Sem#2 Chap 5 Flashcards
Inland floods in non-coastal regions result from:
- Oversupply of rainfall
- Oversupply of melting snow
Stream floods:
- When water spills over the banks of a stream channel
- Most inland flooding results from stream flooding.
Areal floods:
- When low areas collect water and become submerged
Urban floods:
- When cities are inundated due to insufficient drainage
Floods also occur when _____
and ______fail.
dams and levees fail.
Hydrologists:
- Scientists who study water on and below the land
- Distinguish between flood types
- Research stream discharge, competence, and capacity
- Use spatial data to create flood probability maps
- Use models to predict flood interval probabilities
Slow-onset floods (development time)
- Develop over days or weeks
Flash floods (development time)
Develop over minutes or hours
Runoff:
water flowing from an area in response to gravity
- Overland flow: water flows across land as a thin layer (sheetwash).
- Stream flow: water flows down a trough or channel.
Stream
any flowing body of water in a channel
* Medium-sized streams are called creeks or brooks.
* Large-sized streams are called rivers.
Streams form when ?
runoff carves a trough into the ground.
* Downcutting deepens stream channels.
* Headwater erosion lengthens stream channels.
Slide #5 Chap #5
Slide #5
Headwater
where a stream begins to flow
Mouth
where a stream empties into another body of water
Gradient
a stream’s slope, in the downstream direction
Longitudinal profile:
- Plots elevation on the vertical axis
- Plots distance from mouth on the horizontal axis
Floodplains
- Wider than stream channel
- Submerged during a flood
- Often contain fertile soil
Base level:
- The elevation below which a stream surface will not drop
- Local base levels lie upstream from mouth.
- Lakes and tributaries are local base levels.
Stream flow creates ______ and dissolves _______.
sediment and dissolves minerals.
Dissolved load
mineral ions in solution
Suspended load
silt and clay particles within a stream
Competence
the maximum clast size a stream can carry
* Faster flowing streams have greater competence.
* Muddy (denser) streams have greater competence.
Capacity
the total quantity of sediment a stream can carry
* Competence and water Alluviumvolume dictate capacity
Alluvium
a layer of settled stream sediment
Coarse alluvium settles from _______
faster-flowing streams
Fine alluvium settles from __________
slower-flowing streams
Bar
an accumulation of gravel and sand along or within a channel
Alluvial fan
a wedge of sediment deposited at a canyon mouth
Floodplain deposits consist largely of _____ and _______.
largely of silt and clay.
Natural levees
- Sediment ridges on the banks of a stream channel
- Form as water slows and spills into a floodplain
Deltas
- Sediment wedges at the mouth of a stream
- Form as water empties into a standing body of water
Distributaries
- Small channels cutting across a delta
Delta plains
are broad lowlands liable to flooding.
Permanent Streams
Fed by sufficient overland flow, tributaries, and springs
Stream bed remains submerged
Water table lies above stream bed
Common in temperate and tropical regions
Flow all year
Ephemeral Streams
Fed by insufficient overland flow, tributaries, and springs
Stream bed is often exposed
Water table lies below stream bed
Common in semiarid and arid regions
Flow only part of the year
Dry wash/arroyo/wadi: a completely dried up ephemeral stream
Meandering Streams
Often flow across broad floodplains with deep alluvium
May carve meandering canyons:
* Base level must drop relative to land surface.
* Stream downcuts into bedrock.
Course changes are common:
* Water flows faster on outer curves, creating cut banks.
* Water flows slower on inner curves, creating point bars.
Oxbow lakes form when a meandering neck is eroded away.
Evolving Meanders
A channel’s position changes within a floodplain as meanders evolve.
Meandering River
river with a single channel and high to moderate sinuosity.
Cutbanks (CB)
form on the outside of meander bends where the water is accelerated along the outside wall and erodes into the bank.
Point bars (PB)
form as sediment is deposited in the
slower water on the inside of the
meander bends.
Thalweg
the deepest parts of the channel along the length of the stream bed
Evidence of channel position changes:
- Abandoned meanders, relict point bars, and oxbow lakes
Braided Streams
Carry large quantities of sediment when flow is high
When flow slows, sediment settles into elongated bars.
Stream separates into small channels flowing around bars.
Channels resemble braided hair.
Braided River
a river characterized by multiple,
frequently shifting channels.
