某水电站地应力场数值模拟与工程区稳定性分析

根据黄河上游某水电站地下厂房区工程地质勘探资料和原地应力测量资料,用Marc Mentat软件建立了有限元计算模型.利用应力回归分析方法模拟该地区地应力场:首先确定自重和构造应力因素为待回归因素,然后采用回归法原理计算待回归因素的系数,从而模拟出工程区地应力场.对比实测值和模拟值,表明两者在数值和方位上差距均不大.厂房...     

Preliminary hydrodynamic results of a field experiment on a barred beach, Truc Vert beach on October 2001

A field experiment conducted on a sandy barred beach, situated on the southern part of the French Atlantic coastline, allowed us to investigate the impact of the intertidal bar on the wave-energy dissipation on the beach face in presence of a high-energy long-incoming swell (significant wave height of about 1.7 to 3.0 m in 56 m water depth and significant wave period about 12 s). Data were collected along three parallel cross-shore transects deployed along an intertidal ridge and runnel system. Wave heights in the inner surf zone are depth-limited, consistent with previous works, and the wave-energy dissipation in the inner surf zone appears to be relatively independent of the offshore energy level. On the other hand, the presence of the bar seems to scatter the data. In models of surf-zone hydrodynamics, wave-energy dissipation is often parameterized in terms of , the ratio of the sea-swell significant wave height to the local mean water depth. The observed values of are not constant along a cross-shore transect, and increase onshore. Furthermore, the observed values observed onshore the intertidal bar are higher than those observed outside the influence of the intertidal bar, and this cannot be fully explained by the different local beach slope.Responsible Editor: Iris Grabemann     

Least action principle,equilibrium states,iterative adjustment and the stability of alluvial channels

The energy based least action principle (LAP) has proven to be very successful for explaining natural phenomena in both classical and modern physics. This paper briefly reviews its historical development and details how, in three ways, it governs the behaviour and stability of alluvial rivers. First, the LAP embodies the special stationary equilibrium state of motion and so its incorporation with the principle of energy conservation explains why so many optimizing hypotheses have been proposed in fluvial geomorphology. Second, the variational approach underlying the LAP provides a more straightforward and simpler fuzzy‐object orientated method for solving river regime problems than do the various complex Newtonian formulations. Third, it is shown that in fluvial systems with surplus energy the surplus can be expended with slope and/or channel geometry adjustments, with the degree of channel geometry adjustment quantified by the dimensionless numbers F for depth dominated adjustment and H for width/depth dominated adjustment. Different planforms are preferred at different energy levels, with H providing a quantitative measure of the flow's efficiency for moving sediment. In rivers with insufficient energy, the interactions of endogenous and exogenous factors are shown to be capable, in certain circumstances, of achieving a stationary equilibrium condition which acts as the attractor state. Importantly, this study describes how iterative changes enable systems to achieve such a stable equilibrium. Copyright © 2007 John Wiley & Sons, Ltd.     

Dynamics of the magnetic field and velocity field in an emerging active region

The dynamics of the magnetic field and velocity field during the birth and an early development stage of a major active region is studied. SOHO MDI longitudinal-component magnetograms, Dopplergrams, and continuum images are used. The presence of an enhanced material upflow in the photosphere during the passing of the top of the magnetic flux loop, forming the active region, is revealed. The maximum upflow velosity is 2 km/s and the maximum size of the upflow area exceeds 20000 km. The lifetime of the upflow is about two hours. The undulating form of the magnetic flux tubes crossing the photosphere is confirmed. A structural-analysis technique is used to show that the trailing polarity field at the formation stage of the active region is more highly structured than the leading polarity field.     

Motion of particles in turbulent flow: Hyperbolicity,strange attractors and dynamic stochasticity of a system

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The stability of dissipative magnetohydrodynamic shear flow in a parallel magnetic field

Abstract

Strong decay bounds are obtained for linearized perturbations to an unbounded, plane Couette flow in a parallel magnetic field. Finite conductivity and molecular viscosity are found to be stabilizing. Those modes decaying most slowly have the form of rolls aligned with the shear flow. The non-aligned rolls decay at an enhanced rate. Stability bounds at finite amplitude are obtained for flows bounded in one direction using energy methods.     

Dynamics of groundwater chemistry in the water-rock system in a megalopolis

The methodological approach developed in this study allows the assessment of the degree of groundwater pollution. It is based on the comparative characteristic of the factual data on water chemistry with model data obtained by simulating the interaction in water-rock system. Prerequisites for the determination of the background composition in a megalopolis are presented. The equilibrium state is calculated for a system consisting of 29 chemical elements with 113 potentially possible minerals and 159 simple and complex particles in an aqueous solution. This equilibrium state is in optimal correspondence with the natural conditions of the groundwater horizon considered.     

