50 terms

(15) Floods

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flooding
an overflow or accumulation of an expanse of water that submerges land
flooding
affects more people worldwide than any other disaster
flooding
- Damage buildings, roads, bridges, canals, sewer systems, cars
- Drown people
- Contaminate water supplies
- Spread unhygenic conditions and water-borne diseases
- Destroy crops, drown livestock, kill trees, lead to famine
flooding in Canada
causes relatively few deaths but extensive damage
more people living in flood prone areas and better flood reporting
number of reported Canadian flood disasters increased since 1960s for two reasons
flood deaths
- Under-reported prior in first half of 20th C
- 1950s particularly low
- 1960s high (5 deadly floods)
flooding in Canada
occur in every month of the year... max in early spring/summer (heavy rains and snow melt) and min in fall/early winter
flooding in Canada
occur in every province/territory...
most in ON and QB,
a lot in NB
small number in north
moving away from bodies of water
would eliminate all death and damaged from floods
hydrometeorological floods
flood type caused by specific weather conditions, such as...
- rainfall floods
- snowmelt floods
- rain-on-snow floods
- icejam floods
outburst floods
flood type that results from obstruction of waterflow flood landing, and catastrophic failure of natural dams
rainfall floods
occur when precipitation rate exceeds infilltration rate of ground and carrying capacity of streams/rivers
slow rainfall floods
Sustained heavy rains over a large area for one or more days that exceed stream/river runoff capacity
flash floods
Torrential rainfall over a short time period (< 6 hours) and often a small area (e.g., intense thunderstorm) that saturate ground with runoff over surface
Vancouver Island Flood, 2009
example of a slow flood...
- 300 homes evacuated
- schools closed, high tide flooded duncan
- caused by "pineapple express"
- several days of heavy rain on Vancouver Island, including 250 mm in 24 hrs on N island
- 8 rivers flooded
pinapple express
warm, moist, sub-tropical pacific air flow from hawaii bringing warm temperature and heavy rain to pacific NW... causes vancouver island floods in 2009
stages in flash floods
1) heavy rain falls onto waterlogged ground
2) rainfall cannot soak in so runs down into river
3) river rises dramatically and bursts its banks, flooding valley floor
Buffalo Gap, SK, 1961
example of flash flood...
- candian record
- 254 mm rail/hair <1 hr
- pigpens, gas barrels, toilets, timbers, CPR ties and telegraph poles went down like sail boats
Toronto, 2005
example of flash flood...
- 153 rain plus hail
- $500 M damage
dangers of flash floods
Half of flash-flood fatalities from driving through flooded intersections:
- Water 0.3 m deep can move vehicle laterally
- Water 0.6 m deep can float car, allowing it to be swept/rolled away
- Road bed could be washed out under flood waters
- Abandon vehicles stalled in flood waters
area prone to flash floods
desert areas with dry impenetrable soil, and occasional torrential rain
snowmelt flooding
most common flood type in canada
snowmelt flooding
- generally occurs in spring, but can also occur in winter thaw
- due to melt of unusually heavy snow or unusually fast melt
- more likely with late thaws
- melt water unable to penetrate frozen ground and runs quickly over surface
- can affect large areas (climatic factors often widespread)
rain-on-snow floods
combine rainfall and snowmelt floods...
- heavy rains on snow at 0degC
- rain and snow melt cannot penetrate frozen ground so runs over surface
- heavy water-saturated snow pack can damage roofs, buildings, and cause avalanche hazards
icejam flood
major cause of flooding on Canadian rivers...
- Result from accumulation of ice fragments that act as a temporary obstruction, restricting water flow
- Can occur at freeze-up or break-up, but worst flooding usually occurs at break-up
- Annual peak level of most Canadian rivers due to icejams
icejam flood
can occur at river bends or changes in slope, or at bridges or piers
south to north
rivers flowing this way are particularly vulnerable because breakup starts earlier in one part and sends ice where river is still frozen
natural dams
formed by:
- glaciers
- mass movements
- lava or pyroclastic flows from volcanoes
outburst floods
catastrophic failure of natural dams causes these
Jokulhlaups
outburst floods of glacial origin are referred to as these (in Iceland, are caused by sub-glacial volcanism)
Vatnajokill Jokulhlaups
icelandic volcanism beneath this icecap maintains sub-glacial lake, which discharges every few years as an outburst flood
Vatnajokill Jokulhlaups (1996)
a 13-day eruption of Bardarbunga volcano filled lake Grimsvotn with...
- 3.6km3 of water discharged over 20 hrs
- peak rate of 55,000 m3/s (20x rate of nigara falls)
- 1,000 ton iceblocks carried
- 9m of sediment deposited
damage:
- 6 km highway destroyed
- power and telephone lines destroyed
- two concrete and steel bridges destroyed that were built to withstand these
flood frequency plot
shows estimate of flood return periods based on historical data... also estimates given size of flood occurring within a given period
flood frequency plot
used for:
- determining land use
- designing roads, bridges, and buildings
100-year flood
has 1% probability of occurring in any year, assuming floods are random and uncorrelated