- Common in regions where there is a strong seasonally and monthly variation in stream discharge
- During short periods of high discharge a braided river carries the coarsest sediment
- Develop in regions where sediment is readily available
Watershed
the land area from which water drains
* Also called catchments or drainage basins
Watershed tributaries feed water into a ______.
trunk stream
Trunk stream drains water from ______.
watershed
Drainage network includes:
- Watershed’s tributaries
- Watershed’s trunk stream
Drainage divide
- Elevated land separating drainage networks
Continental Divide
- Separates drainage networks flowing into different oceans
The Amazon Drainage Network
The largest drainage network in the world
Drains much of South America
Continental divide follows the crest of the Andes
1st-order streams:
- Small streams
- No tributaries
- Near margins of drainage network
Low-order streams drain _____ areas
small
High-order streams drain _____ areas.
large
Two 1st-order steams merge to
form a 2nd-order stream
(just a knowledge thing, repeat for every #stream take 2 to from the next)
A trunk streams is a network’s _______
highest-order stream.
Stream Discharge
The water volume passing a cross-sectional area
Cross-sectional area
- A plane drawn perpendicular to the banks of the stream
Cross-sectional area formula
𝐷𝐷 = 𝐴𝐴 × 𝑣𝑣
* D = discharge
* A = cross-sectional area
* v = average water velocity
Reported in m3 or ft3 per second
Gauging stations
record stage and average velocity.
Stage
- A reference elevation just below the streambed
Temperate and tropical
places(Discharge Variations):
- Tributaries and springs add water to flow.
- Discharge often increases downstream.
Arid and semi-arid places(Discharge Variations):
- Water seeps into ground, evaporates, or is removed.
- Discharge often decreases downstream.
Trunk stream discharge (what does it represent)
- Reflects watershed size and climate
Turbulence develops from ________
shearing water flows
(Discharge Variations): Calculations are inexact due to _________
friction and turbulence.
Flood stage:
- When water rises above a stream bank
- Significant areas outside channel are submerged.
- Cross-sectional area and average flow velocity increase.
Large elevation increase in flood stage:
- When stream channel lies within a narrow valley
Small elevation increase in flood stage:
- When stream channel lies within a low-lying floodplain
Discharge changes during a
flood.(increase and decrease explain)
- Increases as flood develops
- Decreases as flood recedes
Discharge resembles a ________
bellshaped curve
Peak discharge:
the highest point on a hydrograph
Flood crest:
when water reaches highest stage
Lag time is the difference between:
- The event triggering flooding
- Flood crest
Slow-Onset Floods
Water rises for days or weeks.
Water recedes over weeks or months.
Affect high-order trunk streams in downstream places
Hydrographs display wide curves.
Common in Bangladesh, within the Ganges River Delta
~80% of Bangladesh’s people live on the Ganges River Delta.
Flooding requires millions of people to evacuate
Flash flood characteristics
- Discharge becomes hazardous in < 6 hours.
- Discharge increases after a short lag time.
- Floodwaters quickly recede.
- Affect small areas
- Affect low-order streams
Hydrographs have a _______
narrow, bell-shaped curve
Flash Flood Dangers
People and motorists are often caught by surprise.
Water may flow faster than people can run or drive.
Helicopters may be required to save people.
Turbulent water causes rapid erosion.
Dynamic pressure of sedimentladen water destroys structures.
Heavy debris batters and buries areas downstream.
Rapid melting snow may cause ________
flash floods
- Spring rain melts winter ice.
- Volcanic eruptions melt glaciers and snow caps.
Ice dams:
natural walls of ice holding back water
Outburst floods result when ______
ice dams fail
Iceland’s “Jökulhlaup” floods:
- Meltwater accumulates beneath a glacier.
- Outburst flood results when water breaches glacial toe.
Glacial Torrents
Immense outburst floods from melting glaciers
Ice Jams
Form when river ice breaks up during the spring
Ice encounters an obstacle and piles up.
Ice blocks water from flowing downstream.
Water floods into floodplain.
Outburst flooding may result when ice jam breaks
Areal Flooding
Land is submerged due to heavy rain or rapid snowmelt.
Flooding occurs without input from a nearby stream.
May delay agricultural planting
Farmland and low-lying communities at risk
Urban Flooding
A flood that inundates a built
environment
May occur during slow-onset and flash-flood events
Flood control channels may expedite water removal.
Urban Flooding: Two causes:
Impermeable surfaces that prevent water infiltration
* Inadequate drainage
Urban Flooding: Potential impacts:
- Toxic runoff causing environmental damage
- Higher flood crest
Before urbanization
- Rainwater infiltrates the ground.
- Less discharge, and peak occurs after a long lag time
After urbanization:
- Rainwater flows directly into streams.