Calculations of hydrodynamic time parameters in a semi-opened coastal zone using a 3D hydrodynamic model

Hydrodynamic time parameters (HTs) in a semi-opened aquatic ecosystem are synthetic indicators offering the opportunity to bring out the links between its physical functioning and its biology. The generic term “residence time” is frequently used through literature to mention HTs resulting in various calculation methods. This article presents different computing methods relying on the use of a 3D numerical hydrodynamic model and the HTs to which they give access. Several large-scale (water exchange time, average water export time, e-flushing time) and local time parameters (export time, flushing lag, local e-flushing time) are defined. The applications presented are carried out within the south-west lagoon of New Caledonia (SLNC), on three embedded control volumes. The definition of the control volume is more important for the values of local HTs than for their comparative distribution. The comparison of the global hydrodynamic time scales applied to a control volume provides information on the mixing processes inside the control volume.     

Dynamics,sediment transport,and morphology in a tide-dominated embayment

Western Port, Victoria, Australia is a tide-dominated embayment with an unusual and complex shape. Bottom currents and circulation and their effects on sediment processes were examined using instrumented tripods to measure currents, tides and wave activity, and to estimate sediment transport at nine locations in the bay. Overall bottom water movement patterns were determined by use of seabed drifters. The characteristics of the bay reflect a small catchment and low freshwater and sediment input. A complex system of channels is flanked by extensive intertidal areas. Tidal range varies up to more than 3 m, generating bottom currents up to 70cm s^?1. Flow directions generally conform to channel alignments but major deviations are important. Net circulation in the bay is clockwise around the large central island. The ratio of tidal range to half-tidal period (ΔH/ΔT) when compared with measured currents gave a method of prediction of the annual frequency distribution of maximum bottom current velocities. Determination of threshold current velocities enabled prediction of annual frequency of bedload movement (generally 50–100 per cent of tide cycles). Bedload mass transport for all observed tide cycles was calculated, and estimates of annual mass transport capacity (between 10² and 10⁶ g cm^?1 a^?1( were obtained from a relationship between predicted tidal conditions and mass transport. Seabed drifters delineated the major bottom water movements in the bay and adjacent Bass Strait, and also detailed circulation patterns. This linked the data from the tripod stations, in particular patterns of ebb- and flood-dominance. A coherent picture of the processes operating in Western Port is presented by integrating these studies with corroborative studies of sediment distribution and morphology, hydrochemistry, and mathematical modelling. Some morphological characteristics related to tide-dominance are discussed.     

鄱阳湖湖泊流域系统水文水动力联合模拟

本文以鄱阳湖湖泊流域系统为研究对象,鉴于该湖泊流域系统尺度较大,下垫面自然属性呈现高度空间异质性且具有流域-平原区-湖泊不同机制的水文水动力过程,为了真实描述湖泊流域间的水文水动力联系及反映不同过程间的作用机制,构建了鄱阳湖湖泊流域联合模拟模型.该模型基于自主研发的流域分布式水文模型WATLAC和湖滨平原区产流模型以及水动力模型MIKE 21 3个不同功能子模型的连接来实现该复杂系统的模拟.模型的联合采用输入-输出驱动及子模型的顺序执行进程,即将五大子流域与平原区入湖径流量作为输入条件来驱动湖泊水动力模型,模拟湖泊水位对流域入湖径流量的响应.以2000-2005年鄱阳湖流域6个水文站点的河道径流量、流域基流指数以及湖泊4个站点的水位资料来率定模型,其中各站点日径流量拟合的纳希效率系数E_ns为0.71~0.84,确定性系数R²介于0.70~0.88之间,而湖泊各站点水位拟合的纳希效率系数E_ns变化为0.88~0.98,确定性系数R²为0.96~0.98,均取得令人满意的率定结果.本文提出的鄱阳湖湖泊流域系统水文水动力联合模拟模型能较为理想再现湖泊水位对流域降雨-径流过程的响应.水位模拟结果进一步表明,该联合模型能用来获取重要的水动力空间变化特征.该模型可作为有效工具定量揭示湖泊流域系统水文水动力过程对气候变化和流域人类活动的响应.     

Dynamics of escape of high-energy electrons from a spherical plasma volume in the geomagnetic field

Using results of numerical modeling, the dynamics of escape of electrons, produced as a result beta-decay, to the external magnetic field from a spherical plasma volume with an expelled magnetic field is studied. The dependence of the fraction of escaped electrons on time and radius of the plasma volume has been obtained for two kinds of electron sources: a point isotropic source, located at the center of the sphere, and a volume isotropic source. It is shown that for a point source some part of electrons remains in the sphere, whereas for a volume source all electrons, at different values of the magnetic cavern radius, leave it at an identical relative escape rate.     

