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1.
The paper presents the development of a morphological model and its application to experimental model rivers.The model takes into account the key processes of channel migration,including bed deformation,bank failure and wetting and drying.Secondary flows in bends play an important role in lateral sediment transport,which further affects channel migration.A new formula has been derived to predict the near-bed secondary flow speed,in which the magnitude of the speed is linked to the lateral water level gradient.Since only non-cohesive sediment is considered in the current study,the bank failure is modelled based on the concept of submerged angle of repose.The wetting and drying process is modelled using an existing method.Comparisons between the numerical model predictions and experimental observations for various discharges have been made.It is found that the model predicted channel planform and cross-sectional shapes agree generally well with the laboratory observations.A scenario analysis is also carried out to investigate the impact of secondary flow on the channel migration process.It shows that if the effect of secondary flow is ignored,the channel size in the lateral direction will be seriously underestimated. 相似文献
2.
This study systematically investigates the entire parameter space of the Green–Ampt model for water infiltration and allows for arbitrary constant hydraulic head boundary conditions, any column inclination, hydrophilic and hydrophobic porous media, and a nonzero initial cumulative infiltration. There are five infiltration and five liquid-withdrawal scenarios which differ with respect to the direction of flow and the sign of the acceleration of the liquid. We present analytical solutions for these flow scenarios and 2D phase diagrams that show which flow scenario is predicted depending on the model parameters. All liquid infiltration scenarios, except accelerating downward infiltration (ADI), have already been reported by Green and Ampt. For liquid withdrawal, however, only decelerating downward flow has previously been reported, which is representative of drainage to the water table. We present experimental evidence that ADI may occur in hydrophobic media. 相似文献
3.
含裂缝多孔介质渗透率预测是非常规油气资源勘探开发的一个紧迫问题.现有多孔介质岩石物理模型通常利用圆形孔管模拟宏观岩石孔隙空间,难以定量描述软孔隙/裂缝在压力作用下的闭合情况,缺乏裂缝/孔隙间流量交换的连通机制.本文提出含三维裂缝/软孔隙网络多孔介质模型,将储层岩石裂缝/软孔隙表示为椭圆截面微管,建立了周期性压力作用下微观裂缝流量表达式,通过网络模型和流量守恒条件,得到含有三维裂缝/软孔隙网络的多孔介质渗透率计算方法.数值算例表明,预测结果与实验数据分布范围吻合很好,能够给出不同类型岩心对应孔隙纵横比的分布图.三维裂缝/软孔隙网络模型建立了宏观可观测量与裂缝参数之间关系,能够定量分析岩石渗透率随裂缝体密度、纵横比、孔隙流体类型和围压等因素的变化规律,为复杂条件下储层渗透率预测提供了一种有效方法. 相似文献
4.
A. Thorolfsson J.-C. Cerisier M. Lockwood P. E. Sandholt C. Senior M. Lester 《Annales Geophysicae》2000,18(9):1054-1066
Dayside poleward moving auroral forms (PMAFs) were detected between 06:30 and 07:00 UT on December 16, 1998, by the meridian scanning photometer and the all-sky camera at Ny Ålesund, Svalbard. Simultaneous SuperDARN HF radar measurements permitted the study of the associated ionospheric velocity pattern. A good general agreement is observed between the location and movement of velocity enhancements (flow channels) and the PMAFs. Clear signatures of equatorward flow were detected in the vicinity of PMAFs. This flow is believed to be the signature of a return flow outside the reconnected flux tube, as predicted by the Southwood model. The simulated signatures of this model reproduce globally the measured signatures, and differences with the experimental data can be explained by the simplifications of the model. Proposed schemes of the flow modification due to the presence of several flow channels and the modification of cusp and region 1 field-aligned currents at the time of sporadic reconnection events are shown to fit well with the observations. 相似文献
5.
A large-eddy simulation study has been undertaken to investigate the turbulent structure of open-channel flow in an asymmetric compound channel. The dynamic sub-grid scale model has been employed in the model, with the partial cell treatment being implemented using a Cartesian grid structure to deal with the floodplain. The numerical model was used to predict the: primary velocity and secondary currents, boundary shear stress, turbulence intensities, turbulent kinetic energy, and Reynolds stresses. These parameters were compared with experimental measurements published in the literature, with relatively close agreement being obtained between both sets of results. Furthermore, instantaneous flow fields and large-scale vortical structures were predicted and are presented herein. These vortical structures were found to be responsible for the significant lateral exchange of mass and momentum in compound channels. 相似文献
6.
