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Hydro 2 Flashcards

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1
Q

Unconfined Aquifer

A

Aquifer that is close to the land surface, with layers of high intrinsic permeability extending from the land surface to the base of the aquifer.

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2
Q

How are unconfined aquifers recharged?

A

Either from downward seepage, lateral flow, or upward seepage.

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3
Q

Confined Aquifers

A

Aquifers with an overlying confining layer

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4
Q

How are unconfined aquifers recharged?

A

Can occur either from a recharge area where the aquifer crops out or from leaking of confining layer.

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5
Q

Perched Aquifer

A

A layer of saturated soil perched above the main aquifer.

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6
Q

How does hydraulic conductivity affect discharge?

A

Q=kiA, therefore a higher K value will result in higher discharge and a lower K value will result in a lower discharge.

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7
Q

How does grain size affect K?

A

Typically, the larger the sediments and thus the pore space, the higher the hydraulic conductivity (but also dependent on sorting), so for well sorted sediments, K will increase with grain size. Also, larger sediments show a greater decrease in permeability with an increase in standard deviation.

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8
Q

How does sorting affect K?

A

The poorer sorted a material is, the lower the hydraulic conductivity.

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9
Q

What is meant by “ K is a material constant?

A

1

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10
Q

What is the relationship between K and fluid Dynamic Viscosity?

A

K=Ki[(specific weight)/(dynamic viscosity)]

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11
Q

What is Ki?

A

Ki is intrinsic permeability, which represents the properties of the porous media alone.

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12
Q

What is dynamic viscosity?

A

A measure of the resistance of the fluid to the shearing that is necessary for fluid flow

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13
Q

What is the relationship between K and intrinsic permiability?

A

K=Ki[(specific weight)/(dynamic viscosity)], the higher the intrinsic permeability, the higher hydraulic conductivity will be.

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14
Q

What is the relationship between K and fluid density?

A

K=Ki(density*g/dynamic viscosity), the higher the density, the higher the hydraulic conductivity.

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15
Q

What is the relationship between K and fluid Dynamic Viscosity?

A

K=Ki[(specific weight)/(dynamic viscosity)] or K=ki[gamma/mu], so the higher the viscosity the lower the hydraulic conductivity.

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16
Q

What is the relationship between K and temperature?

A

K=Ki[(specific weight)/(dynamic viscosity)], dynamic viscosity is a partially determined by temperature, and the lower the temperature the higher the viscosity. So the lower the temperature, the lower hydraulic conductivity will be.

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17
Q

Storativity (S)

A

The volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head. It is dimensionless

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18
Q

Transmissivity

A

A measure of the amount of water that can be transmitted horizontally through a unit width by the full saturated thickness of the aquifer under a hydraulic gradient. T=bK

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19
Q

What is the transmissivity of multi layer aquifers?

A

The sum of the transmissivity of each layer.

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20
Q

Specific Storage

A

The amount of water per unit volume of a saturated formation that is stored or expelled from storage owing to the compressibility of the mineral skeleton and the pore water per unit change in head. Also called the elastic storage coefficient.

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21
Q

Homgenous

A

contains the same properties at all locations. Ex. Same grain size, porosity, degree of cementation, thickness, only variable within small limits. Transmisivity and storativity are the same everywhere.

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22
Q

Heterogeneous

A

Hydraulic properties change spacially.

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23
Q

isotropic

A

The intrinsic permeability is the same in all DIRECTIONS.

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24
Q

anisotropic

A

The intrinsic permeability may different in one direction then from another.

