Tornadoes and Floods

Cold Waves and Polar Vortex

Cold wave: a relatively rapid, prolonged fall in temperature that requires people to take precautions against the cold

• There is no standard criteria for this

Polar vortex: flow of cold air (a fluid) circulating around the North Pole

• If mid-latitude cyclone occurs in N. America, the polar front under these conditions, can extend far to the south and may pass over the Gulf Coast into central Florida

• This can last for days

Winter Storms Intensity

• In the U.S., snowfall intensity depends on visibility

  • Light snow, when visibility is greater than 1km

  • Moderate snow, between 1km and 0.5km

  • Heavy snow, less than 0.5km

Winter Storms - Blizzards

• Associated with snowstorms

• National Weather Service (U.S)

  • Blizzards when winds exceed 56km/h and visibility less than ¼ mile for 3 hours

• Ground blizzards extend 10-15 above the ground and can persists for hours. This is not new precipitation but movement of snow by wind

Winter Storms - Ice Storm

• Ice Strom: a winter storm produced an accumulation of ice at the ground surface as a result prolonged freezing rain

• In the winter, the 0C isotherm, the invisible boundary that separates warmer air (above freezing) from cooler air (below freezing), envelops a wedge or tongue of warmer air that crosses the warm front

• The character of the precipitation that reaches the ground at a given location varies depending on the position of the location relative to this wave

Winter Storms - Ice Storm - Locations

• Location A: now passes down across the 0C isotherm at an elevation of 0.5-1.5km and descends into warmer air, where it melts

• Location B: falling rain crosses the 0C isotherm again, close to the ground. It then descends in coddler air, so raindrops become supercooled, meaning that they remain in liquid form, even though they are below freezing. These freeze on contact on a cold solid surface - freezing rain

• Location C: the wedge of wamr air aloft is not as deep and its temperature, although above freezing is not as warm. Snow falling through the warm air wedge and the location melts only partially, and as it falls into cold airs, it refreezes

  • Frozen raindrops are called sleet or ice

• Location D: snow call falls through cold air all the way to the ground

The ITCZ is visible as a band of clouds encircling Earth near the Equator

Tornados - Regina Tornado - June 30, 1912

• Considered deadliest tornado in Canadian history (F4)

• 28 killed, 300 injured, ~2500 displaced

• 1.2 million CAD in damages ($30.5 million in 2022)

Tornadoes

• Tornado Alley region in central U.S> and Canada associated with frequent tornadoes

  • Oklahoma, Kansas, Nebraska, South Dakota, Iowa, Minnesota, etc.

• Research suggests that the main alley may be shifting eastward away from the Great Plain

• Tornadoes are also becoming more frequent in the northern and eastern parts of Tornado Alley where it reaches the Canadian Praries, Ohio, michigan, and Southern Ontario

• Scenario looks at tornadoes forming from SW to NE U.S.

  • A supercall thunderstorm wil contain a strong rotating updraft, called a mesocyclone, and that strong doendrafts form on both the forward (NE) flank and the rear (SW) flank of mesocyclone

Tornado development occurs in association with the rear-flank downdraft

• As the rapidly downward-flowing air of this downdraft approaches the round, it spreads outward and produced a roll of rotating air near the ground along the downdraft’s speaking outer boundary

• The air in this role spins around horizontal  axis

• A tornado forms when the part of the roll that passes under the base of the mesocyclone gets pulled upward into the mesocyclone

• Initially, the roll warps into a tight arch, but the clockwise-rotating arm, rotating in the opposite direction of the larger mesocycone, quickly weakens and dissipates, while the counter clockwise-rotatinf arm, the rotating in the same direction as the meocyclone, gets carried into the updraft, where it stretches and narrows to become with the ground, it forms a tornado

• If the spinning vortex is in contact with the ground, a tornado will form

Multiple-vortex tornadoes:

can form when the extremely low pressure that develops at the base of the vortex can draw air downward within the tornado core

• This downdraft forms aloft within the tornado circulation and descends inside the tornado toward the ground

• This causes the tornado to become wider

The different wind direction between the outflow and the rotating winds of the tornado cause the main tornado to break down into several small vortices around the main tornado

The Enhanced Fujita (EF)

scale is used to classify the tornado based on the damage it causes using correlation of observed damage and wind speed

Rivers and Floods - What is a stream?

