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1.
A rigorous and practical approach for interpretation of impeller flow log data to determine vertical variations in hydraulic conductivity is presented and applied to two well logs from a Chalk aquifer in England. Impeller flow logging involves measuring vertical flow speed in a pumped well and using changes in flow with depth to infer the locations and magnitudes of inflows into the well. However, the measured flow logs are typically noisy, which leads to spurious hydraulic conductivity values where simplistic interpretation approaches are applied. In this study, a new method for interpretation is presented, which first defines a series of physical models for hydraulic conductivity variation with depth and then fits the models to the data, using a regression technique. Some of the models will be rejected as they are physically unrealistic. The best model is then selected from the remaining models using a maximum likelihood approach. This balances model complexity against fit, for example, using Akaike's Information Criterion.  相似文献   

2.
Ditch blocking in blanket peatlands is common as part of peatland restoration. The effects of ditch blocking on flow regimes and nearby water tables were examined in a field trial. After an initial 6‐month monitoring period, eight ditches had peat dams installed 10 m apart along their entire length (dammed), four of these ditches were also partially infilled through bank reprofiling (reprofiled). Four ditches were left open with no dams or reprofiling (open). These 12 ditches and the surrounding peat were monitored for 4 more years. An initial five‐fold reduction in discharge occurred in the dammed and the reprofiled ditches with the displaced water being diverted to overland flow and pathways away from the ditches. However, there was a gradual change over time in ditch flow regime in subsequent years, with the overall volume of water leaving the dammed and the reprofiled ditches increasing per unit of rainfall to around twice that which occurred in the first year after blocking. Hence, monitoring for greater than one year is important for understanding hydrological impacts of peatland restoration. Overland flow and flow in the upper ~4 cm of peat was common and occurred in the inter‐ditch areas for over half of the time after ditch blocking. There was strong evidence that topographic boundaries of small ditch catchments, despite being defined using a high‐resolution Light Detection And Ranging‐based terrain model, were not always equivalent to actual catchment areas. Hence, caution is needed when upscaling area‐based fluxes, such as aquatic carbon fluxes, from smaller scale studies including those using ditches and small streams. The effect of ditch blocking on local water tables was spatially highly variable but small overall (time‐weighted mean effect <2 cm). Practitioners seeking to raise water tables through peatland restoration should first be informed either by prior measurement of water tables or by spatial modelling to show whether the peatland already has shallow water tables or whether there are locations that could potentially undergo large water‐table recoveries.  相似文献   

3.
A novel technique for visualizing turbulent flow data from a gravel-bed river is presented. The time development of flow velocity and shear stress at three heights is displayed using a computer program. This can be used to observe how the fluctuations of velocity and shear stress interact both spatially and temporally. We highlight examples of flow events which are important for the understanding of flow dynamics. The visualization suggests that the turbulent flow-field is characterized not only by coherence over time at a point, but also by spatial interdependence between points. We suggest that this new visualization approach will assist further interpretation of statistical analyses of turbulent signals, as well as focusing future measurement strategies by providing a clearer spatio-temporal picture of the flow structure.  相似文献   

4.
The interaction of ash flows with ridges   总被引：1，自引：0，他引：1  
 Using both laboratory experiments and theoretical models, we examine the different flow regimes that may develop when an ash flow encounters a ridge. For very small ridges, all the flow may pass over the ridge. For intermediate-size ridges, the flow may be partially blocked, with a fraction of the flow reflected upstream as a travelling bore. In this case, the remainder of the flow, which does pass over the ridge, is hydraulically controlled at the ridge crest. Finally, if the ridge is sufficiently high, then the flow will be totally blocked. New laboratory experiments show that the sedimentation patterns associated with these flow regimes may be very different. Most importantly, flows that involve partial blocking and the formation of upstream propagating bores display enhanced sedimentation upstream of the ridge, analogous to valley-ponded and caldera-fill deposits. In contrast, under some circumstances, if the flow is able to scale a ridge, the deposit may be relatively unaffected by the presence of the ridge. The minimum ridge height that leads to total blocking of the flow increases with mass eruption rate and has a complex variation with distance from the source. In a one-dimensional channel, the minimum ridge height that causes blocking increases with distance downstream. This is because the flow becomes less dense through sedimentation of particles and entrainment of air and so requires less energy to scale a ridge of a particular height. In axisymmetric flow, the minimum ridge height initially decreases with distance downstream as the flow spreads radially, but subsequently increases as the flow becomes less dense through sedimentation and entrainment. A new quantitative model of dilute ash flows propagating over ridges indicates that flows with mass fluxes in excess of 108–109 kg/s can partially scale barriers as high as 1000 m at distances of tens of kilometres from the source, whereas smaller flows are likely to be totally blocked by such an obstacle. Our results shed new insight on the possible long-range transport mechanism of several large flows including the Ata, Fisher and Aniakchak pyroclastic flows. Received: 20 December 1996 / Accepted: 15 March 1998  相似文献   