Vegetative filter strips (VFSs) are a commonly used conservation measure to remove pollutants from agricultural runoff. The effectiveness of VFSs has been widely studied at the plot scale, yet researchers generally agree that field scale implementations are far less effective. The purpose of this research was to develop a field scale VFS submodel for the Soil and Water Assessment Tool (SWAT). A model for the retention of sediments and nutrients in VFSs was developed from experimental observations derived from 22 publications. A runoff retention model was developed from Vegetative Filter Strip MODel (VFSMOD) simulations. This model was adapted to operate at the field scale by considering the effects of flow concentration generally absent from plot scale experiments. Flow concentration through 10 hypothetical VFSs was evaluated using high resolution (2 m) topographical data and multipath flow accumulation. Significant flow concentration was predicted at all sites, on average 10% of the VFS received half of the field runoff. As implemented in SWAT, the VFS model contains two sections, a large section receiving relatively modest flow densities and a smaller section treating more concentrated flow. This field scale model was incorporated into SWAT and verified for proper function. This model enhances the ability of SWAT to evaluate the effectiveness of VFSs at the watershed scale. Published in 2009 by John Wiley & Sons, Ltd. 相似文献
7.
Previous experimental studies of capillary barriers have identified highly hysteretic soil moisture retention characteristics in the materials used. In this study, numerical modelling is used to analyse the role of soil moisture hysteresis in capillary barrier functioning. Comparisons between published experimental results and model simulations indicate that soil moisture hysteresis was a necessary inclusion in the modelling approach to adequately reproduce pore water pressure distributions and the timing of breakthrough occurrences. Under hypothetical intermittent infiltration and evaporation conditions, the predicted volumetric water content in the moisture retention layer was significantly different for hysteretic and non‐hysteretic models. The hysteresis effect was found to be dependent on the nature of infiltration–evaporation cycling, although the predicted volume of flow through the hysteretic barrier was lower than that of the non‐hysteretic case, regardless of the nature of the cyclic upper boundary conditions. For practical engineering designs, where the water leakage through the barrier is the primary concern, the inclusion of soil moisture hysteresis in numerical modelling is needed. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
8.
In this part, the parameter functions for clay brick masonry appearing in the non-linear model are established for the wall material used in the experiments by means of experimental data and a particular type of optimization. This special optimization makes use of the fact that the wall behaves linearly at the intensity level of each excitation, as described in Part 1, and involves matching in frequency space the experimental and theoretical complex frequency response functions relating the Fourier transforms of the top and base accelerations of the wall. It is found that the envelope curves for the parameter functions are bilinear and that the dynamic values of mechanical properties of masonry differ greatly from their static values. The completed model is appraised by comparing how the wall will respond to strong earthquake excitations when predicted using the model and how it actually responded on the shaking table. The predicted response is remarkably close to the experimental. 相似文献
9.
Predicting average annual groundwater levels from climatic variables: an empirical model 总被引：8，自引：0，他引：8
On the basis of one-dimensional theoretical water flow model, we demonstrate that the groundwater level variation follows a pattern similar to recharge fluctuation, with a time delay that depends on the characteristics of aquifer, recharge pattern as well as the distance between the recharge and observation locations. On the basis of a water budget model and the groundwater flow model, we propose an empirical model that links climatic variables to groundwater level. The empirical model is tested using a partial data set from historical records of water levels from more than 80 wells in a monitoring network for the carbonate rock aquifer, southern Manitoba, Canada. The testing results show that the predicted groundwater levels are very close to the observed ones in most cases. The overall average correlation coefficient between the predicted and observed water levels is 0.92. This proposed empirical statistical model could be used to predict variations in groundwater level in response to different climate scenarios in a climate change impact assessment. 相似文献
10.