Application of a wind-driven hydrodynamic model for the quantification of transport in an open boundary canal-lakes system in sw Friesland,The Netherlands

The south western lake district is a part of the boezem, a system of interconnected lakes and canals in the province of Friesland. The lake district has open boundaries with the other part of the boezem system. However, discharges in the boundary canals are unknown. These discharges are needed for modelling the phosphorus dynamics in the study area. Incidental water flow measurements gave a good indication of the complex water transport in the study area, but continuous water flow recording was not possible. Consequently, discharges could not be measured directly. In order to quantify the discharges, the water transport in the area was modelled by the application of a detailed wind-driven hydrodynamic model. In the model hourly mean values of wind data and water levels at the boundary locations were used as forcing functions. Model tuning was done by comparing observed and computed water levels of three stations within the system. This approach is new in surface water systems in The Netherlands. Therefore, a sensitivity analysis was done and it was verified whether the model results were reliable.The sensitivity analysis showed that the sensitivity was low for modifications of the wind exponent value and rather high for the bottom roughness coefficient. Simulations with daily or weekly mean wind and water level data resulted in an undesirable loss of detail. The sensitivity for noise at the imposed water levels at the boundary locations was moderate. The calculated discharges were used as forcing functions for a chloride mass balance model. Calculated chloride concentrations coincided with measured concentrations in three lakes, during three periods. From this it was concluded that the discharges were reliable. The simulations also lead to the quantification of the water balance and water residence times in the lakes.     

Kaguyak dome field and its Holocene caldera,Alaska Peninsula

Kaguyak Caldera lies in a remote corner of Katmai National Park, 375 km SW of Anchorage, Alaska. The 2.5-by-3-km caldera collapsed ~ 5.8 ± 0.2 ka (¹⁴C age) during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61–67% SiO₂). Proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling the 150-m-deep caldera lake, emplacement of two intracaldera domes (61.5–64.5% SiO₂), and phreatic ejection of lakefloor sediments onto the caldera rim. CO₂ and H₂S bubble up through the lake, weakly but widely. Geochemical analyses (n = 148), including pre-and post-caldera lavas (53–74% SiO₂), define one of the lowest-K arc suites in Alaska. The precaldera edifice was not a stratocone but was, instead, nine contiguous but discrete clusters of lava domes, themselves stacks of rhyolite to basalt exogenous lobes and flows. Four extracaldera clusters are mid-to-late Pleistocene, but the other five are younger than 60 ka, were truncated by the collapse, and now make up the steep inner walls. The climactic ignimbrite was preceded by ~ 200 years by radial emplacement of a 100-m-thick sheet of block-rich glassy lava breccia (62–65.5% SiO₂). Filling the notches between the truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. They were probably shed by a large lava dome extruding where the lake is today.     

Non-adiabatic motion of charged particles traversing a weak magnetic field: Pitch angle scattering

Summary A charged particle moves with velocityv in a constant non-uniform magnetic fieldH, spiralling with Larmor radiusR. IfR is small compared with the scale lengthL of the field, the magnetic moment associated with the Larmor motion of the particle is nearly constant. Consequently , the (pitch) angle betweenv andH, varies as arcsinH ^1/2. Hence in such adiabatic motion is approximately the same at points on the path whereH has the same value. But the magnetic moment and the pitch angle may differ materially at two such points, each in the region whereR/L is small, if between them the particle traverses a region whereR/L is not small. This region of non-adiabatic motion scatters the pitch angles.Such scattering is investigated for regions of weak field (R large), with and without the presence of a neutral line along whichH=0. Either type of region, it is found, can scatter the pitch angles. This gives support to the theory proposed byAkasofu andChapman to explain why auroral arcs and bands are very thin.The scattering here examined is of interest also in connection with magnetic mirror devices for nuclear energy transformation. It may also have applications to phenomena of solar and stellar atmospheres.     

The chaotic attractor of the precursory field of the seimogenic system and its fractal dimension

In this paper we outlined the chaotic attractor of the precursory field evolution of the seismogenic system and its fractal dimension of the precursory time and space distribution. We developed the calculative method of reconstruction complex system dynamics from single time series and analysed the descent dimension phenomena of the precursory distribution before large earthquakes. We also showed the time-space synthesis method constructed complex system dynamics by many stations or many methodes in the seismogenic system consists of large area tectonic network. This method can describe the self-organization behavior of the system more accurately and get rid of the uncertainty and randomness caused by single station or single method. As an example, we calculated the chaotic attractor of the precursory field evolution and the fractal dimension of the precursory time and space distribution and its change tendencies before large earthquakes in Beijing-Tianjing area. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 463–469, 1993.     

