Final Study Guide 8

Question 1

Sketch the hydrologic cycle. What factors determine whether rainfall will run off or infiltrate?

Answer 1

Whether the water runs off or infiltrates is controlled by a number of mechanisms including precipitation rate (how hard it’s raining), soil water content (is it already saturated?), slope (steeper = more run-off) and vegetation (more veg. holds water).

Question 2

Draw a graph showing the equilibrium water content of a parcel of air vs. temperature. Use this graph to explain why it rains (2 different causes).

Answer 2

1. Two air masses near saturation, but with different temperatures, are mixed
2. Air rises, causing it to cool, so cant hold as much water, so precipitates

Question 3

Draw a cross-section across a river channel in an area of gentle topography. Label the following: bank, levee, flood plain, channel.

Answer 3

• Alter hydrologic budget through changes in vegetation, impermeable area, construction, dams, flood control basins
• Channelization = straighten (and shorten) channels to improve drainage. This increases gradient in channel, leading to higher velocity and more erosion
• Impact on sediment delivery to deltas, beaches. Reduction in flow cuts sediment delivery. Debris basins intercept sand.

Question 4

Which portion of a stream meander will be eroded? Why? What is the most critical factor for determining whether erosion or deposition will occur?

Answer 4

• the outer side of the curve, because the velocity is faster on the outer side than on the inner curve. The speed/velocity of the river in relation to the curve.

Question 5

Why are dams constructed?

Answer 5

• Millions of people depend on dams to help provide the right amount of water in the right place at the right time. Lots of dams provide water for growing crops and for farm animals to drink. They also store water for fighting fires and can help control floods. Dams also give us hydroelectric power.
• Renewable energy that does not pollute. A dam can cause floods and thus kill people.

Question 6

What problems can each solve or cause?

Answer 6

Solve:

• Flood control
• Hydroelectric power
• Water storage for dry times

Cause:

• Loss of water through evaporation and infiltration
• Loss of land and some recreational opportunities
• Seismicity
• Downstream impacts: less water, salinization and erosion
• Siltation in reservoirs
• Barrier to fish migration for spawning

Question 7

What is the meaning of recurrence interval? How are event frequency and recurrence interval related?

Answer 7

• An estimate of the interval of time between events like an earthquake, flood or a river discharge flow of certain intensity or size
• When geologic events are random or quasi-random, it is helpful to represent their frequency as an average time between past events, a “recurrence interval” also known as a “return time.”

Question 8

Do you expect floods to occur at regularly spaced intervals? Explain why or why not. How can we predict the size of a “100 year flood” on the basis of 20 years of stream gauging?

Answer 8

• We cannot expected them to be at regularly spaced intervals. Certain environmental, natural phenomena can occur, disrupting interval times
• human activities can also cause floods
• In the 1960’s, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100 years, it often is referred to as the “100-year flood”.

Question 9

What is residence time? How can it be calculated, and what assumption is involved? What are the major reservoirs of water in the earth’s crust, and approximately what is the residence time of water in each?

Answer 9

• If a dynamic system is gaining and losing material at equal rates, the mass of the system is in a steady state, meaning that it doesn’t change over time.
• dM/dt=Input - Output=0 (Input is equal to output)
• Residence Time=System Mass/Input or Output
• Reservoirs of Water:
  > Atmospheres can accumulate water quickly to produce a new storm with a residence time of 2 weeks.
  > Groundwater has long residence times and is not quickly replaced, with a residence time of 10-10,000 y.
  > Oceans have the longest residence times at 4,000 y. (10,000 y > 4,000 y)

Question 10

What factors can lead to flooding?

Answer 10

• Heavy rain
• Snowmelt
• Dam failure
• Modification of flood plains and levees
• Coastal floods from storm surge

Question 11

How can we try to mitigate the impact of floods? Can upstream flood control increase downstream flooding? Briefly explain. What is channelization?

Answer 11

• Channelization = Straighten (and shorten) channels to improve drainage–modify the channel so that it more efficiently transports water–thus decreasing the chance of flooding
• This increases gradient in channel, leading to higher velocity and more erosion

Question 12

Write Darcy’s Law and identify terms.