孔隙尺度的喷射流流动是引起地震波速度频散和衰减的重要机制之一.目前,大多数喷射流模型仅考虑硬孔隙与微裂隙之间的局部流动,而忽略了具有不同孔隙纵横比微裂隙间的喷射流作用.为了研究各种类型孔隙间的流体流动效应,本文对经典喷射流模型进行了扩展,通过结合等效介质理论和孔隙结构模型,根据从干燥岩石超声速度-压力曲线中提取的微裂隙孔隙纵横比分布,求取出岩石中各种微裂隙的体积压缩系数,并在此基础上,利用孔隙空间的压力松弛效应对微裂隙间的喷射流效应进行了模拟,并运用Biot理论描述了硬孔隙间的宏观流动效应.扩展后的理论模型不仅考虑了微裂隙与硬孔隙间的局部流动、硬孔隙与硬孔隙间的Biot宏观流,还加入了微裂隙与微裂隙间的喷射流作用,且模型的高、低频极限始终与Mavko-Jizba理论和Gassmann方程保持一致.模型应用分析发现,对于砂岩和大部分致密灰岩样品,扩展模型均能给出与超声实验测量数据吻合良好的估计结果.此外,扩展模型预测的速度频散及衰减表明,喷射流机制在地震和测井频段发挥着重要作用,其速度频散曲线由低频至高频呈逐渐增大趋势,不具有明显的快速变化特征,与经典频散曲线形态存在显著差异;在低有效压力下,频散和衰减程度较大,喷射流机制发挥主要作用,而随着有效压力的增加,Biot宏观流机制开始占主导,频散和衰减程度逐渐减小. 相似文献
11.
Mark R. Welford 《地球表面变化过程与地形》1994,19(4):287-297
This study investigates the fluvial dynamics of straight natural stream channels. In particular, this experimental field study quantitatively assesses a physically based non-linear mathematical theory of alternate bar formation under unsteady natural flow conditions within a straight alluvial stream. The study site is an artificially straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, bed sediment size and flow stage over a 2 year study period. Both linear and non-linear steady flow hydrodynamic theories suggest that alternate bars are critical to the process of meander development. But these theories do not predict bar development for unsteady flow conditions, which typically occur in natural alluvial channels. Tubino (1991) suggests that bar evolution for a flood hydrograph can be divided into three parts: (1) a period of limited bar growth during the rising stage of the flood; (2) a stage of modest bar decay near the peak of the flood; and (3) a stage of non-linear bar growth during the prolonged falling stage of the flood. Bars developed during the falling limb of a hydrograph, and exhibited sequential development rather than the uniform growth along the reach predicted by Tubino's model. As flow stage decreased, short, low, fine-grained bars were superimposed on long, high and coarser-grained bars that developed under preceding high flow stages. These results suggest that the process of bar formation in artificially straightened natural streams with heterogeneous bed material may occur under different flow conditions and in a different manner than predicted by theoretical models. Further work should focus on attempting to isolate the physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy hydrologic flow regimes. 相似文献
12.
Snow drift transport may cause avalanches on the roads during the periods of snowfall and strong wind. To better understand the factors influencing transport we have developed a theoretical model. This model is based on the boundary layer theory, where the particle mass conservation is considered. Assuming that the saturation is reached, the concentration profile can be represented by a negative exponential law. By means of this analysis, the influence of particle characteristics is explored through the roles of threshold friction velocity and fall velocity. Using fluid mechanics laws, an analysis of the concentration profile resulting from the effect of the wind on a particle bed was also developed. For several velocities of flow and for different kinds of particles an experimental determination of the concentration profile was achieved. We used a laser visualisation and image processing technique to carry out these experiments. The obtained results fit with the values predicted by the theoretical model. 相似文献
13.
14.
A methodology to derive solute transport models at any flow rate is presented. The novelty of the proposed approach lies in the assessment of uncertainty of predictions that incorporate parameterisation based on flow rate. A simple treatment of uncertainty takes into account heteroscedastic modelling errors related to tracer experiments performed over a range of flow rates, as well as the uncertainty of the observed flow rates themselves. The proposed approach is illustrated using two models for the transport of a conservative solute: a physically based, deterministic, advection-dispersion model (ADE), and a stochastic, transfer function based, active mixing volume model (AMV). For both models the uncertainty of any parameter increases with increasing flow rate (reflecting the heteroscedastic treatment of modelling errors at different observed flow rates), but in contrast the uncertainty of travel time, computed from the predicted model parameters, was found to decrease with increasing flow rate. 相似文献
15.
