Main Characteristics of an Aquifer The main function of the aquifer is to provide underground storage for the retention and release of gravitational water. Aquifers can be characterized by indices that reflect their ability to recover moisture held in pores in the earth (only the large pores give up their water easily). These indices are related to the volume of exploitable water. Other aquifer characteristics include: • Effective porosity corresponds to the ratio of the volume of “gravitational” water at saturation, which is released under the effect of gravity, to the total volume of the medium containing this water. It generally varies between 0.1% and 30%. Effective porosity is a parameter determined in the laboratory or in the field. • Storage coefficient is the ratio of the water volume released or stored, per unit of area of the aquifer, to the corresponding variations in hydraulic head 'h. The storage coefficient is used to characterize the volume of useable water more precisely, and governs the storage of gravitational water in the reservoir voids. This coefficient is extremely low for confined groundwater; in fact, it represents the degree of the water compression. • Hydraulic conductivity at saturation relates to Darcy’s law and characterizes the effect of resistance to flow due to friction forces. These forces are a function of the characteristics of the soil matrix, and of the fluid viscosity. It is determined in the laboratory or directly in the field by a pumping test. • Transmissivity is the discharge of water that flows from an aquifer per unit width under the effect of a unit of hydraulic gradient. It is equal to the product of the saturation hydraulic conductivity and of the thickness (height) of the groundwater. • Diffusivity characterizes the speed of the aquifer response to a disturbance: (variations in the water level of a river or the groundwater, pumping). It is expressed by the ratio between the transmissivity and the storage coefficient. Effective and Fictitious Flow Velocity: Groundwater Discharge As we saw earlier in this chapter, water flow through permeable layers in saturated zones is governed by Darcy’s Law. The flow velocity is in reality the fictitious velocity of the water flowing through the total flow section. Bearing in mind that a section is not necessarily representative of the entire soil mass, Figure 7.7 illustrates how flow does not follow a straight path through a section; in fact, the water flows much more rapidly through the available pathways (the tortuosity effect). The groundwater discharge Q is the volume of water per unit of time that flows through a cross-section of aquifer under the effect of a given hydraulic gradient. The discharge of a groundwater aquifer through a specified soil section can be expressed by the equation:

Hydrology ◽  
2010 ◽  
pp. 229-230
Keyword(s):  
Hydraulic Conductivity ◽  
Flow Velocity ◽  
Darcy’S Law ◽  
Groundwater Discharge ◽  
Effective Porosity ◽  
Darcy's Law ◽  
Hydraulic Gradient ◽  
Water Volume ◽  
Storage Coefficient ◽  
Groundwater Aquifer

Incorporation of Interfacial Areas in Models of Two-Phase Flow

1999 ◽  
Author(s):  
William G. Gray ◽  
Michael A. Celia
Keyword(s):  
Porous Media ◽  
Hydraulic Conductivity ◽  
Single Phase ◽  
Darcy’S Law ◽  
Darcy's Law ◽  
Hydraulic Gradient ◽  
Phase Flow ◽  
Single Phase Flow ◽  
Α Phase ◽  
Flow Through

The mathematical study of flow in porous media is typically based on the 1856 empirical result of Henri Darcy. This result, known as Darcy’s law, states that the velocity of a single-phase flow through a porous medium is proportional to the hydraulic gradient. The publication of Darcy’s work has been referred to as “the birth of groundwater hydrology as a quantitative science” (Freeze and Cherry, 1979). Although Darcy’s original equation was found to be valid for slow, steady, one-dimensional, single-phase flow through a homogeneous and isotropic sand, it has been applied in the succeeding 140 years to complex transient flows that involve multiple phases in heterogeneous media. To attain this generality, a modification has been made to the original formula, such that the constant of proportionality between flow and hydraulic gradient is allowed to be a spatially varying function of the system properties. The extended version of Darcy’s law is expressed in the following form: qα=-Kα . Jα (2.1) where qα is the volumetric flow rate per unit area vector of the α-phase fluid, Kα is the hydraulic conductivity tensor of the α-phase and is a function of the viscosity and saturation of the α-phase and of the solid matrix, and Jα is the vector hydraulic gradient that drives the flow. The quantities Jα and Kα account for pressure and gravitational effects as well as the interactions that occur between adjacent phases. Although this generalization is occasionally criticized for its shortcomings, equation (2.1) is considered today to be a fundamental principle in analysis of porous media flows (e.g., McWhorter and Sunada, 1977). If, indeed, Darcy’s experimental result is the birth of quantitative hydrology, a need still remains to build quantitative analysis of porous media flow on a strong theoretical foundation. The problem of unsaturated flow of water has been attacked using experimental and theoretical tools since the early part of this century. Sposito (1986) attributes the beginnings of the study of soil water flow as a subdiscipline of physics to the fundamental work of Buckingham (1907), which uses a saturation-dependent hydraulic conductivity and a capillary potential for the hydraulic gradient.

