Showing papers on "Groundwater flow published in 1975"

A Stochastic-Conceptual Analysis of One-Dimensional Groundwater Flow in Nonuniform Homogeneous Media

Abstract: The most realistic representation of a naturally occurring porous medium is a stochastic set of macroscopic elements in which the values of the three basic hydrogeologic parameters (hydraulic conductivity K, compressibility α, and porosity n) are defined by frequency distributions. A homogeneous formation under this representation is one in which the frequency distributions do not change through space. All soils and geologic formations, even the ones that are homogeneous, show random variations in the values of the hydrogeological parameters through space; that is, they are nonuniform, and a measure of the nonuniformity is provided by the standard deviation of the frequency distributions. If K and α are log normally distributed and n is normally distributed, and if we define Y = log K and C = log α, then the parameters Y, C, and n can be generated from a multivariate normal density function with means μy, μc, and μn, standard deviations σy, σc, and σn, and correlation coefficients ρyc, ρyn, and ρcn The analysis of groundwater flow in nonuniform media requires a stochastic-conceptual approach in which the effects of stochastic parameter distributions on predicted hydraulic heads are analyzed with the aid of a set of Monte Carlo solutions to the pertinent boundary value problems. In this study, two one-dimensional saturated flow problems are analyzed: steady state flow between two specified heads and transient consolidation of a clay layer. The primary output is the statistical distribution of hydraulic head ϕ, through space and time, as indicated by the mean values and their standard deviations Sϕ¯(x, t) Results show that the standard deviations of the input hydrogeologic parameters, particularly σy and σc, are important index properties; changes in their values lead to different responses for even when the means μy, μc, and μn are fixed. The degree of uncertainty associated with hydraulic head predictions increases as the degree of nonuniformity of the porous medium increases. For large values of σy and σc it becomes virtually impossible to obtain meaningful hydraulic head predictions. For transient flow the output distribution of hydraulic head values is almost never normal; in some cases it approaches a uniform distribution. The results of this study throw into question the validity of the hidden assumption that underlies all deterministic groundwater modeling; namely, that it is possible to select a single value for each flow parameter in a homogeneous but nonuniform medium that is somehow representative and hence define an ‘equivalent’ uniform porous medium. For transient flow there may be no way to define an equivalent medium. The fact that nine index parameters rather than three are required to describe a nonuniform geologic formation, the large uncertainties in predicted hydraulic heads for relatively simple flow problems in nonuniform soils, and the contention that there may be no simple way to define an equivalent uniform porous medium all have important implications in the development of groundwater flow theory and in its most fundamental applications.

Drainage of groundwater resting on a sloping bed with uniform rainfall

Abstract: The differential equation derived by Childs for groundwater flow over a sloping bed, the streamlines being assumed to be parallel to the slope, is integrated for the case of ditches and uniform rainfall. Expressions are obtained for the maximum vertical water table height, the location of this maximum and of the watershed, and the water table shape. Calculated water table heights are in much better agreement with previously published experimental data than those calculated from an earlier theory based on the assumption that streamlines are horizontal. Thus Childs' revised assumption is confirmed. An approximate solution for the form of the water table under small rainfall rates is also derived.

Identification of parameters in a linear equation of groundwater flow

Abstract: An inductive method is presented for evaluating parameters in a two-dimensional linear equation describing groundwater flow. The approach employs finite difference approximations, which can be simply programed for calculation by computer. Before illustrating the method, which is applicable to both stationary and time-dependent problems, the various types of data required for evaluation in general are systematically enumerated. An assumption is introduced limiting the local variability of T, the transmissivity, and its relation to apparently similar assumptions used elsewhere is discussed. The particular aquifer chosen for illustration is the chalk of the South Downs between the rivers Adur and Ouse. The parameters are calculated on the basis of cyclic data in which annual abstraction is about 30% of the total infiltration for the year. The method gives a match to the minimum and maximum water levels during the year to within 7%.

