lecture 19: flooding

Question 1

flooding definition

Answer 1

any overflow or accumulation of water that submerges land that is normally dry

• occurs due to water escaping from water bodies or rain/meltwater accumulating on impermable ground
• affects more people worldwide than anything

Question 2

3 long lasting impacts of flooding

Answer 2

1. damage or destruction of buildings, roads, bridges, sewer system
2. contamination of water supplies, spreading of water born disease
3. destruction of crops, trees and other vegetation

Question 3

flooding in canada

Answer 3

killed few people (200 in 100 years) but caused extensive damage and can be costliest disaster
increasing numbers due to more ppl living in flood prone areas

Question 4

drainage basin

Answer 4

can be thought of as a large net that catches and channels rain or snow melt
water on hillslopes either infiltrates soil/rock and flows as groundwater or if saturated it flows rapidly as surface runoff feeding stream network to draniage outlet

Question 5

when does flooding occur in channels

Answer 5

river has carrying capacity (discharge in m3/s), when exceeded flooding will occur
repeated flooding produces flat floodplain over time

Question 6

4 reasons people drawn to building and living on floodplains

Answer 6

1. access to fresh water
2. fertile land for farming
3. cheap transportation (rivers/railroads)
4. ease of development of flat land

Question 7

how are floodplains classified

Answer 7

by the frequency by which they naturally flood, 100 year flood plain has 1% probability of flooding in any year

Question 8

how do we estimate flood return periods

Answer 8

using historical events

- for zoning and designing buildings

Question 9

2 flood types

Answer 9

1. hydrometeorological: caused by weather
   - rainfall floods (flash floods), snowmelt, rain on snow, ice jam and storm surges
2. outburst floods: caused by catastrophic failure naturally

Question 10

rainfall floods

Answer 10

occur when precipitation exceeds ground infiltration and increase in surface runoff overcomes carrying capacity of rivers
slow: sustained heavy rain over one or more days

Question 11

flash floods

Answer 11

torrential rainfall over short period (<6h) and in a small area
- desert areas with poorly absorbant soils are prone

Question 12

where do flash-flood fatalities occur

Answer 12

driving through flooded roads

0. 3m of water move a vehicle laterally
1. 6m of water float a car

Question 13

snowmelt floods

Answer 13

melting of heavy snow pack or from rapid thaws
meltwater unable to penetrate the frozen or water-saturated grounds, generating surface runoff which overwhelms rivers
- pinebeetle causes floods to be larger b/c living trees soak up meltwater and shade

Question 14

what is the most common type of flooding in Canada

Answer 14

snowmelt floods: normally occur during the spring

Question 15

rain on snow floods

Answer 15

heavy rainfalls on snowback, water cannot infiltrate the ground and mixes with snow to form surface runoff
heavy water saturated snow can damage roofs and buildings and increase avalanche hazards
- double the end of the century from climate change

Question 16

ice jam floods

Answer 16

major cause of flooding on canadian rivers at both freeze up (winter) and break up (spring)

Question 17

red river floods

Answer 17

red river drains north dakoda, saskatchewan into lake winnebeg
has low gradient and has not had time to carve valley
underlain by clay rich impermeble glacial till
cold winter make it prone to snowmelt and icejam flooding

Question 18

what caused red river floods

Answer 18

1950 snowy winter and late thaw caused flooding over 1600km
winnipeg had largest evacuation in canadian history
red river floodway was built to divert floodwater

Question 19

storm surges

Answer 19

rise in sea level caused by

1. strong onshore winds that pile up ocean water towards land
2. low atmospheric pressure beneath the storm eye pulls up mound of seawater

Question 20

when do storm surges arrive

Answer 20

at the coast before the hurricane reaches land
most dangeorus when high tide
record is 42 feet in australia
in northern hemisphere: occur 15-30km to the right of eye

Question 21

outburst floods

Answer 21

when failure of a natural dam results in sudden deluge of water downstream

Question 22

3 ways natural dams can be formed

Answer 22

1. mass movements (landslides)
2. volcanic materials (lava flows)
3. glaciers (proglacial- above or subglacial- below lakes)

Question 23

outburst floods from glaciers

Answer 23

jokulhlaups, triggered by subglacial volcanic eruptions which melt ice on top of volcano

Question 24

thunderstorms

Answer 24

kill injure and damage through: lightning (majority of deahts), fires by lightning, hail, winds and rain/flash floods

Question 25

thunderstorm formation

Answer 25

early stage: warm, moist air heated near ground surface rises, water vapour condenses to liquid droplets and forms cloud column, condensation releases latent heat of vaporization which uplifts air
mature stage: droplets and/or crystals become heavy enough to precipitate causing cold downdrafts
dissipating stage: downdraft drag in surrounding air thunderclouds evaporate

Question 26

frequency of thunderstorms

Answer 26

approximately 20 days per year in canada

highest in ontario (30) lowest at coasts (5) and north

Question 27

lightning occurances

Answer 27

worldwife: 100 lightning strikes/second 9 mill per day
canada: 3 million flashes per year, kills 10/yr
male death rate 5x female, most in Ontario and praries
least near coasts and north
most deaths in summer when ground is heated

Question 28

what causes lightning

Answer 28

electrical discharge between opposite charge accumulations
clouds/ground
rapid thermal expansion of air produces thunder sounds