Answer 12

• Q(volume of water)=A(cross sectional area flow) x (K(permeability)x H(vertical drop)/ L(flow distance))
• A law in geology describing the rate at which a fluid flows through a permeable medium. Darcy’s law states that this rate is directly proportional to the drop in vertical elevation between two places in the medium and indirectly proportional to the distance between them. The law is used to describe the flow of water from one part of an aquifer to another and the flow of petroleum through sandstone and gravel.

Question 13

Which rock types have the greatest hydraulic conductivity? Why?

Answer 13

• Gravel, because they have a high intrinsic permeability

Question 14

What geologic characteristics make a formation a good aquifer? Why?

Answer 14

An aquifer is a rock material with high permeability that acts as a transport system.

• Sands and gravels
• Jointed Units
• Water occupies pours
• Sandstones and limestones
• Low permeability layers

Question 15

How is hydraulic head (liquid pressure) measured?

Answer 15

It is usually measured as a liquid surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer

Question 16

Flood Plain

Answer 16

• an area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding

Question 17

Erosion

Answer 17

wearing away, abrasion, attrition; weathering; dissolution, corrosion, decay; deterioration, disintegration, destruction. the process of wearing off or being eroded by wind, water, or other natural agents the gradual destruction or diminution of something

Question 18

Delta

Answer 18

a landform that is formed at the mouth of a river, where the river flows into an ocean, sea, estuary, lake, or reservoir, Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river.

Question 19

Ogallala Aquifer

Answer 19

Giant aquifer in Midwest, whose water level changes since 1940 have appeared to be between a decrease in 15 meters and an increase in 3 meters

Question 20

Drainage basin

Answer 20

catchment area; area of land where surface water from rain converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean

Question 21

Natural levee

Answer 21

An elongated embankment compounded of sand and silt and deposited along both banks of a river channel during times of flood.

Question 22

Saltation

Answer 22

Transport of a sediment in which the particles are moved forward in a series of short intermittent bounces from a bottom surface.

Question 23

Antecedent Stream

Answer 23

A stream that has retained its early course in spite of geologic changes since its course was assumed.

Question 24

River Terrace

Answer 24

• a step like landform. A terrace consists of a flat or gently sloping geomorphic surface, called a tread that is typically bounded one side by a steeper ascending slope
• throughout time there are different levels of floodplains and forms terraces

Question 25

Stream Gradient

Answer 25

the grade measured by the ratio of drop in elevation of a stream per unit horizontal distance, usually expressed as feet per mile or meters per kilometer

Question 26

Rain Shadow

Answer 26

An area having relatively little precipitation due to the effect of a barrier, such as a mountain range, that causes the prevailing winds to lose their moisture before reaching it.

Question 27

Laminar Flow

Answer 27

also known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers

Question 28

Permeability

Answer 28

How easy to move water through rock

Question 29

Meanders

Answer 29

a winding curve or bend of a river or road

Question 30

Hydraulic Conductivity

Answer 30

a property of vascular plants, soils and rocks, that describes the ease with which a fluid (usually water) can move through pore spaces or fractures

Question 31

Competence

Answer 31

the ability to carry a load of a given size; faster rivers have a high competence.

Question 32

Spring

Answer 32

the source of the river usually at high elevations, where the water starts to collect, arises from a perched water table/aquifer.

Question 33

Recurrence interval

Answer 33

- RI = (N + 1)/ M > N=number of years on record > M=number of recorded occurrences of the event being considered - the time between events such as floods or earthquakes or other natural processes. Often we are interested in the average recurrence interval, which is determined by finding the mean of a series of recurrence intervals between events

Question 34

Divide

Answer 34

waters on each side of the divide never meet, but flow into the same ocean

Question 35

Gradient

Answer 35

- the grade measured by the ratio of drop in elevation of a stream per unit horizontal distance - high gradient means steep slope and rapid flow of water

Question 36

Surface runoff

Answer 36

the water flow that occurs when the soil is infiltrated to full capacity and excess water from rain, melt water, or other sources flows over the land

Question 37

Aquitard

Answer 37

- a zone within the earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge. Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity - same as a confining layer

Question 38

Floodplain

Answer 38

- an area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge

Question 39

Alluvial fan

Answer 39

- sediments deposited as the river widens dramatically (and the velocity of the river decreases) upon going from mountainous valley to plain.

