lecture 19: flooding Flashcards
flooding definition
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
3 long lasting impacts of flooding
- damage or destruction of buildings, roads, bridges, sewer system
- contamination of water supplies, spreading of water born disease
- destruction of crops, trees and other vegetation
flooding in canada
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
drainage basin
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
when does flooding occur in channels
river has carrying capacity (discharge in m3/s), when exceeded flooding will occur
repeated flooding produces flat floodplain over time
4 reasons people drawn to building and living on floodplains
- access to fresh water
- fertile land for farming
- cheap transportation (rivers/railroads)
- ease of development of flat land
how are floodplains classified
by the frequency by which they naturally flood, 100 year flood plain has 1% probability of flooding in any year
how do we estimate flood return periods
using historical events
- for zoning and designing buildings
2 flood types
- hydrometeorological: caused by weather
- rainfall floods (flash floods), snowmelt, rain on snow, ice jam and storm surges - outburst floods: caused by catastrophic failure naturally
rainfall floods
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
flash floods
torrential rainfall over short period (<6h) and in a small area
- desert areas with poorly absorbant soils are prone
where do flash-flood fatalities occur
driving through flooded roads
- 3m of water move a vehicle laterally
- 6m of water float a car
snowmelt floods
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
what is the most common type of flooding in Canada
snowmelt floods: normally occur during the spring
rain on snow floods
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
ice jam floods
major cause of flooding on canadian rivers at both freeze up (winter) and break up (spring)
red river floods
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
what caused red river floods
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
storm surges
rise in sea level caused by
- strong onshore winds that pile up ocean water towards land
- low atmospheric pressure beneath the storm eye pulls up mound of seawater
when do storm surges arrive
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
outburst floods
when failure of a natural dam results in sudden deluge of water downstream
3 ways natural dams can be formed
- mass movements (landslides)
- volcanic materials (lava flows)
- glaciers (proglacial- above or subglacial- below lakes)
outburst floods from glaciers
jokulhlaups, triggered by subglacial volcanic eruptions which melt ice on top of volcano
thunderstorms
kill injure and damage through: lightning (majority of deahts), fires by lightning, hail, winds and rain/flash floods
thunderstorm formation
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
frequency of thunderstorms
approximately 20 days per year in canada
highest in ontario (30) lowest at coasts (5) and north
lightning occurances
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
what causes lightning
electrical discharge between opposite charge accumulations
clouds/ground
rapid thermal expansion of air produces thunder sounds
