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EOS 170 > Lecture 19 - Flooding > Flashcards

Lecture 19 - Flooding Flashcards

1
Q

What is flooding defined as?

A

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

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2
Q

When does flooding occur?

A

when water escapes from water bodies (rivers, lakes, oceans) or when rainwater or meltwater accumulates on impermeable ground

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3
Q

When do rivers flood?

A

when their carrying capacity is exceeded, often because the rate of precipitation in the upstream drainage basin exceeds the rate of infiltration, leading to excess overland surface runoff

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4
Q

What is a landform associated with flooding?

A

In lowland river systems, repeated flooding produces a flat floodplain perched above the river banks.

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5
Q

Why are people drawn to living on floodplains?

A

access to fresh water
fertile soils
cheap transportation via rivers and railroads
ease of developing flat land

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6
Q

Why does flooding affect more people worldwide than any other type of natural disaster?

A

people are drawn to living on floodplains - more and more people live in flood-prone areas

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7
Q

What are the main dangers of flooding?

A

people drowning
damage to buildings, roads, bridges, canals and sewer systems
contamination of water supplies
spreading of water-borne diseases
destruction of crops, trees and other vegetation

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8
Q

What are the two overarching categories of floods?

A

hydrometeorological floods

outburst floods

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9
Q

What are hydrometeorological floods caused by?

A

adverse weather conditions

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10
Q

What are outburst floods caused by?

A

failure of natural dams

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11
Q

What are some types of hydrometeorological floods?

A 
rainfall floods
snowmelt floods
rain-on-snow floods
ice jam floods
storm surges
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12
Q

What is the difference between slow rainfall floods and flash floods?

A

Slow rainfall floods are those caused by heavy rainfall sustained over a large area for a long period of time, whereas flash floods are formed by torrential rainfall over a shorter period (<6 hours) and often a much smaller area.

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13
Q

Where are flash floods particularly dangerous?

A

They are dangerous in arid regions because they take people by surprise, they have dry, poorly absorbent soil and occasional torrential rain.
Be careful when hiking and do not camp in the bottom of narrow canyons or on dry stream beds.

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14
Q

What are snowmelt floods caused by?

A

They are caused by melting of heavy snow pack or late, rapid thaws. The meltwater is unable to penetrate the frozen or water-saturated ground, generating surface runoff which overwhelms rivers.

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15
Q

When do rain-on-snow floods occur?

A

They occur when heavy rain falls on snowpack, generating large quantities of surface runoff, damaging buildings from the sheer weight of the wet snow, and raising avalanche hazards.

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16
Q

When do ice jam floods happen?

A

They happen when pieces of floating ice carried downstream accumulate at bends in the river, changes in river gradient, or upstream of bridges, and start to obstruct the water flow. The water that is held back may cause flooding upstream, and the sudden failure of the ice jam can cause flooding downstream.

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17
Q

Where are ice jams a major cause of flooding?

A

They are a major cause of flooding in Canada, both at river ‘freeze-up’ in early winter and at ice ‘break-up’ in the spring.

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18
Q

What is one of the most flood prone rivers in Canada?

A

The Red River

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19
Q

What types of flooding is the Red river particularly susceptible to?

A

snowmelt and ice jam flooding

20
Q

What areas does the Red river drain?

A

It drains parts of Minnesota, North Dakota, Saskatchewan and Manitoba, meandering northwards into Lake Winnipeg across an extremely flat, broad floodplain.

21
Q

What caused severe flooding of Winnipeg by the Red River in 1950?

A

A snowy winter and late thaw caused flooding of a 1600 km² area.

22
Q

How many people were evacuated in the Winnipeg flood of 1950?

A

> 100,000 people (one third of Winnipeg’s population) this was the largest evacuation in Canadian history

23
Q

What was done in response to the Winnipeg flood of 1950?

A

A massive artificial flood-control waterway (the Red River Floodway) was built in the 1960s to divert floodwaters around the city. This has saved Winnipeg on several occasions since.

