Exam 5 - Groundwater

Question 1

hydrologic cycle

Answer 1

low of water on/above the earth

Geologists are most interested in runoff over Earth’s surface and within earth

Water running over Earth’s does geologic ‘work’ ◦E.g. Erosion, transport of sediments

Question 2

stream

Answer 2

In Geology, any channelized flow of water

rivers, creeks, streams, brooks, runs, etc.

Question 3

stream velocity

Answer 3

Stream velocity or speed determined how much geologic work can be accomplished

Stream velocity is directly proportional to erosional and transport work
I.e. the faster the stream, the more work it can do

Question 4

stream gradient

Answer 4

Slope of the stream channel

Also = the vertical drop of the stream over a fixed distance
In general, high gradient near mountains, lower near basins (like oceans)

Measured as a unit of length (vertical drop) by a unit of distance (overland distance)

E.g. 1000ft/mile in the mountains and 10 ft.mile downstream

Question 5

calculating gradient

Answer 5

Step1 –measure the distance between 2 points
Step2 –determine the elevations at the high point (upstream) and the low point (downstream)
Step3-subtract the low elevation from the high elevation to get the difference
Step4 –divide the distance from step1 by the difference from step3

Question 6

roughness of channel

Answer 6

Water flowing without obstruction will move in straight lines as laminar flow

In most streams there are many obstructions (rocks, logs, boats, bridges, etc.) that make the flow divert into SLOWER, turbulent flow

Question 7

shape of channel

Answer 7

Related to the cross-sectional area of the stream

Frictional contact of the water with the bottom shape of the channel can be calculated

The more frictional contact that water in a stream has with the bottom of the channel, the lower the stream velocity

Question 8

frictional contact of a stream

Answer 8

Stream A is 1 foot deep and 10 feet across
Frictional contact is calculating by adding the depth + width + depth again
Here» 1 + 10 + 1 = 12 feet contact
NOTE the area of Stream A is 1 X 10ft or 10ft

lower contact feet results in higher stream velocity because of lower frictional contact with the channel

Question 9

discharge

Answer 9

Amount of water flowing past a certain point, in a given unit of time

Units are usually in ft3/second or m3/second

Discharge (ft3/s) = channel width (ft) X channel depth (ft) X stream velocity (ft/s)

E.g. Stream A: 10ft X 1ft X 5ft/s = 50ft3/s

So 50 cubic feet flow by any point in stream A each second.

Seems like a high number, BUT actual discharge of the Mississippi River in Louisiana is over 17000ft3/s!!

Question 10

longitudinal profile

Answer 10

A cross sectional view of a stream from its source to the mouth

Near source» high gradients, more erosion, BUT lower discharge

Near mouth» low gradients, more deposition, high discharge

Question 11

stream velocity review

Answer 11

As stream velocity increases, the amount of geologic work (e.g. erosion and deposition of sediments) also increases

Stream velocity is primarily affected by gradient, channel roughness, channel shape and stream discharge

As velocity varies downstream, different sedimentary environments are created by the stream

Question 12

alluvial fans

Answer 12

Created as streams first leave the high mountainous areas where their source is located

So… found in high gradient, low discharge areas

Sediments are dumped into big piles called alluvial fans

Question 13

braided streams

Answer 13

Downstream from alluvial fans

Gradient is decreasing as discharge is increasing

Lots of gravel bars deposited in channel as sands and smaller clasts are carried

Question 14

meandering streams

Answer 14

Well downstream from braided streams

Low gradient (flat) and high discharge

Nearer to mouth

Stream moves back and forth within a broad, flat floodplain

Question 15

why do streams meander?

Answer 15

As water moves downstream, it spirals around in the channel, creating areas of higher velocity

More erosion occurs on one side of stream, causing meander

Question 16

meandering stream features

Answer 16

High velocity erodes steep cut banks

Lower velocity deposit sands as point bars

Each outer curve of a stream channel has a point bar, while each inner curve has a point bar

Meandering caused by erosion keeps continuing until meanders cause a cutoff to occur

Creates an oxbow lake + a new, straighter channel

Keeps meandering

Question 17

flood plains

Answer 17

Meandering streams keep cutting off and migrating, but remain within the boundaries of a flood plain

Flood Plain = flat area near mouth, bounded laterally by low highlands

Can be many kilometers wide

During flood situations, the plain can/does fill with water

Question 18

stream load

Answer 18

=the weathered materials carried by the stream

1) dissolved load = dissolved ions (atoms) in solution (from chemical weathering)
2) suspended load = small size clasts (silts & muds) carried up within the flowing stream
3) bed load = larger clasts dragged or bouncing along bottom of channel

Question 19

stream load capacity

Answer 19

Capacity= maximum load a stream can transport

Must measure all 3 components of stream load

Difficult to do, so instead we measure…

Question 20

stream load competence

Answer 20

Competence= maximum size clast a stream is capable of transporting

I.e. just measure the biggest thing moving in the stream!