5.
Experiments are conducted in a laboratory flume on the propagation of a surface wave against unidirectional flow with a sediment bed. This article presents the spatial variation of bedforms induced by the wave-blocking phenomenon by a suitably tuned uniform fluid flow and a counter-propagating wave. The occurrence of wave-blocking is confirmed by finding a critical wave frequency in a particular flow discharge in which the waves are effectively blocked and is established using the linear dispersion relation. The purpose of this work is to identify wave-blocking and its influence on the development of bedforms over the sediment bed. Interestingly bedform signatures are observed at a transition of bedforms in three zones, with asymmetric ripples having a steeper slope downstream face induced by the incoming current, followed by flat sand bars beneath the wave-blocking zone and more symmetric ripples below the wave-dominated region at the downstream. This phenomenon suggests that the sediment bed is segmented into three different regions of bed geometry along the flow. The deviations of mean flows, Reynolds stresses, turbulent kinetic energy, and power spectral density due to the wave-blocking phenomenon are presented along the non-uniform flow over sediment bed. The bottom shear stress, bed roughness and stochastic nature of the bedform features are also discussed. The results are of relevance to engineers and geoscientists concerned with contemporary process as well as those interested in the interpretation of palaeoenvironmental conditions from fossil bedforms. © 2019 John Wiley & Sons, Ltd.  相似文献   

6.
We investigated the effects of ditch blocking on fluvial carbon concentrations and fluxes at a 5‐year, replicated, control‐intervention field experiment on a blanket peatland in North Wales, UK. The site was hydrologically instrumented, and run‐off via open and blocked ditches was analysed for dissolved organic carbon (DOC), particulate organic carbon, dissolved carbon dioxide, and dissolved methane. DOC was also analysed in peat porewater and overland flow. The hillslope experiment was embedded within a paired control‐intervention catchment study, with 3 years of preblocking and 6 years of postblocking data. Results from the hillslope showed large reductions in discharge via blocked ditches, with water partly redirected into hillslope surface and subsurface flows, and partly into remaining open ditches. We observed no impacts of ditch blocking on DOC, particulate organic carbon, dissolved carbon dioxide or methane in ditch waters, DOC in porewaters or overland flow, or stream water DOC at the paired catchment scale. Similar DOC concentrations in ditch water, overland flow, and porewater suggest that diverting flow from the ditch network to surface or subsurface flow had a limited impact on concentrations or fluxes of DOC entering the stream network. The subdued response of fluvial carbon to ditch blocking in our study may be attributable to the relatively low susceptibility of blanket peatlands to drainage, or to physical alterations of the peat since drainage. We conclude that ditch blocking cannot be always be expected to deliver reductions in fluvial carbon loss, or improvements in the quality of drinking water supplies.  相似文献   

7.
A discontinuous Galerkin (DG) finite element method is described for the two-dimensional, depth-integrated shallow water equations (SWEs). This method is based on formulating the SWEs as a system of conservation laws, or advection–diffusion equations. A weak formulation is obtained by integrating the equations over a single element, and approximating the unknowns by piecewise, possibly discontinuous, polynomials. Because of its local nature, the DG method easily allows for varying the polynomial order of approximation. It is also “locally conservative”, and incorporates upwinded numerical fluxes for modeling problems with high flow gradients. Numerical results are presented for several test cases, including supercritical flow, river inflow and standard tidal flow in complex domains, and a contaminant transport scenario where we have coupled the shallow water flow equations with a transport equation for a chemical species.  相似文献   

