Quiz #2 -- Floods
Levees and dams do protect from floodsfor some time They tend to give a false
sense of security because they are subject to failure
Flash Floods
Local intense storms in areas of steep topography •Flash floods typically occur in deserts and travel many miles away from storm; narrow canyons (obstruct view) •Most flood related deathsby flash-floods; about 50% car-related •more people drownin deserts than die of thirst!
Match each of the letters on this diagram with the correct cause of levee failure.
Overtopping Wave attack Slumping Subsurface erosion (piping)
How levees fail
Overtopping •Undercutting and slumping from erosion from river currents •Piping or seepagethrough levee, sometimes reaching surface as sand boil
As more people move to _____ they inhabit more land that is prone to flooding.
cities
Our climate zone here dominantly has ______ - water enters stream from groundwater
gaining streams
Why do people like to live on floodplains?
Agriculture
tophat question: braided streams are typical of ___ regions
high sediment load and strongly variable flow rate
According to the green flood frequency curve in this figure,
larger floods have a longer recurrence interval. small floods happen every year or so. small floods occur more frequently.
Arid climate zones (such as western US) have many_______ - water enters groundwater from stream
losing streams Most streams have some aspect of both.
These natural levees are often the safest place to be during
low -level floods.
Dam failure can occur by:
overtopping erosion of foundation poor design or construction seepage through earthen dams
problem worked out in class (tophat) - if you buy a house with a 30 year mortgage, how likely is there to be a 100 year flood while you still owe money on the house?
1% chance per year of it occurring = 99% chance of it not occurring each year over 30 years thats (.99)^30 =.74; chance of the 100 year flood sometime in 30 years is 1-.74 =.26 —>26 percent chance or 1 in 4
correct descriptions to explain how a stream breaches a natural dam.
Stream fills lake basin with sediment. Landslide creates natural dam. Water flows over dam, eroding it.
What is the best way to predict flooding in an area?
Study the history of flooding in that area.
"Weibull formula"
T = n+1/m where nis the total number of years in record,and m is the rankof the flood
Recurrence Interval (T)
T is the average time between floods of a given discharge
Wildfires can also 'seal' soil surface, reducing its ability to contrast, wetland areas tend to
absorb water reduce flooding
Bedrock Streams
are fast moving, high energy streams that occur in steep mountainous areas where the streambed is solid rock. High gradient bed and high flow removes all sediment and erodes down into bedrock
Meandering Streams
are most common and sweep from side to side in wide turns
Braided Streams
are much less common and typically occur in glaciated terrains where there is an abundance of coarse sediments. - high sediment loads and deposition are typical- gradient decreases out of the mountains- strongly variable flow moves a lot of sediment intermittently
A stream's cross-sectional profile flattens out as it approaches ______ because it cannot erode below it.
base level
A 66-year flood occurs on the Flushing River in 2014. What is the probability of a 66-year flood occurring on the same river in 2015?
1.5%
According to the flood recurrence interval equation, a flood that occurs after 90 years of record keeping, and is the seventh largest ever recorded has a recurrence interval of _______ years.
13
When designing roads, buildings, and bridges in a floodplain, engineers consider the statistical probability of a flood of a given size happening in a given year. A 50-year flood has a ____ probability of occurring in a given year.
2% (1/50)
According to the flood recurrence interval equation, a flood that occurs in 2014, after 149 years of record keeping, and is the third largest ever recorded has a recurrence interval of _______ years.
50
River Floodplain Floods
Caused by prolonged (heavy) rain and/or snow melt •Large river valleys with low topography •High water inundating extensive region for weeks •Few deathsbut extensive damage
reconstruction of stream channel to straighten and shorten distance
Channelization Increases slope to drain area of flood waters more rapidly can create downstream floods bc of increased erosion and faster flow
Which of the following are considered to be structural responses to flood hazards?
Dams Channel widening Levees Sandbagging
Increased Sediment Load causes
Deforestation by logging, fire, or overgrazing causes excess sediment to reach river,increasing sediment load and flood risk
True or false: Flash floods are smaller than regional floods and therefore result in fewer fatalities and less economic loss.
False Rationale: Flash floods are deadlier than regional floods.
Urbanization via surface alterations (building and pavement) causes
Increased Runoff Loss of vegetation due to deforestation, agriculture, or overgrazing
Which of the following accurately describe the cross-sectional profile of a typical stream when the graph of the profile is vertically exaggerated?
It is steeper at or near the source of the stream. It has a lower gradient near the mouth of the stream.
What happens to a channel during flood?
