Floods

What are floods?

Overflowing of water onto land that is normally dry

What are types of floods?

River floods, flash floods, coastal floods

What are some properties of floods?

Can occur quickly or over long periods, may last days, weeks, or longer, most common of all weather related disasters

What are regional floods?

Flooding under high water for weeks, few deaths, extensive damage, occur in floodplains/large river valleys with low topographies

What causes river flooding?

Seasonal snowmelt, excessive rainfall

How long do river floods last?

Days, weeks and months

What causes flash flooding?

Excessive rainfall (thunderstorms, hurricanes, atmospheric river), levee or dam failure, release or water from ice dam or debris

How long do flash floods last?

Less than 6 hours between rainfall and flooding

What are some properties of flash floods?

Heavy rainfall in steep or dry areas, cause most deaths, especially vehicle related

Why is it dangerous to drive in floodwater?

Only 2ft of water required to lift and carry average car, lots of uplift and lateral force

What are atmospheric rivers?

Up to thousands of km long, hundreds of km wide bands of warm air with voluminous water vapour

Where to atmospheric rivers come from?

Originates in tropics, travels at low altitude between larger air masses

How do atmospheric rivers contribute to flooding?

Carry exceptionally large volumes of water vapour, falls intensely, especially after orographic lifting, extreme rainfall to western coasts in North America, North Africa and Europe

What causes coastal flooding?

Tsunami, seiches, storm surges, high tides

How long does coastal flooding take?

Very quick, catastrophic even

What is a drainage basin?

Total area from which water flows into a stream

What does a small drainage basin mean for floods?

Short lasting (minutes), fast moving, flash floods

What does a large drainage basin mean for floods?

Long lasting (weeks), regional floods

What is flood frequency analysis?

Uses annual maximum discharges to predict the magnitude and frequency of flood discharges

What can we say with confidence about flood size and frequency?

Future floods are likely to be bigger than those in the past

What does a flood frequency curve do?

Extrapolate how often floods of given discharges occur using historical flood records, unique to each stream

How do you plot a flood frequency curve?

Size over recurrence interval, volume on Y axis, recurrence interval on X axis

What is the probability for bigger floods?

Longer return period, lower likelihood in given year

How do you find flood frequency as probability?

1/Recurrence Interval

What is yearly probability of floods?

Probability of a flood occurring each year the same regardless when the last flood happened: 100 year flood has 1% chance of occurring each year

What is cumulative probability?

The longer you wait for a flood of a certain magnitude, the more likely it will happen: 100 year flood has a 63% chance of occuring once in 100 years

What increases the risk of flooding of rivers (particularly the Red River)?

Soil saturation at time of freeze, amount of snow, amount of water content in snow, amount of water available prior to spring runoff, soil frost depth, future rain

What are the primary hazards of floods?

Direct contact with flood water, damage to infrastructure, massive amounts of erosion, water damage to buildings, furniture, cars, deposited mud and debris, mudflows, crop loss from flooded farmland, drowning, concentrating of garbage and pollutants

What are the secondary effects of floods?

Disruption of services, drinking water polluted, disease, gas and electrical disruption, transportation systems disrupted, shortages of food and supplies

What are the tertiary effects of floods?

Avulsion (location of river channel changes), deposited sediments destroy or increase farmland quality, job loss, insurance rate increase, corruption, destruction of habitat

What are structural responses to flood damages?

Dams, levees, sandbagging, raising buildings, straightening/widening/deepening/clearing channel to increase water carrying stability

What are limitations to dams?

Life span of dams are limited by construction materials, construction style, rate at which sediment fills reservoir

How do dams flood?

Overtopping, heavy rainfall below dam, dam failure

What is a levee?

An embarkment built to prevent overflow of river

What is the argument against levees?

Cost more than the value of structures intended to protect

What is the argument for levees?

Reduced frequencies of flooding saves billions of dollars

What are ways that levees can be compromised?

Wave attack, erosion by overtopping, failure by slumping, undermining by piping

What is the value of sandbagging?

Sometimes lessen damage but more therapeutic

What are non-structural responses to floods?

More accurate flood forecasting, zoning and land use policies, insurance programs, evacuation planning, education

Does flood forecasting help with deaths and damages?

Reduce fatalities but damages are only increasing

Do insurance ad campaigns seem effective?

No

What is a hydrograph?

Plots volume of water (stream depth also) against time

What flood processes does a hydrograph show?

Time lag after rainfall for runoff to reach stream channel then stream surface height rises quickly, stream level falls more slowly as underground flow of water continues to feed stream

How does urbanization affect flooding?

Floods last 20% as long as regular floods, but can be four times higher, higher frequency of floods, more flash floods

What is channelization?

Trying to control floodwaters by making channels clear of debris, deeper, wider and straighter

Why does urbanization affect floods in these ways?

More storm sewers, more roofing and paving, increased surface runoff of rainwater

Does channelization work?

Sometimes, but often streams regain equilibrium by erosion to pick up sediment and decrease gradient