Two models, one linear and one non‐linear, were employed for the prediction of flow discharge hydrographs at sites receiving significant lateral inflow. The linear model is based on a rating curve and permits a quick estimation of flow at a downstream site. The non‐linear model is based on a multilayer feed‐forward back propagation (FFBP) artificial neural network (ANN) and uses flow‐stage data measured at the upstream and downstream stations. ANN predicted the real‐time storm hydrographs satisfactorily and better than did the linear model. The results of sensitivity analysis indicated that when the lateral inflow contribution to the channel reach was insignificant, ANN, using only the flow‐stage data at the upstream station, satisfactorily predicted the hydrograph at the downstream station. The prediction error of ANN increases exponentially with the difference between the peak discharge used in training and that used in testing. ANN was also employed for flood forecasting and was compared with the modified Muskingum model (MMM). For a 4‐h lead time, MMM forecasts the floods reliably but could not be applied to reaches for lead times greater than the wave travel time. Although ANN and MMM had comparable performances for an 8‐h lead time, ANN is capable of forecasting floods with lead times longer than the wave travel time. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
16.
by Tom Clemo 《Ground water》2010,48(1):68-78
A model coupling fluid hydraulics in a borehole with fluid flow in an aquifer is developed in this paper. Conservation of momentum is used to create a one-dimensional steady-state model of vertical flow in an open borehole combined with radially symmetric flow in an aquifer and with inflow to the well through the wellbore screen. Both laminar and turbulent wellbore conditions are treated. The influence of inflow through the wellbore screen on vertical flow in the wellbore is included, using a relation developed by Siwoń (1987) . The influence of inflow reduces the predicted vertical variation in head up to 15% compared to a calculation of head losses due to fluid acceleration and the conventional Colebrook-White formulation of friction losses in a circular pipe. The wellbore flow model is embedded into the MODFLOW-2000 ground water flow code. The nonlinear conservation of momentum equations are iteratively linearized to calculate the conductance terms for vertical flow in the wellbore. The resulting simulations agree favorably with previously published results when the model is adjusted to meet the assumptions of the previous coupled models. 相似文献
17.
In shallow water table‐controlled environments, surface water management impacts groundwater table levels and soil water dynamics. The study goal was to simulate soil water dynamics in response to canal stage raises considering uncertainty in measured soil water content. Water and Agrochemicals in the soil, crop and Vadose Environment (WAVE) was applied to simulate unsaturated flow above a shallow aquifer. Global sensitivity analysis was performed to identify model input factors with the greatest influence on predicted soil water content. Nash–Sutcliffe increased and Root Mean Square Error reduced when uncertainties in measured data were considered in goodness‐of‐fit calculations using measurement probability distributions and probable asymmetric error boundaries, implying that appropriate model performance evaluation should be carried out using uncertainty ranges instead of single values. Although uncertainty in the experimental measured data limited evaluation of the absolute predictions by the model, WAVE was found a useful exploratory tool for estimating temporal variation in soil water content. Visual analysis of soil water content time series under proposed changes in canal stage management indicated that sites with land surface elevation of less than 2.0‐m NGVD29 were predicted to periodically experience saturated conditions in the root zone and shortening of the growing season if canal stage is raised more than 9 cm and maintained at this level. The models developed could be combined with high‐resolution digital elevation models in future studies to identify areas with the greatest risk of experiencing saturated root zone. The study also highlighted the need to incorporate measurement uncertainty when evaluating performance of unsaturated flow models. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
18.
In this work, the influence of non-equilibrium effects on solute transport in a weakly heterogeneous medium is discussed. Three macro-scale models (upscaled via the volume averaging technique) are investigated: (i) the two-equation non-equilibrium model, (ii) the one-equation asymptotic model and (iii) the one-equation local equilibrium model. The relevance of each of these models to the experimental system conditions (duration of the pulse injection, dispersivity values…) is analyzed. The numerical results predicted by these macroscale models are compared directly with the experimental data (breakthrough curves). Our results suggest that the preasymptotic zone (for which a non-Fickian model is required) increases as the solute input pulse time decreases. Beyond this limit, the asymptotic regime is recovered. A comparison with the results issued from the stochastic theory for this regime is performed. Results predicted by both approaches (volume averaging method and stochastic analysis) are found to be consistent. 相似文献
19.
