GHC Ch 13: Floods Flashcards
Profile for almost any stream is…?
smooth, concave upward, with steeper slope near source and flatter slope near mouth.
Base level
level below which stream can not erode
Profiles are similar for all streams because of…?
equilibrium processes.
Equilibrium
state of balance
What are the 4 factors that affect equilibrium?
Discharge, available sediment (load), gradient, channel pattern (sinuosity)
What happens if there is too much water, or discharge? How does the stream return to equilibrium?
The stream will flow more rapidly and energetically. The excess energy will erode stream bottom and banks. Erosion of stream bottom results in less vertical drop, meaning a flatter gradient and a slower, less energetic water flow. OR Erosion into stream banks creates meandering pattern, meaning a longer stream path, lower gradient, and slower, less energetic water flow.
What happens if there is too much sediment, or load? How does the stream return to equilibrium?
The stream becomes choked. Excess sediment builds up on stream bottom, which results in increased gradient. Water flows faster and more energetically and can carry away more sediment. OR Channel pattern becomes straighter, and only minimum energy is needed to flow distance. Islands of sediment form within the channel, creating braided stream pattern. OR Similar to stream overflowing and eroding away landslide dam, it slowly erodes sediment buildup.
Graded Stream Theory
An equilibrium stream with an evenly sloping bottom is adjusted to efficiently handle water flow (discharge) and sediment (load) transport.
What happens if there is too much load, and too little discharge in upstream portion?
braided pattern
What happens if there is too much discharge, and less load in downstream portion?
meandering pattern
Stream changes in response to…? (3)
seasonal changes, changes in global sea level, tectonic events
Rainfall varies in…?
intensity and duration
How do we forecast of future events?
Look at events of the past.
Floors of streams during floods are built by what?
erosion and deposition
The terraces on the flanks of the floodplain are what?
an abandoned floodplain due to geologic change.
What are the two main causes of floods, and what kind of flood does each lead to?
A thunderstorm leads to flash (UPSTREAM) flood in steep topography which lasts a few hours. Rainfall over days leads to regional (DOWNSTREAM) floods lasting weeks.
What are 5 other kinds of floods?
Storm surge of hurricane, ice-jam flood (Broken ice on rivers can dam up, block water flow and then fail), Hot weather causing rapid melting of snow, Short-lived natural dams (landslide) fail in flood, Human-built levees or dams fail in flood.
Which floods cause most flood-related deaths?
flash floods
50% of flood-related deaths are related to…?
vehicles
How many feet of moving water is required to lift and carry away average car?
only 2 ft
Regional Floods (Downstream)
Inundation of area under high water for weeks. Few deaths, extensive damage.
Where do regional floods occur?
Large river valleys with low topography.
What conditions lead to regional floods?
Widespread cyclonic systems lead to prolonged, heavy rains.
Where are the greatest inundation floods in the U.S.?
Mississippi River
Third largest river basin in world?
Mississippi River
Average water flow in lower reaches of the Mississippi is…?
18,250 m3/sec
Water flow can increase how much during a flood?
fourfold
Mississippi drains all or part of…?
31 states, two Canadian provinces
Mississippi river system includes almost how many of the major rivers in U.S.?
about half
Where does the Mississippi river system carry sediment to?
deposits it in bird-foot delta lobes in the Gulf of Mexico
Describe avulsion.
River builds up channel bottom over time, until channel bottom is higher than surrounding floodplain. Avulsion occurs in next major flood: river adopts new, lower elevation channel, and abandons old channel
Mississippi River overdue for…?
avulsion – current channel unstably high (above downtown New Orleans). Should undergo avulsion to channel of Atchafalaya River.
What is New Orleans’s response to floods?
Built levees
Continuous efforts to build levees to prevent flooding result in what when the levees fail in the next flood?
more destruction
What is the cause of each of the largest floods on the Mississippi?
wet preceding autumn and winter, saturated ground for spring, followed by wet summer, caused by low pressure from bend in jet stream
Describe the Mississippi River System 2011 Flood Management.
Agonizing decisions needed to be made regarding diverting flows from upstream areas onto historic floodplains to reduce downstream flooding in highly populated and industrial areas
What problem causes flooding in the Yellow River of China?
Sediment deposition on channel floor builds up channel in elevation and eventually may be higher than surrounding floodplain.
How is the Huang River kept in place today?
by levees
What are 4 structural responses to floods?
Dam construction, Building levees, Straightening, widening, deepening and clearing channel to increase water-carrying ability, Sandbagging
What are 5 nonstructural responses to floods?
More accurate flood forecasting, Zoning and land-use policies, Insurance programs, Evacuation planning, Education
Name 3 societal responses to floods.
Dams, Reservoirs and Natural Storage Areas.
Life spans of dams are limited by what three things?
construction materials, construction style, and rate at which sediment fills reservoir
Major dam floods occur downstream due to…?
Overtopping, Heavy rainfall below dam, Dam failure
Cost of building levees vs value of structures intended to protect.
Cost of building levees may be more than value of structures intended to protect.
Saturated levees can be compromised by what 4 things?
wave attack, erosion by overtopping, failing by slumping, undermining by piping
Sandbagging
Temporary levees of bags of sand and mud
How has forecasting affected flood damage?
Forecasts of height and timing of regional floodwaters have significantly reduced loss of life.
Where has the National Flood Insurance Program, FEMA, banned building?
on floodplain covered by 100-year flood
Presidential Disaster Declarations
Disastrous floods caused 64% of PDDs in 50 years
Hydrographs
Plots volume of water (or stream depth) against time. Shows time lag after rainfall for runoff to reach stream channel, after which stream surface height rises quickly (steep rising limb of hydrograph).
Urbanization changes shape of hydrograph how?
makes curve much steeper
Good news and bad news about urban floods?
Good news: urban flood might only last 20% as long. Bad news: urban flood could be four times higher
How does urbanization affect floods?
Urbanization increases surface runoff of rainwater leading to higher stream levels in shorter times (flash floods).
Channelization
Attempt to control floodwaters by making channels clear of debris, deeper, wider and straighter.
Problem with channelization?
Pushes stream into “too much discharge” case.
The Hit-and-Miss Approach: Tucson (Desert floods)
Protective walls concentrate erosion at end of wall. Creates stronger discharge at gaps, though keeps bank from eroding.
Desert floods damage by… not …?
bank erosion, not inundation
Ice-Dam Failure Floods: Glacial Lake Missoula
Biggest floods during melting of continental ice sheets. Lakes behind ice dams failed suddenly.
Potential problem of Ice-Dam Failure Floods?
Huge volume of meltwater: has changed paths of rivers and could change global circulation of deep ocean water, which would mean a change in global climate
