Exam 5 - Groundwater Flashcards
hydrologic cycle
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
stream
In Geology, any channelized flow of water
rivers, creeks, streams, brooks, runs, etc.
stream velocity
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
stream gradient
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
calculating gradient
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
roughness of channel
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
shape of channel
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
frictional contact of a stream
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
discharge
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!!
longitudinal profile
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
stream velocity review
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
alluvial fans
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
braided streams
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
meandering streams
Well downstream from braided streams
Low gradient (flat) and high discharge
Nearer to mouth
Stream moves back and forth within a broad, flat floodplain
why do streams meander?
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
meandering stream features
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
flood plains
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
stream load
=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
stream load capacity
Capacity= maximum load a stream can transport
Must measure all 3 components of stream load
Difficult to do, so instead we measure…
stream load competence
Competence= maximum size clast a stream is capable of transporting
I.e. just measure the biggest thing moving in the stream!
floods
= flow of water greater than normal, average discharge
BOTH capacity and competence increase during flood situations
upstream floods
Sudden, high velocity
Narrow channels in mountainous areas
Affecting alluvial fans, braided streams
= ‘flash’ floods
downstream floods
Slow, filling of flood plains
Time scale is days, weeks, months
Can radically change meandering stream patterns
recurrence interval
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)
recurrence interval example
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
flood probability
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 %)
flood probability example
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
flood mitigation
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
water table
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
effluent streams
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)
influent streams
Also called losing streams Dry climatesLow water tables (WT) Streams do not intersect Earth surface WT is the inverse of topography So... topographic lows (valleys) are WT highs (=recharge mound)
porosity
Porosity = total volume of rocks/sediments that consists of pore spaces
Sediments usually ~ 10 to 50% porosity
Rocks usually much less
calculating porosity
Porosity = volume of pore space/ total volume of sample X 100
E.g. 250cm3/1000cm3= 0.25 X 100
= 25% porosity
permeability
= 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
specific yield
That portion of the groundwater that will drain due to gravity
specific retention
That portion of the groundwater that is retained around the clasts
aquifer
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
aquiclude
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
artesian
Any situation in which groundwater under pressure rises above the aquifer
Can have artesian wells or natural springs
recharge and discharge
Recharge area is where precipitation is “soaked into the aquifer
Discharge is where groundwater is removed ◦Natural or artificial
cone of depression
Conical shapes of water table developed when groundwater is removed by active pimping
Can drawdownentire water table forcing well deepening
saltwater intrusion
Excessive drawdown in coastal areas
Brings ocean groundwaters inland, eventually up into wells
Need to reinject freshwater force back towards ocean◦Time & $ consuming
residence time
= 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!
groundwater pollution
Multiple point sources of pollution
Chemical and biological
Extremely expensive $$ to correct