A reduced 3D hydrodynamic model of a shallow,long, and weakly curved stream

A reduced three-dimensional mathematical model of a free-flow stream in nondeformable channels (a model of a long shallow flow), proposed earlier, has been studied analytically and numerically. The reduced model has been verified by direct numerical simulation of the flow by full hydrodynamic models in COMSOL finite-element software complex for laminar and turbulent flows of a viscous fluid. The obtained results show that the proposed reduced model of a shallow weakly curved stream flow adequately describes its hydrodynamics, so it can be used in systems of complex simulation of the ecology of water objects and the use of water resources.     

Dynamics of a headwater system and peatland under current conditions and with climate change

Interactions between headwater aquifers and peatlands have received limited scientific attention. Hydrological stresses, including those related to climate change, may adversely impact these interactions. In this study, the dynamics of a southern Québec headwater system where a peatland is present is simulated under current conditions and with climate change. The model is calibrated in steady state on field‐measured data and provides satisfactory results for transient‐state conditions. Under current conditions, simulations confirm that the peatland is fed by the fractured bedrock aquifer year‐round and provides continuous baseflow to its outlets. Climate change is simulated through its impact on groundwater recharge. Predicted precipitation and temperature data from a suite of regional climate model scenarios provide a net precipitation variation range from +10% to ?30% for the 2041–2070 horizon. Calibrated recharge is modified within this range to perform a sensitivity analysis of the headwater model to recharge variations (+10%, ?15% and ?30%). Total contribution from the aquifer to rivers and streams varies from +14% to ?44% of the baseline for +10% to ?30% recharge changes from spring 2010 data, for example. With higher recharge, the peatland receives more groundwater, which could significantly change its vegetation pattern and eventually ecosystem functions. For a ?30% recharge, the peatland becomes perched above the aquifer during the summer, fall and winter. Recharge reductions also induce sharp declines in groundwater levels and drying streams. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamics of phosphorus compounds in a lowland river system: Importance of retention and non-point sources

Intensive measurements of the fluxes of phosphorus (P) and of P retention were carried out in a Danish lowland watershed (Gjern Å) during two hydrological years (June 1987 to May 1989). Seasonal and short-term variations of P concentrations were measured by intensive automatic sampling covering P fluxes from the entire Gjern Å watershed and two subcatchments: the Gelbæk and Lake Søbygård. Moreover, infrequent sampling was performed in all major tributaries of the river system. Transport of P compounds (dissolved and particulate P) exposed a seasonal pattern which was highly affected by flow conditions, instream processes, as well as point and non-point sources. Stormflow P transport during the two study years (1987-8 and 1988-9) constituted 56 and 61% in the Gelbæk, respectively, compared with 25 and 23% in the Gjern Å, respectively. Particulate P transport in the Gelbæk constituted 56 and 75% of annual total P transport during the two years compared with 77 and 75% in the Gjern Å. The magnitude of monthly baseflow and stormflow total P loads was significantly correlated with the mean monthly discharge during baseflow and stormflow in the Gelbæk, whereas only stormflow loads were correlated to discharge in the Gjern Å. In situ measurements of retention showed that the maximum amount of P stored in the Gelbæk during summer (June to August) was 22gPm^?2 (1988) compared with 27gPm^?2 (1987) in the Gjern Å. Lake Søbygård was a P source during summer but a P sink during winter and on an annual basis the net P released was 810 and 1420 kg, which corresponded to 7 and 20% of the annual P export from the watershed, respectively. Retention constituted about 30% of gross P transport in the Gelbæk during summer compared with 20% in the Gjern Å. Resuspension of retained P during stormflows in September 1987 constituted 94% of the stormflow P transport and 54% of the total P transport. Monthly total P and dissolved reactive P (DRP) mass balances for the main channel of the Gjern Å revealed a significant DRP retention over 17 months (p < 0.05) caused by P adsorption on sediments and biological uptake. On an annual basis the main channel was a sink of DRP (1220 and 1660 kg P) but a source of total P (3440 and 1000 kg P). implying that the channel is an important source of particulate P. Bank erosion proved to be a significant P source in the main channel of Gjern Å, whereas P delivery from soil erosion was possibly of minor importance. The annual non-point P export from two intensively sampled catchments was significantly higher (0.89 and 0.45 kg Pha^?1 yr^?1) than from the five extensively sampled subbasins (0.43 and 0.17 kg Pha^?1 yr^?1). No significant relationship could be established between non-point P export and the proportion of arable land and soil type. Only for the wet year of 1987-8 was a significant relationship established between non-point P export and the P load from scattered dwellings outside sewage areas in five subbasins.