Question 40

Turbulent Flow

Answer 40

- the motion of a fluid having local velocities and pressures that fluctuate randomly

Question 41

Aquifer

Answer 41

- a body of water that is held under ground and is created by the fact that the rocks that surround it are impermeable by water

Question 42

Longitudinal Profile

Answer 42

Streamflow everywhere balances input and output (besides during floods); longer term dynamic equilibrium between erosion and sedimentation.

Question 43

Braided stream

Answer 43

multiple “braids” (paths) to a stream, formed before meandering streams and occur when the river has a high velocity and sediment content.

Question 44

Base level

Answer 44

the theoretical lowest elevation, to which a river may erode, generally is at sea level

Question 45

Bed Load

Answer 45

particles in a flowing fluid (usually water) that are transported along the bed. Bed load moves by rolling, sliding, and/or saltating (hopping)

Question 46

Oxbow lake

Answer 46

formed when a river creates a meander, due to the river’s eroding the bank through hydraulic action, abrasion, and corrosion. over time, the meander becomes very curved, and eventually the neck of the meander will become narrower and the river will cut through the neck at a time of flood, cutting off the meander and forming the oxbow lake

Question 47

Water table

Answer 47

the ground water table is the boundary between the saturated and non-saturated zones of the Earth.

Question 48

Infiltration

Answer 48

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Question 49

Porosity

Answer 49

the water storage capability in rocks. For example, non-cemented sandstone has more porosity than cemented sandstone.

Question 50

point bar

Answer 50

a depositional feature made of sand and gravel that accumulates on the inside bend of streams and rivers below the slip-off slope

Question 51

recharge

Answer 51

- groundwater recharge, a hydrologic process where water moves to groundwater - surface water recharge, a hydrologic process where water runs off to surface watercourses

Question 52

Hydraulic Head

Answer 52

a specific measurement of liquid pressure above a geodetic datum

Question 53

Darcy’s Law

Answer 53

Q(volume of water)=A(cross sectional area flow) x (K(permeability)x H(vertical drop)/ L(flow distance))

Question 54

Terrace

Answer 54

a step-like landform that borders a shoreline or river floodplain

Question 55

Vadose Zone

Answer 55

- the part of Earth between the land surface and the top of the phreatric zone, the position at which groundwater is atmospheric pressure - aka: the ground between the surface and the groundwater

Question 56

Saturated Zone

Answer 56

the zone in rock layers where all the pores are filled with water

Question 57

Perched Aquifer

Answer 57

an aquifer that rests on a layer of impermeable rock at a higher elevation.

Question 58

Settling Velocity

Answer 58

- The fastest settling particles are huge, heavy, spherical molecules. The slowest settling particles, which sometimes cannot be settled accurately or properly, are tiny, light, irregularly shaped molecules. And for anything in between, here is a general guide as to what characteristics increase the rate of thickening

Question 59

Artesian Aquifer

Answer 59

an artesian aquifer contains very high pressure, so when a well taps into it, the water squirts out without being pumped

Question 60

Confined Aquifer

Answer 60

those in which an impermeable dirt/rock layer exists that prevents water from seeping into the aquifer from the ground surface located directly above; instead, water seeps into confined aquifers from farther away where the impermeable layer doesn't exist

Question 61

Unconfined Aquifer

Answer 61

those into which water seeps from the ground surface directly above the aquifer

Question 62

Capacity

Answer 62

the total sediment carried by the river.

Question 63

Groundwater Overdraft

Answer 63

the process of extracting groundwater beyond the safe yield or equilibrium yield of the aquifer

Question 64

Cone of depression

Answer 64

- occurs in an aquifer when groundwater is pumped from a well - In an unconfined aquifer (water table), this is an actual depression of the water levels - xIn confined aquifers (artesian), the cone of depression is a reduction in the pressure head surrounding the pumped well

Question 65

Tributary

Answer 65

stream/river that flows into a main stream, river or lake but not into the ocean. Deals with mountain divides and drainage networks that eventually lead to oceans.

Question 66

Dendritic

Answer 66

tree-like, most common form of drainage system. Twigs of the tree are the contributing streams that join together into a river.

Question 67

Drainage

Answer 67

system where smaller and larger rivers merge into streams, leads to the formation of a divide.

Question 68

Trellis drainage

Answer 68

characteristic of folded mountains, where smaller tributaries feed into a river valley from steep slopes of the mountains, usually at 90 degrees

Question 69

Grade

Answer 69

measures steepness of river.