Experimental Study on the Piping Erosion Mechanism of Gap-Graded Soils Under a Supercritical Hydraulic Gradient

2021 ◽  
Author(s):  
Liang Chen ◽  
Yu Wan ◽  
Jian-Jian He ◽  
Chun-Mu Luo ◽  
Shu-fa Yan ◽  
...  
Keyword(s):  
Hydraulic Conductivity ◽  
Unsteady Flow ◽  
Relative Density ◽  
Flow Velocity ◽  
Steady Flow ◽  
Fine Particle ◽  
Failure Process ◽  
Hydraulic Gradient ◽  
Particle Content ◽  
Piping Erosion

Abstract Seepage-induced piping erosion is observed in many geotechnical structures. This paper studies the piping mechanism of gap-graded soils during the whole piping erosion failure process under a supercritical hydraulic gradient. We define the supercritical ratio Ri and study the change in the parameters such as the flow velocity, hydraulic conductivity, and fine particle loss with Ri. Under steady flow, a formula for determining the flow velocity state of the sample with Ri according to the fine particle content and relative density of the sample was proposed; during the piping failure process, the influence of Rimax on the rate at which the flow velocity and hydraulic conductivity of the sample increase as Ri decreases was greater than that of the initial relative density and the initial fine particle content of the sample. Under unsteady flow, a larger initial relative density corresponds to a smaller amplitude of increase in the average value of the peak flow velocity with increasing Ri. Compared with the test under steady flow, the flow velocity under unsteady flow would experience abrupt changes. The relative position of the trend line L of the flow velocity varying with Ri under unsteady flow and the fixed peak water head height point A under steady flow were related to the relative density of the sample.

Measuring the saturated hydraulic conductivity of peat substrates in nursery containers

1994 ◽  
Vol 74 (4) ◽  
pp. 431-437 ◽  
Author(s):  
S. E. Allaire ◽  
J. Caron ◽  
J. Gallichand
Keyword(s):  
Hydraulic Conductivity ◽  
Correction Factor ◽  
Darcy’S Law ◽  
Water Flux ◽  
Darcy's Law ◽  
Saturated Hydraulic Conductivity ◽  
Disruptive Effect ◽  
Flow Measurements ◽  
Flux Measurements ◽  
Made In

Pore size, distribution and continuity are important characteristics for the exchange and storage of air and water in artificial mixes. Saturated hydraulic conductivity (Ks) measurements can be used to obtain such a characterization. However, two difficulties are encountered when using Ks in potting media. First, the validity of Ks may be limited because it may not apply in media composed of coarse material or peat. Second, the structure of peat substrates is very sensitive and in situ measurements of potted peat substrates (i.e. measurements made directly in the pots) should be carried out to avoid any disruptive effect due to handling. Such a measurement, when made in pots, may require the evaluation of the water flux reduction resulting from the container outflow configuration. The objectives of this study were therefore to check the validity of Darcy’s law for peat substrates and to propose an approach for estimating the saturated hydraulic conductivity from flow measurements made in nursery containers. For three different substrates, water flow in artificial mixes followed Darcy’s law for hydraulic gradients ranging from 1.1 to 1.6 cm cm−1. Experimental results showed that the measured fluxes in 5-L nursery container filled at five different substrate heights (9, 11.5, 14, 16.5 and 19 cm) with laterally located drainage holes were significantly different from those measured in pots with the bottom removed (therefore equivalent to measurement currently made in cylinders) at P = 0.0022. Fluxes in containers with bottoms removed were 7–31% higher than in intact pots. Water flux measurements may therefore need to be corrected for this flux reduction in order to accurately estimate hydraulic conductivity from flow experiments run in pots. A correction factor based on the results obtained from a finite difference model was derived and calibrated. Then, this correction factor was used to convert flux measurements made in pots with lateral holes into equivalent flux that would have been obtained had the pot had an open bottom. After correction, no significant flux reductions were found between pots with open bottoms and pots with lateral holes (P = 0.55). A correction factor estimated from Laplace’s equation, once calibrated, can therefore be applied to flux measurements obtained from pots to obtain estimates of Ks of undisturbed potted media. Key words: Hydraulic conductivity, peat substrates, container

scholarly journals Evapotranspiration computed by Darcy’s Law: Sudan case study

2005 ◽  
Vol 2 (4) ◽  
pp. 1787-1806 ◽  
Author(s):  
O. A. E. Abdalla
Keyword(s):  
Groundwater Level ◽  
Darcy’S Law ◽  
Flow Direction ◽  
Cost Effective ◽  
Effective Porosity ◽  
Darcy's Law ◽  
Present System ◽  
Central Sudan ◽  
White Nile ◽  
Semi Arid

Abstract. The present study applies Darcy's Law to compute evapotranspiration in the arid to semi-arid central Sudan. The average decline in groundwater level (s) along a distance (L) of the aquifer's cross section was calculated. Such decline is a function of discharge Q at any point across the unit width of the aquifer and effective porosity. Groundwater in the study area generally flows from NW to the SE along basin axial trough and is characterized by variable hydraulic gradient. As the aquifer discharge is directly proportional to the gradient, different values of groundwater level decline were calculated along the flow direction. The hydrogeological map constructed during this study indicates that the system is hydrologicaly closed and groundwater doesn't discharge in the neighboring White Nile River. Geological, hydrological and climatological settings of the discharge area demonstrate that evapotranspiration is the main mechanism of groundwater discharge and reveals that the area is suited for the application of Darcy's Law to compute evapotranspiration. Evapotranspiration was estimated from Darcy's law to be 1.2 mm/a and is sufficient to balance the present system. Greater similarity in geology, hydrology, climate and vegetation encourages the application of Darcy's Law in the Sahara and sub-Sahara to compute for evapotranspiration. Such cost effective method can be applied in arid to semi-arid areas if conditions are favorable.

Sign in / Sign up

Export Citation Format

Share Document