8.
A new theoretical approach for the wave-induced setup over a sloping beach is presented that takes into consideration the explicit variations of the surface waves due to bottom slope and viscosity. In this way, the wave forcing of the mean Lagrangian volume fluxes is calculated without assuming that the local depth is constant. The analysis is valid in the region outside the surf zone and is based on the shallow-water assumption. A novel approach for separating the viscous damping of the waves from the frictional damping of the mean flow is introduced, where the mean Eulerian velocity is applied in the bottom stress for the mean fluxes. In the case where the onshore Lagrangian mean transport is zero, a new formula is derived for the Eulerian mean free surface slope, in which the effects of bottom slope, viscous wave damping and frictional bottom drag on the mean flow are clearly identified. The analysis suggests that viscous damping of the waves and frictional dissipation of the Eulerian near-bed return flow could lead to setup outside the surf zone.  相似文献   

9.
Measurement and interpretation of mass fluxes in favor of concentrations is gaining more and more interest, especially within the framework of the characterization and management of large-scale volatile organic carbon (VOC) groundwater contamination (source zones and plumes). Traditional methods of estimating contaminant fluxes and discharges involve individual measurements/calculations of the Darcy water flux and the contaminant concentrations. However, taken into account the spatially and temporally varying hydrologic conditions in complex, heterogeneous aquifers, higher uncertainty arises from such indirect estimation of contaminant fluxes. Therefore, the potential use of passive sampling devices for the direct measurement of groundwater-related VOC mass fluxes is examined. A review of current passive samplers for the measurement of organic contaminants in water yielded the selection of 18 samplers that were screened for a number of criteria. These criteria are related to the possible application of the sampler for the measurement of VOC mass fluxes in groundwater. This screening study indicates that direct measurement of VOC mass fluxes in groundwater is possible with very few passive samplers. Currently, the passive flux meter (PFM) is the only passive sampler which has proven to effectively measure mass fluxes in near source groundwater. A passive sampler for mass flux measurement in plume zones with regard to long-term monitoring (several months to a year) still needs to be developed or optimized. A passive sampler for long-term monitoring of contaminant mass fluxes in groundwater would be of considerable value in the development of risk-based assessment and management of soil and groundwater pollutions.  相似文献   

10.
The solution of second order elliptic partial differential equations typically arising for flow problems, e.g., in porous media, is frequently expressed as face fluxes of a flow field at hexahedral cells. In this letter, we show by example that the following properties are incompatible: (1) A local velocity reconstruction, (2) reproduction of uniform flow for general hexahedral cells, and (3) a flow field in H(div). This is in particular relevant to mixed finite element methods approximating H(div), and to other methods trying to reconstruct a flow field in a hexahedron using only local face fluxes, e.g., streamline tracing methods for reservoir simulation.  相似文献   

11.
This study presents an experimental analysis from aircraft measurements above the Pyrenees chain during the PYREX experiment. The Pyrenees chain, roughly WE oriented, is a major barrier for northerly and southerly airflows. We present a case of southerly flow (15 October 1990) and three successive cases of northerly flows above the Pyrenees (14, 15 and 16 November 1990) documented by two aircraft. The aircraft have described a vertical cross section perpendicular to the Pyrenean ridge. This area is described via the thermodynamical and dynamical fields which have a horizontal resolution of 10 km. Three methods for computing the vertical velocity of the air are presented. The horizontal advection terms which play a role in the budget equations are also evaluated. The altitude turbulence zone of 15 October are shown via turbulent fluxes, turbulent kinetic energy (TKE), dissipation rate of TKE and inertial length-scale. A comparison of results obtained by eddy-correlation and inertial-dissi-pation method is presented. The experimental results show a warm and dry downdraft for the southerly flow with large values for advection terms. All the mountain wave cases are also shown to present an important dynamical perturbation just above the Pyrenees at upper altitudes.  相似文献   

