologic alteration are relatively slight with a mean HA value of 0·48, ranking in the last place among the four large reservoirs. (2) Xiaolangdi reservoir has significantly changed the natural flow regime downstream with mean HA value of 0·56, ranking it in first place among the large reservoirs. (3) The results of ranked median degrees of 33 hydrologic alteration indicators for 10 stations in the Yellow River show that the hydrologic alteration of Huayuankou ranks the highest among 10 stream gauges. (4) Impacts of reservoirs on hydrological processes downstream of the dams are closely associated with the regulating activities of the reservoirs. At the same time, alterations of streamflow regimes resulting from climatic changes (e.g. precipitation variability) make the situation more complicated and more hydrological observations will be necessary for further analysis. The results of the current study will be greatly beneficial to the regional water resources management and restoration of eco‐environmental systems in the middle and lower Yellow River characterized by intensified dam construction under a changing environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Generalized extreme value shape parameter and its nature for extreme precipitation using long time series and the Bayesian approach

Assessing the probability of extreme precipitation events is consequential in civil planning. This requires an understanding of how return values change with return periods, which is essentially described by the generalized extreme value (GEV) shape parameter. Some works in the field suggest a constant shape parameter, while our analysis indicates a non-universal value. We re-analysed an older precipitation dataset (169 stations) extended by Norwegian data (71 stations). We showed that while each set seems to have a constant shape parameter, it differs between the two datasets, indicating regional differences. For a more comprehensive analysis of spatial effects, we examined a global dataset (1495 stations). We provided shape parameter maps for two models and found clear evidence that the shape parameter depends on elevation, while the effect of latitude remains uncertain. Our results confirm an explanation in terms of dominating precipitation systems based on a proxy derived from the Köppen-Geiger climate classification.

EDITOR D. Koutsoyiannis

ASSOCIATE EDITOR not assigned     

Ocean contributes to the melting of the Jakobshavn Glacier front

The Jakobshavn Glacier(JG)in Greenland is one of the most active glaciers in the world.It was close to balance before 1997 but this was followed by a sudden transition to rapid thinning.The reason for the change remains unclear.In this study,The NASA Pre-IceBridge ice thickness data are collected to monitor the melting of JG front.The surface elevation decreased by around 90 m from 1995 to 2002 on the floating front.A distributed energy balance model is developed to estimate the energy balance of JG front in the past 30 years(1986-2016).The results indicate that multi-year average energy fluxes absorbed by the floating front of JG from the ocean were about 500 Wm^-2 from 1986 to 2016.This is approximately two times of the energy fluxes from atmosphere during the same period.The energy fluxes from the ocean increased from 200 to 600 Wm^-2during the period from 1990 to 1998 while energy fluxes from the atmosphere remained stable at about 250 Wm^-2.These results demonstrate that ocean contributes more to the melting of the JG front,and suggest that bottom surface melting must have a profound influence on marine-terminating glacier dynamics.     

Analytic scaling laws in planetary dynamo models

ABSTRACT

Numerical models of planetary MHD dynamos have led to many advances in the last twenty years. However, the accessible parameter space of these models is somewhat limited due to computational constraints. A number of numerically based scaling laws have been proposed, but it has been difficult to reliably confirm them from the simulation data alone. Since these scaling laws have to be used to extend the numerical models into the planetary parameter regime, getting correct scalings is an important issue. Here some possible balances between the relevant terms in the dynamo equations are considered, and a number of different known and new scaling laws are proposed and justified from analytical point of view. Some of these laws are compatible with the widely known Christensen and Aubert (Geophys. J. Int. 2006, 166) scaling for the magnetic field strength. Plausible estimates of the typical values of the important quantities in the Earth's core and other Terrestrial planets are made, and the extent to which the various scaling laws are consistent with these estimates is discussed.     