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25
What are some reasons that anisotropy might occur?
Non-uniform grain geometry, Grain shape may be more like a book shape and thus permeability is greater parallel to the grains. Fractures- Direction of flow constrained to direction of fractures. In basalt units, flow direction is parallel to the dip of the flow as it moves in the interflow zones. Shrinkage cracks.
26
What is the difference between specific discharge and seepage velocity?
Specific Discharge - represents the velocity at which water would move through an aquifer if the aquifer were an open conduit. Speepage Velocity - a velocity representing the average rate at which the water moves between two points.
27
What is the equation for seepage velocity?
v(x) = Q/nA = - (K/n)(dh/dl)
28
What is the equation for Darcy's law?
V= -K(dh/dl)
29
What is Specific Retention?
the ratio of the volume of water a rock can retain against gravity drainage to the total volume of the rock. n = sy + sr
30
Specific Yield
The ratio of the volume of water that drains from a saturated rock owing to the attraction of gravity to the total volume of the rock.
31
Piezometer
Device used to measure the total energy of the fluid flowing through a pipe packed with sand. Water rises within the pipe in direct proportion to the total fluid energy at the point at which the bottom of the piezometer is open to the sand.
32
What is a "darcy" equivalent to?
1 darcy = 10^(-8) cm^2
33
How do you calculate the storativity for a confined aquifer?
S=bSs
34
How do you calculate the storativity for an unconfined aquifer?
S=Sy+bSs
35
What are the values for storativity for a confined aquifer?
Generally 0.005 or less
36
What are the values for storativity for an unconfined aquifer?
Ranges from 0.02 to 0.03
37
What is hydraulic head?
The total mechanical energy per unit weight. h
38
What does z represent in Bernoulli's equation?
The gravitational potential energy
39
What does P/(rho)g represent in Bernoulli's equation?
Potential energy due to pressure
40
What does V^2/2g represent in Bernoulli's equation.
The kinetic energy due to flow.
41
How is hydraulic gradient calculated for a well nest?
It is the difference in total head divided by the vertical distance between the piezometer.
42
Compare and contrast the hydraulic head in a confined aquifer vs. unconfined aquifer.
The hydraulic head in an unconfined aquifer is the same as the water table. In a confined aquifer it is not. In a confined aquifer, the water level in a piezometer represents the potentiometric surface.
43
What is a potentiometric surface?
In a confined aquifer it represents the level in which the water would rise in a cased well.
44
How is hydraulic gradient calculated?
The hydraulic gradient represents the slope of the potentiometric surface b/w two points.
45
In flow map, what is the direction of the gradient?
The direction of increasing head and perpendicular to contours.
46
In an unconfined aquifer, what is the storativity equivalent to?
The storativity is usually taken to be equivalent to the specific yield because the specific storage is usually very small.
47
What is the porosity range for well sorted sand or gravel?
25-50%
48
What is the porosity range for sand and gravel mixed?
20-35%
49
What is the porosity range for well sorted silt?
35-50%
50
What is the porosity range for clay?
33-60%
51
How do you find Q moving through a strip of soil.
Multiply velocity times the area times. Note, that seepage velocity isn't used. Or could use seepage velocity and then multiply by porosity to account only for area of pore space.
52
water table
The undulating surface where pore water pressure is equal to water pressure.
53
When is Darcy's law valid?
Flow is laminar and resistive forces of viscosity predominate, Re less then 1 and to ten. Thus applicable with very slow moving ground waters. May not be applicable in steep hydraulic gradients, such as in areas of pumping
54
How do you calculate the consolidation in a confined aquifer?
Use alpha = (db/b)(1/dp)
55
What does the negative sign in Darcy's equation indicate?
The flow is flowing from a higher hydraulic gradient to a lower one.
56
Which elements of storativity are important in unconfined aquifers?
The specific yield
57
Which elements are important in storativity of confined aquifers?
All the water released is accounted for by the compressibility of the mineral skeleton and the pore water pressure. Water comes from the entire thickness of the aquifer.
58
What are the components of specific storage.
water released due to the compression of the water skeleton and water released from pore water pressure.
59
What happens when the head increases in an aquifer?
The mineral skeleton will expand while the water will contract.
60
How is flow calculated in an aquifer.
Q=kb(dh/dl)*width, dont do seepage velocity.
61
What are the Dupuit assumptions?
1. ) The hydraulic gradient in in the direction of the water table 2. ) For small water table gradients, the streamlines are horizontal and the equipotential is vertical.
62
How can you tell if an aquifer is confined from the DE's of flow?
h^2 will be present.
63
How can you tell if a flow DE is not homogenous/ isotropic?
Kxx, Kyy, Kzz, will be present
64
How can you tell if a DE is steady state?
It will be equal to zero.
65
What is the difference between steady state flow and non-steady state flow?
In steady state flow, there is no change in head with time.
66
How could geological formations affect homogeneity.
The thickness of the unit may change and thus is heterogenous. A facies change may occur. Sedimentary units may be deposited in layers and the properties of each layer may be different.
67
What will happen in a unit with lower k?
Lower K units use more energy to move water, have a faster head loss.
68
What is K in horizontal in comparison to K vertical?
Kx is usually greater then Kv. Kv is about 1/10 to 1/3 of Kx.
69
For an unconfined aquifer, how do you find the water extracted?
Use specific yield. Sy=Vdrained/Vtotal
70
How do you find volume removed in confined aquifer?
Use S equation and set bs equal to the equation on sheet.
71
What are values of j in texturally mature sediments for shepherds method?
j=2 for glass spheres, j =1.5 for consolidated sediments. | Texturally mature have j of 1.75 or greater. Immature can be as low as 1.5
72
Effective grain size
d10
73
Uniformity Coefficient
d60/d10
74
What is the permeability range for clay?
-9 to -6
75
What is the permeability range for well sorted sand
-3 to -1
76
What is the permeability range for silty sands clayey sands, mixed.
-6 to -4
77
What is the permeability range for find sands
-5 to -3

Hydrogeology (6 decks)

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