Most flow is in the channel • Floods onto the floodplain • Erodes deeper channel during flood
stream discharge equation
Q= VA Q is the discharge or total flow (m^3/second) V is the average velocity (cm/second) A is the cross-sectional area of the steam, or width times depth (m^2)
Shallow section of a river with rapid current and a surface broken by gravel, rubble or boulder
Riffle
LONGITUDINAL PROFILEOF A STREAM:
STEEP HEADWATERS, GENTLER BUT GREATER FLOW DOWNSTREAM
the amount of sediment that a river can carry
Sediment Load Load increases with discharge (flow velocity and X-sectional area). •When the water level is higher a stream is capable of carrying more material than normal.
water that flows downslope in a channel carrying a load of sediment and dissolved matter
stream/river/creek
Actions that alter the natural river system, such as building dams or straightening channels, are considered to be ______ to flood hazards
structural responses
streams accumulate
surface runoff and groundwater seepage from within their watersheds—also called drainage basin
Floods occur when the channel cannot accommodate
the discharge / flow Input: Precipitation amountand rateare both important
When designing bridges, engineers will take into account ________.
the size of larger floods because, although they are uncommon, the cost of a bridge failing during a large event would be very high
what is the basic cause of a flood?
the water gets high enough to overtop the sides of the bank, channels fill up and cannot accommodate the amount of water that is coming into them more water entering the channel than it can move out so the water level goes up eventually the water spilled over the top
What does it mean to say streams are equilibrium systems?
they "seek" an equilibrium by compensating for changes in their systems.
Flash floods are sometimes called______ floods because they start near the headwaters of a river, whereas regional floods are sometimes called ______ floods because the flood depth increases toward the mouth of the river.
upstream downstream
Once a dam fails, shape of downstream valley becomes importantIn broad open valleys but narrow valleys
water flow drops quickly can maintain high water speeds further downstream
Stream velocity depends on
channel shape
Flash floods are commonly caused by ______.
convective thunderstorms
As water flows down into higher order streams,the
crest height diminishes, but the durationof the flood increases.
The longer the wait for a large flood, the more likely its occurrence becomes. This is referred to as _______ probability
cumulative
Every time a levee is built, also reduce
width of flood-flow part of river and raises water level during flooding higher velocity and erosion, can make flooding worse
Probability of exceeding a particular magnitude flood in a given year so it is 1/T.
•Annual Exceedance Probability
Flood size can be controlled through the construction of ______ , which store floodwater in reservoirs.
dams
the average volume (amount) of water flowing per unit of time at any point along the watershed is
discharge
Flood Intensity also depends on the
distance from the storm
Grain size typically decreases
downstream
In semi-arid to arid regions, runoff tends to be
faster
Deep water under bridge flows
faster, causes erosion, undermines pilings supporting bridge
A _____ flood occurs when rain falls intensely for hours in a small area.
flash
Channel is where flow is confined always exceptduring a
flood
Meanders erode outwards and slowly (100s to 10,000s thousands of years) migrate downstream - creating a broad eroded area called a
floodplain. A major flood would fill the floodplain wall-to-wall.
in the US, floods are worldwide..
floods among deadliest and costliest weather related hazards annual costs of flood damage increased tenfold from 1929 to 2003, from $400 million to about $4 billion floods are the most commonly occurring natural disaster worldwide
The ability of river water to carry sediment depends on the
flow velocity/discharge and turbulence of the flow to lift particles
the steepness of the stream bed as it descends
gradient
Besides surface runoff, some of the precipitation in an area percolates through soil to become ______ and then seeps into rivers and streams
groundwater
More importantly, these recordsno longer apply in many areas because of:
human alterations that greatly change runoff and sediment loads- changes in average rainfall rate due to climate change
This type of graph, which plots river discharge against time, is known as a(n)
hydrograph
Heavy rain (or rapid snow melt) cannot all be absorbed into ground, So a lot of
immediate runoff into streams
For a river with a good historical record, 500 year floods coming two years in a row is:
improbable
tophat question: based on the equation Q= A * V: if the width of a stream channel were to increase but the velocity remained the same, what would happen to the discharge in the stream?
increase
Anything that fills river channel with sediment can
increase number and size of future flood events
This photo shows flooding of farmland due to breach of a(n)
levee
How much of the available sediment is moved by a stream as ______ is determined, in part, by the ______ (slope of the stream bottom) and the rate of the flow.
load; gradient
Statistically speaking, larger floods have a _____ recurrence time between them.
longer
Upstream floods tend to have higher In comparison, downstream floods tend to be less
lood crests and can result in more frequent flash floods than downstream areas violentbut can affect far more people
Most streams and rivers fall on a continuum between
meandering and braided.
Successive floods buildup
natural levees
A _______ flood occurs when rain falls over a wide area for days or weeks.
regional
When streams overrun their banks, water velocity
slows dramatically, and coarse sediment drops out to form natural levees.
How do streams breach natural dams?
The stream deposits sediment in the basin created behind the dam until it is filled. It will then flow over the dam, eroding it.