Soil genesis in relation to groundwater and soil moisture regimes near vegreville, alberta1

Abstract: Summary The study was designed to obtain information on flow and salinity of the groundwater, and on moisture regimes above the water table at selected sites near Vegreville, Alberta, and to relate this information to features of soil genesis revealed by soil mapping and analysis. All soils on the lower slopes, unlike those at upper elevations, contained accumulations of salts and gypsum, but the depth of salt accumulation and therefore the kind of soil appeared to be related only partly to current groundwater flow directions and the depth of the water table. The sodium content of the discharging groundwater and past flow conditions also appeared to be important. Lateral flow above the water table is suggested as being of possible importance in desalinization of surface horizons.

Coupled saturated‐unsaturated transient flow in porous media: Experimental and numeric model

Abstract: Experimental data were obtained from a sector tank for the transient flow toward a well. The data included both the saturated and the unsaturated flow region. The data were used to verify the accuracy of a computer solution using an implicit numerical procedure. Agreement between the two sets of data was good. The computer solution can be adapted to a wide variety of groundwater flow problems.

Normal mode analysis of the linear equation of groundwater flow

Abstract: The study of groundwater flow, given equations of motion and boundary conditions which are mathematically linear, can be analyzed by using a normal mode approach For the fully continuous case the simplest example is the familiar double Fourier analysis Analogous results exist for the semidiscrete case, in which only time is treated as a continuous variable, and a method can be given for computing normal modes whose corresponding recession factors lie within prescribed ranges One application of this approach suggests an empirical form for the base flow component of a stream hydrograph The fully discrete case can also be treated and provides a convenient basis for the comparison of the more common approximations to the solution of the transient groundwater flow equation In particular, it can be shown why the study of model problems does not provide a useful guide to the accuracy of the alternating direction implicit method applied to more general situations

Effets des gravières sur le comportement hydrodynamique des nappes d'eau souterraines

Abstract: Alluvial formations are very valuable sources of sand and gravel. They are frequently worked down to groundwater level. The aquifer, which mostly shows promising characteristics, frequently supplies a whole region with drinking water. Quarries in the aquifer system are liable to depress or raise the groundwater level, and especially to affect water quality by chemical or biological factors or by facilitating pollutant inflow. The severity of such effects is directly dependent on the amount of water exchange taking place between the full gravel pit and the aquifer. To evaluate such effects, therefore, it is necessary to establish hydrodynamic relationships existing between the aquifer and surface water. Author reviews the factors to consider in evaluating such hydrodynamic effects, which include warping of the river banks and bed, depth of excavation and position in relation to the aquifer system, relationships with rivers, shape and dimensions of excavations and position with respect to the general direction of groundwater flow. The effects of these factors vary and can be more clearly visualised with the aid of a series of diagrams. River bank and bed warping turns out to be the vital governing factor as regards flow exchange taking place between a gravel pit and aquifers. If warping is taking place the gravel pit is an obstacle to groundwater flow ; if not, it acts as a "privileged" flow path. Present means whereby these parameters can be evaluated are discussed in the second part of the article. Without going into conventional hydrogeological approaches in detail, special reference is made to local measurement of permeability, vertical permeability distribution through the various ground horizons and, especially, evaluation of river bank and bed warping. Closer methodology, however, seems to be required for some of the points considered. Interpretation and prediction methods vary from simple calculation to use of a three-dimensional model. The usefulness of electrical analogies of the conductive paper model type, which are particularly suitable for this type of problem, is also emphasized. Reference is made to two very different types of investigation. One relates to a gravel pit extension scheme in the Crau region of France, more especially its effects on the local water table level. The other is a more basic study of optimal gravel pit and water supply operation in the Toulouse region, in which the warping problem in quarries and its variation over a period of time are considered. Warping measurements have been carried out on site, from the results of which it is expected that the effects of the gravel pits on present and future water supply yields can be predicted.