(could be zero or >1... dont take literally)
flood safety and mitigation (before)
- make home resistant (seal ground floor, install drains around building, and sump pump with one way valves)
- listen for warnings
- IF there is a forecast and time permits... (move valuables above ground level, and turn off basement furnace and outside gas valve)
- IF its imminent (about to happen)... (turn off power and plug ground floor/basement sewer drains and toilets)
flood safety and mitigation (during)
- listen to radio flood reports
- do not cross flooded are by foot or by car
flood safety and mitigation (after)
- wear rubber boots when re-entering home and beware of electrical shocks
- waters are often contaminated...so dispose of all food that touched water and do not move back in until all contaminated rooms have been cleaned, disinfected, and dried (may require professionals)
structural mitigation of floods
- infrastructure to contain waters should have dams, levees, and floodways
- engineering projects to increase river carrying capacity (straightening, deepening, widening river channels, removing debris and obstructions in rivers)
... all costs money!
non-structural mitigation of floods
- zoning and land use policies
- education and evacuation planning
- improved flood forecasting (satellites, river gauges, etc)
... all costs money!
Red River
flows northward, draining parts of North and South Dakota, Minnesota and Manitoba... slowly meandering river with broad floodplain that floods frequently
Red River Flooding
Causes:
- geologically young river (<9000yrs) so has not carved a deep valley
- valley underlain by impermeable red clay
- very low riverbed slope (~8cm/km)
- high snow melt into northward flowing river prone to icejams
1950 Winnipeg Flood
- snowy winter and late thaw led to dike collapse with flooding of 1600 km2 from red river
- largest canadian evacuation (>100,000 people)
- 51 day flood damage
- wrecked 10,000 buildings
- $1.1 B damage
- red river floodway built in response
Red River Floodway
- 48-km long, 140-m wide artificial flood-control waterway around Winnipeg built from 1962-68
- Empty under normal circumstances
In flood conditions, gates opened to divert water around city
- Built for 1-in-225 year flood
- Typically required every 2-3 years
1997 Red River Flood
caused by:
- 1996 rainfall was 4x normal, causing early cold frozen water in soil
- record winter snow pack of 250 cm
- spring blizzard added 50 cm snow with rapid thaw
- flooded N.Dakota... 60,000 evacuated, 120,000 drown, and $1 B damage
- flooded manitoba with 50x100 km lake
Floodway
saved winnipeg from 1997 red river flood by diverting water at peak rate of 1400 m3/s and was within 60 cm of overwhelming dikes
2013 Alberta Floods
- 19 June to 12 July 2013
- 4 confirmed deaths from the flood itself
- Damages about $5 billion ($1.7 billion insured)... Costliest Canadian natural disaster
- Areas affected Southern and central Alberta.... Bow, Elbow, Highwood, Red Deer, Sheep, Little Bow, and South Saskatchewan rivers and their tributaries
- Calgary was directly hit
- 2,200 Canadian Forces troops were deployed
2013 Alberta Floods
- Prior to 19 June, had experienced heavy rainfall that triggered catastrophic flooding described by the provincial government as the worst in its history.
- A total of 32 states of local emergency were declared and 28 emergency operations centres were activated as water levels rose and numerous communities were placed under evacuation orders.
- And then it got worse
2013 Alberta Floods
- semi arid region
- low pressure system south of it was blocked
- resulting circulation from the east pumped moist warm air across up the foothills slopes
- heavy rain over two days fell in many regions, particularly west and southwest of city most affected
... shows hydro-meteorology)
2013 Alberta Floods
hydro-meteorology:
- The ground was already saturated, and coupled with the steep watershed and heavy snow loads, resulted in a rapid increase in the size and flow of several rivers.
- At the peak of the flooding, the Bow and Elbow rivers were flowing through Calgary at three times their peak levels from a 2005 flood that caused almost half a billion in damages.
- At one point the flow rate on the Bow River had reached five times its normal rate .
- The Elbow and Highwood Rivers reached flow rates inside Calgary ten times their normal rate.