12.
The continuous Galerkin finite element method is commonly considered locally nonconservative because a single element with fluxes computed directly from its potential distribution is unable to conserve its mass and fluxes across edges that are discontinuous. Some literature sources have demonstrated that the continuous Galerkin method can be locally conservative with postprocessed fluxes. This paper proposes the concept of a direct conservative domain (DCD), which could conserve mass when fluxes are computed directly from the potential distribution. Also presented here is a method for modifying the advection fluxes to obtain different conservative domains from the DCDs. Furthermore, DCDs are used to analyze the local conservation of several postprocessing algorithms, for which DCDs provide the theoretical basis. The local conservation of DCDs and the proposed method are illustrated and verified by using a hypothetical 2‐D model.  相似文献   

13.
Simulations of quiet time terrestrial H-ENA fluxes are shown. The simulations are obtained by using average proton differential fluxes from AMPTE-CCE/CHEM. A functional form which provides the ring current quiet time proton fluxes as a function of energy, L-shell and magnetic local time is also used. The energy neutral atom (ENA) fluxes are simulated at a specific location along the Geotail spacecraft orbit, where ENA data have been collected by the HEP-LD spectrometer. A detailed analysis of the ENA generation along the instrument look directions is presented, in order to evaluate the significance of the ENA signal with respect to the locations of ion sources and to the instrument energy resolution. At energies above 70 keV, we show that the ENA energy is directly related to the ion source location.  相似文献   

14.
For certain porous media and initial conditions, constant flux infiltrations show a saturation profile which exhibits overshoot. This overshoot is the cause of gravity driven fingering, cannot be described by standard models of unsaturated flow, and is likely controlled by the exact nature of the pore filling at the initial front. Here we report synchrotron X-ray micro-tomography measurements of the porous medium and measure which pores are filled by water and air at the initial wetting front as a function of flux. We find that at high fluxes all the pores are filled with water; for intermediate fluxes, the pores along the edge of the column remain unsaturated; and for low fluxes the pores in the bulk of the experimental column remain unsaturated. This suggests that the unsaturated overshoot conditions observed at higher fluxes are primarily an edge effect of the column. The results can help delineate the correct continuum models that can capture overshoot and gravity driven fingering.  相似文献   

15.
Blocking is one of the important features when a beam dam intercepts debris flow, while self-cleaning is another when managing suspended debris flow. Both features determine the debris flow control benefits of beam dam but the latter often is not considered in practical engineering design. In this paper, a series of specially designed flume experiments were done to simulate blocking and self-cleaning processes. The blocking ratio and deposition features were measured to contrast the blocking and self-cleaning performance before and after artificial self-cleaning. The experimental results reveal that the beam dam net opening, particle diameter of sediment, sediment concentration, and gradient of the channel are the main factors affecting blocking performance. A new criterion of blocking performance of beam dams that considers the interaction of multiple factors and can provide guidance to practical project design is proposed. For all three types of blocking, sediment deposited upstream of a beam dam can be effectively transported downstream by erosion from post-debris-flow floods, Self-cleaning performance is most efficient for temporary blocking, followed by partial-blocking, and total-blocking. The efficiency of self-cleaning largely depends on the change of the sediment deposit due to erosion. Finally, a discussion is given for the optimal design of an open-type check dam and the feasibility of synergistic effects of self-cleaning in combination with artificial cleaning. Some supporting artificial silt-cleaning should be implemented in practice. A beam dam will, thus, have more storage capacity with which to control the next debris flow event.  相似文献   

16.
Abstract

The hydraulic flow of a reduced-gravity fluid of non-negative potential vorticity through a sill is considered. It is shown that for any flow with a reversal of current, another, physically realisable, flow exists which is unidirectional and/or resting, and carries more flux than the original flow. Thus only non-negative flows need be considered when examining maximal hydraulic fluxes. Then, for a simple sill (one which slopes downward on the left and upward on the right, looking downstream), it is shown that zero potential vorticity flow, possibly modified by having a region of motionless fluid at its right, carries the maximum flux possible for that sill shape. This makes the calculation of maximal fluxes for a given sill considerably simpler, and examples of parabolic and V-shaped sills are computed.  相似文献   