24
Q

What are storm surges?

A

They are dramatic escalations in sea-level which accompany some storms, especially tropical cyclones, leading to extensive flooding of low-lying coastal areas.

25
Q

What are sea-level rises (that cause storm surges) caused by?

A

strong onshore winds that shear the ocean surface

low atmospheric pressure in the storm centre which allows the ocean surface to ‘bulge’ upwards

26
Q

Where do maximum storm surges occur during North Atlantic hurricanes?

A

15–30 km to the right of the eye landfall point, where onshore winds are strongest

27
Q

When do storm surges cause the greatest damage?

A

when the storm surge coincides with high tide

28
Q

What are outburst floods?

A

These are a relatively unusual but catastrophic type of flood caused by the failure of natural dams.

29
Q

In what ways do dams form naturally?

A

Dams form from the blockage of rivers or lakes by:
mass movements (especially landslides)
lava flows or pyroclastic deposits
glaciers

30
Q

What are outburst floods from glaciers referred to as?

A

jökulhlaups - originates from Iceland

31
Q

When are jökulhlaups triggered in glaciated volcanic areas?

A

when sub-glacial volcanic eruptions suddenly melt huge quantities of ice

32
Q

What is a drainage basin?

A

It can be thought of as a large net, open to the sky, catching and channeling rain or snow melt. Water on hillslopes either infiltrates the soil and rock, to flow slowly as groundwater or, if the ground is saturated or impermeable, it flows more rapidly overland as surface runoff. This feeds a stream network which channels the water to the drainage outlet.

33
Q

What is a river’s carrying capacity (measured as discharge, in units of m³/s)?

A

Amount that it can transport within the channel confines. When this is exceeded, flooding occurs.

34
Q

What is used to estimate flood return periods?

A

Historical events (though the record may not be long, or even appropriate)

35
Q

What can flood return periods be used for?

A

zoning and for designing buildings and infrastructure

36
Q

When do rainfall floods occur?

A

They occur when precipitation exceeds ground infiltration and the resulting increase in surface runoff overcomes the carrying capacity of rivers.

37
Q

What are half of all flash-flood fatalities due to?

A

people driving through flooded roads

38
Q

Why does the Red River have a very low gradient?

A

It crosses the former bed of Lake Agassiz, a vast lake that formed from melting of the Laurentide ice sheet ~10,000 years ago, and at ~10,000 years old has not had time to carve a deep valley. Instead, it meanders over a broad floodplain.

39
Q

What is the Red River underlain by?

A

clay- rich, impermeable glacial till

40
Q

Why did the Red River flood in 1997?

A

There was a record 250 cm winter snow pack, a spring blizzard which added a further 50 cm of snow, and finally a rapid thaw. A 50 x 100 km lake formed in Manitoba.

41
Q

What happened on on 4th Nov. 1996 in Iceland?

A

The sub-glacial lake in Vatnajökull failed in a massive jökulhlaup.

42
Q

How much water and ice was discharged in just 20 hours during the 1996 Iceland jökulhlaup?

A

3.6 km³ water and 1,000 tonnes of ice

43
Q

What was the peak flow rate during the 1996 Iceland jökulhlaup?

A

55,000 m³/s (20 x the Niagara Falls)

44
Q

How much sediment was deposited on the Skeiðarársandur plains during the 1996 Iceland jökulhlaup?

A

9 m of sediment

45
Q

What did the 1996 Iceland jökulhlaup destroy?

A

6 km of Iceland’s ring road highway, including two concrete-steel bridges built to withstand jökulhlaups

46
Q

How common are outbursts from moraine-dammed lakes?

A

less common but can be just as destructive

47
Q

How do outbursts from moraine-dammed lakes occur?

A

Water either overtops the moraine, cuts through it, or seeps through it once the internal ice core has melted away.

EOS 170 (22 decks)

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