Question 21

floods

Answer 21

= flow of water greater than normal, average discharge

BOTH capacity and competence increase during flood situations

Question 22

upstream floods

Answer 22

Sudden, high velocity

Narrow channels in mountainous areas

Affecting alluvial fans, braided streams

= ‘flash’ floods

Question 23

downstream floods

Answer 23

Slow, filling of flood plains

Time scale is days, weeks, months

Can radically change meandering stream patterns

Question 24

recurrence interval

Answer 24

The length of time between flood ‘events’ of equal magnitude (size)

may be calculated

Predicts how often flooding of various sizes may occur
R (recurrence interval) = (n+1)/M
N= number of years studied
M= rank magnitude of flood (1=biggest)

Question 25

recurrence interval example

Answer 25

For this river (in Ohio) , N = 75 years

What is the recurrence interval (R) for flood as big as the 1892 flood (rank 2)?
R = 75 + 1/ 2 = 38 years
So.. A flood as big as the one in 1892 will occur, on average, every 38 years

Question 26

flood probability

Answer 26

The chance that a flood of a given size (M) will occur within any given year
“What are my odds of having a flood this big … this year?”
** may be calculated
FP = 1/R X 100 (for %)

Question 27

flood probability example

Answer 27

What is the probability of a flood as big as the 1892 flood (M=2)happening to me this year (or any year)?
FP = 1/R X 100
FP = 1/38yrs X 100 = 0.03 X100 = 3% chance
FP = 1/5.1yrs X 100 = 0.20 X100 = 20% chance
FP = 1/1.3yrs X 100 = 0.77 X

Question 28

flood mitigation

Answer 28

Geologic mapping identifies flood plains
R, FP calculated for all parts of flood plain
CHOICE > accept risk and build or avoid
Dams, artificial levees, etc. change but don’t eliminate risk
Flood insurance

Question 29

water table

Answer 29

Level that water creates as it sinks into the Earth’s surface
WT = boundary between 2 zones
Zone of aeration = vadose zone
Pore spaces filled with air
Zone of saturation
Pore spaces filled with water

Not always level (like a ‘table’), but can be very irregular
Lakes and streams (surface water) are places where water table intersects Earth’s surface

Question 30

effluent streams

Answer 30

Also called gaining streams
Wet climates
High water tables (WT)
Streams intersect Earth surface
WT mimics topography◦So... topographic highs (hills) are also water table highs (hills)

Question 31

influent streams

Answer 31

Also called losing streams
Dry climatesLow water tables (WT)
Streams do not intersect Earth surface
WT is the inverse of topography
So... topographic lows (valleys) are WT highs (=recharge mound)

Question 32

porosity

Answer 32

Porosity = total volume of rocks/sediments that consists of pore spaces
Sediments usually ~ 10 to 50% porosity
Rocks usually much less

Question 33

calculating porosity

Answer 33

Porosity = volume of pore space/ total volume of sample X 100
E.g. 250cm3/1000cm3= 0.25 X 100
= 25% porosity

Question 34

permeability

Answer 34

= ability of a material to transmit a fluid
** smaller pore spaces are harder to flow through
So… 2 rocks can have the same porosity, but the one with smaller pores has lower permeability

Question 35

specific yield

Answer 35

That portion of the groundwater that will drain due to gravity

Question 36

specific retention

Answer 36

That portion of the groundwater that is retained around the clasts

Question 37

aquifer

Answer 37

Rock or soil type that transmits groundwater freely
E.g. sandstones, some limestones, most soils
*** have high porosity, high permeability, high specific yield, low specific retention

A “Good” aquifer has:
High porosity
High permeability
High specific yield
Low specific retention

Question 38

aquiclude

Answer 38

Rock or soil type that prevents groundwater flow
Also called aquitards
E.g. shales, most ig/met rocks
*** have low porosity, low permeability, low specific yield, high specific retention (if water gets inside in the first place)

A “Good” aquiclude has:
Low porosity
Low permeability
Low specific yield
Low specific retention

Question 39

artesian

Answer 39

Any situation in which groundwater under pressure rises above the aquifer
Can have artesian wells or natural springs

Question 40

recharge and discharge

Answer 40

Recharge area is where precipitation is “soaked into the aquifer

Discharge is where groundwater is removed ◦Natural or artificial

Question 41

cone of depression

Answer 41

Conical shapes of water table developed when groundwater is removed by active pimping
Can drawdownentire water table forcing well deepening

Question 42

saltwater intrusion

Answer 42

Excessive drawdown in coastal areas
Brings ocean groundwaters inland, eventually up into wells
Need to reinject freshwater force back towards ocean◦Time & $ consuming

Question 43

residence time

Answer 43

= the total time a given pollutant remains within a “system”

Short residence times in atmosphere, river, etc. systems

Res. Time in groundwater can be 1000’s of years!

Question 44

groundwater pollution

Answer 44

Multiple point sources of pollution
Chemical and biological
Extremely expensive $$ to correct