Comparative review of theoretical models for elastic wave attenuation and dispersion in partially saturated rocks 总被引：1，自引：0，他引：1
J. Toms T.M. Müller R. Ciz B. Gurevich 《Soil Dynamics and Earthquake Engineering》2006,26(6-7):548-565
Saturation of porous rocks with a mixture of two fluids (known as partial saturation) has a substantial effect on the seismic waves propagating through these rocks. In particular, partial saturation causes significant attenuation and dispersion of the propagating waves, due to wave-induced fluid flow. Such flow arises when a passing wave induces different fluid pressures in regions of rock saturated by different fluids. As partial fluid saturation can occur on different length scales, attenuation due to wave-induced fluid flow is ubiquitous. In particular, mesoscopic fluid flow due to heterogeneities occurring on a scale greater than porescale, but less than wavelength scale, is responsible for significant attenuation in the frequency range from 10 to 1000 Hz.Most models of attenuation and dispersion due to mesoscopic heterogeneities imply that fluid heterogeneities are distributed in a periodic/regular way. In 1D this corresponds to periodically alternating layering, in 3D as periodically distributed inclusions of a given shape (usually spheres). All these models yield very similar estimates of attenuation and dispersion.Experimental studies show that mesoscopic heterogeneities have less idealized distributions and that the distribution itself affects attenuation and dispersion. Therefore, theoretical models are required which would simulate the effect of more general and realistic fluid distributions.We have developed two theoretical models which simulate the effect of random distributions of mesoscopic fluid heterogeneities. The first model assumes that one fluid forms a random ensemble of spherical inclusions in a porous medium saturated by the other fluid. The attenuation and dispersion predicted by this model are very similar to those predicted for 3D periodic distribution. Attenuation (inverse quality factor) is proportional to ω at low frequencies for both distributions. This is in contrast to the 1D case, where random and periodically alternating layering shows different attenuation behaviour at low frequencies. The second model, which assumes a 3D continuous distribution of fluid heterogeneities, also predicts the same low-frequency asymptote of attenuation. However, the shapes of the frequency dependencies of attenuation are different. As the 3D continuous random approach assumes that there will be a distribution of different patch sizes, it is expected to be better suited to modelling experimental results. Further research is required in order to uncover how to relate the random functions to experimentally significant parameters. 相似文献
20.
Linked hydrologic, hydraulic, and ecological models can facilitate planning and implementing water releases from reservoirs to achieve ecological objectives along rivers. We applied a flow-ecology model, the Ecosystem Functions Model (HEC-EFM), to the Bill Williams River in southwestern USA to estimate areas suitable for recruitment of riparian tree seedlings in the context of managing flow releases from a large dam for riparian restoration. Ecological variables in the model included timing of seed dispersal, tolerable rates of flow recession, and tolerable duration of inundation following germination and early seedling establishment for native Fremont cottonwood and Goodding's willow, and non-native tamarisk. Hydrological variables included peak flow timing, rate of flow recession following the peak, and duration of inundation. A one-dimensional hydraulic model was applied to estimate stage-discharge relationships along ~58 river kilometres. We then used HEC-EFM to apply relationships between seedling ecology and streamflow to link hydrological dynamics with ecological response. We developed and validated HEC-EFM based on an examination of seedling recruitment following an experimental flow release from Alamo Dam in spring 2006. The model predicted the largest area of potential recruitment for cottonwood (280–481 ha), with smaller areas predicted for willow (174–188 ha) and tamarisk (59–60 ha). Correlations between observed and predicted patches with successful seedling recruitment for areas within 40 m of the main channel ranged from 0.66 to 0.94. Finally, we examined arrays of hydrographs to identify which are most conducive to seedling recruitment along the river, given different combinations of peak flow, recession rate, and water volume released. Similar application of this model could be useful for informing reservoir management in the context of riparian restoration along other rivers facing similar challenges. 相似文献