A body of water confined within a channel, supplied with water by its drainage basin

What determines the size of a stream?

• Climate

• Size of drainage basin

• Substrate

• Vegetation

Stream Valley:

 area between tops of slopes on both sides of the river

channel:

trough through which water flows

Thalweg

line connecting deepest parts of the channel

Discharge

volume of water flowing downstream/unit time (m3/sec)

Where would you expect to find the highest flow velocities in a stream?

• Straight areas

  • Center of channel

  • Just below water surface

• Around meanders

  • Outside of meander bend

Valley profiles - How does the shape of a valley change downstream?

• Cross sectional profiles

  • Steep-sided, V-shpaed near source, canyons

  • Broader valleys downstream

• Longitudinal profile

  • Steep gradients near source

  • Low gradients downstream

• Base level

  • Deepest level to which streams can erode

  • Usually sea level (but not always)

• Channel morphology

  • Sinuosity is used to classify channels

    • Ratio of stream length: valley length

Meandering streams

• Common in areas with low slopes

  • Thalweg close to outer bend of meander

• Outside bend of meander - deepest water, highest flow velocities: cutbank

• Inside bend of meander: point bar

• Erosion of cutbank, deposition on point bar = migration of meanders

• Meanders can join - form neck cut off

• Oxbow Lake: crescent shaped lake formed where a meander is cut off from a stream

Flood plain

• Broad strip of land built by sedimantation on each side of the channel

  • Covered with water when river floods

  • Coarset material deposited closest to the channel - forms levees

  • Finger-grained material carried further away - clays and silts

What kinds of sediment do meandering streams deposit?

• On point bar

  • Snad and silts (ripples and dunes)

• On floodplain

  • Silts and clays

• With migration forms

  • Fining upwards sequence

Flood vs. Shore floods vs. Inland floods

• Flood - an event during which water covers significant areas of land that are normally dry, a stream floods when its discharge becomes so great that water overtops its stream’s banks and submerges areas outside its channel

• Shore floods - associated with storm surges, storm waves, tsunamis

• Inland floods - oversupply of freshwater from rainfall or melting snow

Braided Streams

• Multiple channels

• Common on steep clopes

• Common in glaciated or arid regions

• Channels split and rejoin

• Areas of deposition between channels

What kinds of sediments?

• Coarse-grained

  • Gravel and sands

Urban river systems - How does urbanization affect river systems?

• What environmantal hazards are associated with fluvial environments?

  • Ground surface less permeable

  • Less infiltration of rainwater

  • More storm runoff

    • Increased flooding

What happens once water falls to the ground?

• Infiltration 

  • Process through which water on the ground enters into the soil

  • Amount of infiltration dependent on 

    • Soil moisture

    • Substreate

    • Slope

    • Type of vegetation

    • Intensity of precipitation

Flood Hydrograph

• Shows effect of storm event on river discharge

• Urban rivers have ‘flashy’ discharge

  • Water enters river quickly, causes high flood peaks

Hamilton Flooding Risk

• Hamilton is likely to experience flooding this year, as the Lake Ontario water levels are at a historical high

  • City has very few options for prevention due to the length of the shoreline

  • Sandbags would not be effective due to the force of wave action

  • Popular trails are being flooded

  • Parking lots near the harbour

• Hamilton has spent over $700,000 on repairs form 2017 and 2018 flooding

Ottawa Floods

• Ottawa experiences severe flooding in April 2019

  • Data gatherers by CSA between April 20-30

• In this time

  • 2196 homes were flooded or at risk of flooding 

    • 3800 homes and octtages were affected in Gatineau

  • 100km of roads flooded

  • In total 42km2 was flooded in Ottawa

    • 39km2 in Gatineau

• Damaged houses, path, bridges, and shorelines

  • National Capital Commission could spend $10 million in repairs

• Looking to prepare for the future to reduce damages

Urban Systems

• Erosion of stream banks, clogs channels

• Sediment poses problems for fish

Reduced water quality

• Urban runoff contains road salt, grease oil

• Bluffers Park, Scarborough, 1991

Know differences from natural environment to urban environment