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.