- More discharge, and peak occurs after a short lag time
Artificial Levees
Constructed ridges of gravel, sand, and compacted clay
Built along a stream to confine flow to main channel
Prevent flooding along many river channels
Vulnerable to accidental or deliberate damage
* Military conflicts may intentionally damage levees.
Artificial Levees: Construction parameters
Construction parameters:
* Base should be 3 times wider than height
* May be built atop natural levees
Levee Failure: Overtopping
Occurs when water rises high enough to flow over a levee
* Water velocity and volume increase.
* Erosive power increases as water flows over levee.
* Water may scour a channel in levee wall.
May also occur due to:
* Tree roots disturbing soil
* Floating debris colliding with levee wall
* Slumping in levee wall
Levee Failure: Underseeping
Also called undermining
Water pressure difference:
* May force water through levee base
* May force water through sediment beneath levee
Fountaining may develop:
* Water flows through narrow pathways beneath levee.
* Sand volcanoes may appear.
Fountaining indicates that a breach is imminent.
Levee Failure: Structural Damage
Structural weakness may be caused by:
* Bad concrete
* Poorly compacted clay
* Weak foundation
Concrete barriers may crack.
Levee foundation may shift.
Slumping may occur on either side of levee.
Danger Due to Moving Water
Driving across flooded roads can be fatal.
Water’s dynamic pressure rises with the square of its velocity.
Even shallow water can easily push vehicles sideways.
Buoyancy force pushes vehicles upward.
Flash flood casualties result from people trapped in cars.
Debris in floodwater often destroys bridges and buildings.
Floodwaters erode riverbanks.
Floodwater alluvium may bury streets and buildings.
Damage Due to Rising Water
Floodwaters may submerge fields, homes, and businesses.
Transportation and communication networks fail.
Animals drown if unable to escape to higher ground.
Power generation fails, so electricity goes out.
Floodwater residue contains garbage, sewage, and chemicals.
Mud and silt muck are left behind as water recedes.
Secondary disasters of cholera and dysentery are common.
Monsoon
- A seasonal reversal in wind direction
- Wind reversal causes shift in precipitation.
South Asia’s intense monsoonal rainy season:
- Intertropical convergence zone (ITCZ) drifts north.
- Indian Ocean moisture spawns heavy rains.
Atmospheric rivers:
- Narrow bands of moist air flowing inland from Pacific ocean
- Air flows along the south side of a mid latitude cyclone.
Recurrence interval:
Average years between two floods of a given discharge or greater
How to calculate Recurrence Interval
𝑅 = (𝑁/1)÷ 𝑀
* R: recurrence interval
* N: number of years of flood
records
* M: rank of flood, with largest
ranked #1
Hydrologists prefer using ____ to report flood likelihood
AEP
AEP determines likelihood of:
- A flood of a given discharge or greater will occur in a given year
- AEP is reported as a percent.
AEP Formula
𝐴 = (1÷𝑅)𝑥100
Flood Frequency Graphs: Show Relationship Between
- Recurrence interval
- Peak discharge
Lower-discharge floods: (flood graphs)
- More frequent recurrence
- Plot in graph’s lower left
Higher-discharge floods:(flood graphs)
- Less frequent recurrence
- Plot in graph’s upper right
Climate change will modify:
- Recurrence intervals
- Frequency curves
National Weather Service (NWS) officials:
- Track severe weather
- Keeping public informed of potential flooding
Flash flood watch:
- Conditions may lead to flooding within 12–36 hours
Flash flood warning:
Flooding has been detected.
River flood warnings:
- Issued when large rivers are expected to reach flood stage
Predicted Flooding Risks Correlated
Minor: Low ground flooded, and low parts of
roads underwater; no internal house
flooding (maybe a few basements);
water over roads won’t stall vehicles
Moderate:Numerous buildings flooded; some
infrastructure affected; some people
need evacuation; water makes roads
impassable.
Major:Houses destroyed or floated off
foundations; infrastructure
destroyed; bridges washed away;
severe erosion; extensive
evacuations; National Guard
generally called in.
Protecting Communities from Floods
Sandbagging can reinforce and increase levee height.
* May increase flooding risk for downstream areas
Additional efforts:
* Constructing diversion canals
* Restoring wetlands
* Passing laws that prohibit construction in floodways
Adding sandbags to a levee to increase its height
Private flood insurance is ______ available
rarely
National Flood Insurance
Program:
- Provided by the U.S. federal government
- Premiums are expensive
Federal buyout programs:
- Purchasing flood-prone land for parks or wetlands
Community wealth:
- Poor communities often grow in flood-prone areas.
- Building homes on stilts may be possible for some people.
Floodway zoning ensures _____
minimal losses from a flood
