54 terms

GLG 112 Final Exam-Chapter 6 (NAU)

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What is a Levee?
Earthen embankments constructed parallel to the river to contain floodwaters and reduce flooding
What caused the 1993 flood of the Mississippi River?
Rain storms from April through July in the Cedar Rapids region
What caused the 2008 flood of the Mississippi River?
Winter to late spring storms
What is the bottleneck effect produced by floodwalls?
Floodwater is forced through a narrow channel which cause it to back while it is waiting to get through
Which Mississippi flood did the bottleneck effect contribute to?
1993 in St. Louis
True/False: Engineering structures designed to control the Mississippi have actually decreased the long-term flood hazard.
False
Define Floodplain
the flat surface adjacent to the river channel that is periodically inundated by floodwater
How is a floodplain produced?
By flooding
What is one lesson learned from the Mississippi floods in regards to levees?
They provide a false sense of security
What is a drainage basin?
The region drained by a single stream or river
How do you calculate the gradient of a river?
Vertical drop in elevation/ Horizontal distance
What is the base level of the river?
the lowest elevation to which it may erode
Describe the shape of the river at its headwaters
Steeper-sided and deeper valley
Describe the bed load of the river
Large or heavy particles that slide, roll, and bounce along the channel bottom
Suspended Load
sediment that is small/light enough to remain above the stream bottom by turbulent flow for an indefinite period of time
Dissolved Load
dissolved ions produced by chemical weathering of soluble minerals upstream that are carried in a chemical solution
What is discharge?
the volume of water moving through a cross section of a river per minute
How is discharge calculated?
cross-sectional area of the water X velocity of flow
Where is the highest velocity in a river?
when the water flows through a narrow channel
Alluvial fan
large, fan-shaped piles of sediment that usually form where a stream's velocity decreases as it emerges from a narrow mountain
canyon onto a flat plain
Delta
a body of sediment deposited at the mouth of a river when the flow velocity decreases
What is a distributary channel?
a channel that carries floodwaters to different parts of the fan or delta
What features create a braided channel?
sand/gravel bars and islands that divide and reunite the main channel
Where does the water move fastest in a meander?
Along the outside of the meander
What does the water along the outside of a meander create?
Cutbank
Where does the water move slowest in a meander?
Along the inside of the meander
What does the water along the inside of a meander create?
point bar
Flood discharge
discharge of the stream at the point where water overflows the channel banks
Hydrograph
shows stream discharge, water depth, or stage overtime
Flood stage
The term used to indicated that the elevation of the water has reached a level that will most likely cause damage
Recurrence interval
the average time between flood events that are of equal or greater magnitude
What are two positive effects of flooding?
Flushing out of stream channels/removal of debris and keeping elevations above sea level
What human interactions can increase flooding?
Land use change, dam construction, and urbanization
Why is running water so important?
It is the geologic agent for eroding, transporting, and depositing sediment on Earth's surface
Where is the largest amount of water on Earth?
Oceans
What is the hydrologic cycle?
the movement and interchange of water between the sea, air, life and land
What are the stages of the hydrologic cycle?
Evaporation, Transpiration, Precipitation, Runoff, and Infiltration
Stream
a body of running water, confined to a channel, that runs downhill under the influence of gravity
What controls stream velocity?
Gradient, channel shape, and channel roughness
Narrow deeper channels have what type of flow?
Faster
Smoother channels have what type of flow?
Faster
Wider shallower channels have what type of flow?
Slower
Rougher channels have what type of flow?
Slower
What is a bar?
a ridge of sediment (typically sand or gravel) deposited in the middle or along the sides of a stream
What is a meander cutoff?
a new, shorter channel is cut through the narrow neck of a meander
What are the causes of flooding? (5)
• heavy rains
• rapid snow melt
• dam failure
• topography
• soil conditions
What factors influence flood severity? (4)
• Amount and
distribution of
precipitation
• Rate precipitation
soaks into the
ground (infiltration)
• Soil saturation
• Discharge
What affect does urban flooding have on runoff?
Increases
What is a 20 year flood?
on average, the size of the largest flood within a 20-year period of time
What is the likelihood of a 20 year flood?
5% chance of occurring in any given year
What is a 100 year flood?
on average, the size of the largest flood within a 100-year period of time
What is the likelihood of a 100 year flood?
1% chance of occurring in any given year
What are hazards of floods?
Drowning
Strong currents
Debris
Cars swept off road
Water damage
Erosion
Trigger landslides
How do you mitigate flood hazards?
Levees, Dams, Wise land use planning, retention ponds, channelization