17.
A study of solar proton sources indicated that solar flare events responsible for ≥2000 pfu proton fluxes mostly occur in complex active regions (CARs), i.e., in transition structures between active regions and activity complexes. Different classes of similar structures and their relation to solar proton events (SPEs) and evolution, depending on the origination conditions, are considered. Arguments in favor of the fact that sunspot groups with extreme dimensions are CARs are presented. An analysis of the flare activity in a CAR resulted in the detection of “physical” boundaries, which separate magnetic structures of the same polarity and are responsible for the independent development of each structure.  相似文献   

18.
Riparian wetlands as typical aquatic-terrestrial interfaces control, in a very specific way, nonpoint water and related chemical fluxes exchanging between catchment areas to their respective water systems (streams, lakes). The existing groundwater and soilwater flow models reveal gaps in dealing with the complex behaviour of processes and the considerable spatial and temporal heterogeneity of riparian wetlands. Based on long-term experience gained through field observations and the interpretation of model produced data, a multi-box aggregation of processes which determines lateral as well as vertical flows and, as a whole, water balance, is used to discretise a generic riparian wetland transect situated between an upland aquifer and a receiving water body.

The resulting mathematical model, FEUWAnet, endowed also with an original methodology to adapt parameters, has been applied to a riparian alder wetland adjacent to Lake Belau (northern Germany). Results of simulations illustrate a good fit between calculated water levels and observed values and an accordance of calculated water balance to previous independent evaluations. This confirms that the sound simplifications of real situations performed by the FEUWAnet mathematical model are a promising way to deal with hydrological complexity of riparian zones. Moreover, FEUWAnet permits, to a certain extent, one to unravel the spatial heterogeneity and temporal variation of lateral (from catchment area to water systems) and vertical (from canopy to groundwater zone) water fluxes typical of riparian ecosystems: this is the necessary step to undertake when developing integrated models capable of assessing the effectiveness of riparian systems in controlling the fluxes of nonpoint pollution discharging in the open water bodies.  相似文献   


19.
The discontinuous spectral Galerkin method uses a finite-element discretization of the groundwater flow domain with basis functions of arbitrary order in each element. The independent choice of the basis functions in each element permits discontinuities in transmissivity in the flow domain. This formulation is shown to be of high order accuracy and particularly suitable for accurately calculating the flow field in porous media. Simulations are presented in terms of streamlines in a bidimensional aquifer, and compared with the solution calculated with a standard finite-element method and a mixed finite-element method. Numerical simulations show that the discontinuous spectral Galerkin approximation is more efficient than the standard finite-element method (in computing fluxes and streamlines/pathlines) for a given accuracy, and it is more accurate on a given grid. On the other hand the mixed finite-element method ensures the continuity of the fluxes at the cell boundaries and it is particular efficient in representing complicated flow fields with few mesh points. Simulations show that the mixed finite-element method is superior to the discontinuous spectral Galerkin method producing accurate streamlines even if few computational nodes are used. The application of the discontinuous Galerkin method is thus of interest in groundwater problems only when high order and extremely accurate solutions are needed.  相似文献   

20.
This paper investigates the conditions under which a marine ice sheet may adopt a steady profile. The ice is treated as a linear viscous fluid caused to flow from a rigid base to and over water, treated as a denser but inviscid fluid. The solutions in the region around the point of flotation, or ‘transition’ zone, are calculated numerically. In-flow and out-flow conditions appropriate to ice sheet and ice shelf flow are applied at the ends of the transition zone and the rigid base is specified; the flow and steady free surfaces are determined as part of the solutions. The basal stress upstream, and the basal deflection downstream, of the flotation point are examined to determine which of these steady solutions satisfy ‘contact’ conditions that would prevent (i) the steady downstream basal deflection contacting the downstream base, and (ii) the upstream ice commencing to float in the event it was melted at the base. In the case that the upstream bed is allowed to slide, we find only one mass flux that satisfies the contact conditions. When no sliding is allowed at the bed, however, we find a range of mass fluxes satisfy the contact conditions. The effect of ‘backpressure’ on the solutions is investigated, and is found to have no affect on the qualitative behaviour of the junctions. To the extent that the numerical, linearly viscous treatment may be applied to the case of ice flowing out over the ocean, we conclude that when sliding is present, Weertman's ‘instability’ hypothesis holds.  相似文献   

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