Development and Field Testing of a Basin Hydrology Simulator

Abstract: A computer model consisting of both surface water and groundwater phases and their interaction has been developed to simulate the hydrology of a stream basin. Input to the model includes precipitation, climatic conditions, boundary and initial conditions, and basin constants. Provision has been made for modeling withdrawals from both groundwater and surface water for consumptive use. The computer output includes streamflow hydrographs at selected points, groundwater levels, and groundwater recharge and discharge from the basin. The model is designed to be general in nature so that it will be applicable to different basins with similar geographic and geological conditions. Its structure is such that the basin is divided vertically into four layers and a stream network. The four vertical layers are, in order from top to bottom, a surface layer, an upper soil zone, a lower soil zone, and a groundwater layer (aquifer). Provision is also made to subdivide each layer horizontally into blocks, or subbasins, which are determined by considering topography, soil types, and climatic data. The model is described in detail in this paper, and its application to the Little Arkansas River basin of south central Kansas is made. A 25-year period of history in this basin is simulated, and comparisons are made between calculated behavior and measured hydrologie behavior in the basin.

The base flow of the Glenelg river derived from tritium concentrations

Abstract: Water samples collected during June 1966 from the Glenelg River, Vic., had tritium concentrations that decreased monotonically from 35 T.U. at the headwaters to 6 T.U. just above tidal influence. The dilution of tritium, by base flow, was interpreted to yield estimates of groundwater flow to the river from aquifers into which the river has cut its course. In a lower reach of the river approximately two-thirds of the total discharge of 2900 L sec-1 was made up by base flow additions. At a later collection in April 1970 the river was not in such a steady state as it was at the time of the earlier collection. The river was too perturbed for plausible interpretation of the observed tritium concentrations in other than three reaches of the river. Nevertheless the method and technique have general validity in humid and sub-humid regions where stream regimes are effluent.

A case history of landslide caused by confined groundwater

Abstract: An understanding of groundwater regimes is an important prerequiste to effectice stabilization of many landslides. In this study stabilization measures for confined and unconfined groundwater conditions associated with a 30,000 cubic yard translational landslide were evaluated. The effectiveness of temporary stabilization measures including horizontal drains and a drainage trench is discussed. It is concluded that confined groundwater conditions should be considered after determination of a coincidence of the slide surface and the groundwater table in residual soil.

Dear Sir: Rapid Groundwater Flow in Fissures in the Chalk: an Example from South Hampshire T. C. Atkinson and D. I. Smith

Abstract: I read the work on the Chalk groundwater of the Horndean-Havant area with interest. I entirely agree with the authors' claim that their tracer investigations provide unequivocal evidence of the existence of highly-localised, very rapid, probably turbulent, conduit flow of Chalk groundwater, at least at that location. Part of their discussion and conclusions however, namely the suggestion that similar near-karstic features are generally responsible for the widespread existence of zones of high transmissivity, is distorted and likely to lead to the results being quoted out-of-context by non-groundwater specialists. Comparable conditions are only likely to be found at occasional locations mainly along the margin of the Tertiary outcrop where Chalk solution has been accelerated by concentrated run-off or recharge of low pH; certain examples are known from the London Basin (Water Resources Board 1972). Subaerial weathering on the Tertiary erosion surface will also have contributed to the development of secondary permeability. Indeed, is it not possible that the main conduits of the flow system described in the paper are developed along the Cretaceous-Tertiary unconformity? Now high fissure transmissivity is one thing but near-karstic conduits (in which transmissivity is a meaningless concept and for which the basic laws of porous media hydraulics do not even approximately apply) are another. It is fair to say that high fissure transmissivities (over 500 m2/d and often over 1000 m2/d) are developed over wide areas of the Chalk outcrop intake area. It is such areas that are of greatest volumetric importance and most topical interest in