Spatial seismic modeling of base‐isolated buildings pounding against moat walls: effects of ground motion directionality and mass eccentricity

Structural impact between adjacent buildings may induce local and, in some extreme cases, severe damage, especially in the case of seismically isolated buildings. This study parametrically investigates in the three‐dimensional domain the effect of pounding on the peak response of base‐isolated buildings, which are simulated as nonlinear three‐dimensional multi‐degree‐of‐freedom systems. Firstly, it is shown that considering unidirectional, instead of bidirectional, excitations may lead to underestimation of the base drift demands. Subsequently, the peak responses of seismically isolated buildings utilizing lead rubber bearings are studied while varying important parameters, such as the incidence angle of seismic excitations, the available seismic clearance, and mass eccentricities, under the action of bidirectional horizontal excitations. A large number of numerical simulations are performed using a specially developed software that implements an efficient approach to model impacts, taking into account arbitrary locations of contact points. It is found that the peak interstory drift ratio is significantly influenced by the directionality of the ground motion. Therefore, the seismic performance of structures should ideally be assessed examining the peak structural response while bidirectional ground motions are imposed at various incident angles. Furthermore, it is also observed that the interstory drift ratios increase while decreasing the available gap size, up to a certain value. Finally, the parametric analyses indicate that the effects of impact are more severe for structures with mass eccentricities, and in which case, the estimation of the critical incidence angle becomes more laborious. Copyright © 2016 John Wiley & Sons, Ltd.     

基于重力卫星数据监测地表质量变化的三维点质量模型法

利用卫星重力测量手段监测全球质量变化取得了巨大成功,本文基于牛顿万有引力定律在三维空间直角坐标系中导出利用重力卫星观测数据监测全球质量变化的三维点质量模型法,该方法可直接利用重力卫星的轨道和星间观测数据或时变重力场模型计算全球质量变化,由于利用卫星观测数据计算地表质量变化的向下延拓过程以及观测数据噪声的影响,需要采用合适的空间约束方程或正则化技术对解算结果进行约束或平滑处理.利用合成全球质量变化模型模拟一个月的GRACE双星轨道和星间距离变率数据计算全球质量变化,对三维点质量模型法进行分析验证,采用零阶Tikhonov正则化技术处理病态问题.结果表明,三维点质量模型法可有效用于重力卫星观测数据监测全球质量变化,为利用重力卫星观测数据监测全球质量变化提供一种可选的途径.     

Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths (Tok Cenozoic volcanic field, SE Siberia)

Partial melting and reactive melt transport may change the composition, microstructures, and physical properties of mantle rocks. Here we explore the relations between deformation and reactive melt transport through detailed microstructural analysis and crystallographic orientation measurements in spinel peridotite xenoliths that sample the shallow lithospheric mantle beneath the southeastern rim of the Siberian craton. These xenoliths have coarse-grained, annealed microstructures and show petrographic and chemical evidence for variable degrees of reaction with silicate melts and fluids, notably Fe-enrichment and crystallization of metasomatic clinopyroxene (cpx). Olivine crystal preferred orientations (CPO) range from strong to weak. [010]-fiber patterns, characterized by a point concentration of [010] normal to the foliation and by dispersion of [100] in the foliation plane with a weak maximum parallel to the lineation, predominate relative to the [100]-fiber patterns usually observed in lithospheric mantle xenoliths and peridotite massifs. Variations in olivine CPO patterns or intensity are not correlated with modal and chemical compositions. This, together with the analysis of microstructures, suggests that reactive melt percolation postdated both deformation and static recrystallization. Preferential crystallization of metasomatic cpx along (010) olivine grain boundaries points to an influence of the preexisting deformation fabrics on melt transport, with higher permeability along the foliation. Similarity between orthopyroxene (opx) and cpx CPO suggests that cpx orientations may be inherited from those of opx during melt-rock reaction. As observed in previous studies, reactive melt transport does not weaken olivine CPO and seismic anisotropy in the upper mantle, except in melt accumulation domains. In contrast, recovery and selective grain growth during static recrystallization may lead to development of [010]-fiber olivine CPO and, if foliations are horizontal, result in apparent isotropy for vertically propagating SKS waves, but strong anisotropy for horizontally propagating surface waves.