Impact of Global Warming on Floods

Figure TS.10

1951-2003 trends in intense rainfall or very wet days (i.e., corresponding

to the 95th percentile and above). White land areas have insufficient data

for trend determination.

IPCC (2007)

Europe Floods

Number

Deaths

Flooding Types

River Flood: Rainfall over extensive areas. Also snow melt in the spring. Some floods occur

seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.

Coastal Flood: Winds generated from tropical storms and hurricanes or intense offshore low

pressure systems can drive ocean water inland and cause significant flooding. Escape routes can be cut off and blocked by high water. Coastal flooding can also be produced by sea waves called tsunamis, sometimes referred to as tidal waves. These waves are produced by earthquakes or volcanic activity.

Urban Flood: As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases runoff 2 to 6 times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements can become death traps as they fill with water.

Flash Flooding: Result of heavy rainfall in a short time, and in a small area. Has a short lifetime

of a few hours and can be very destructive due to the power of the water flow. The flash flood start within a few hours of the heavy rainfall.

Ice Jam: Floating ice can accumulate at a natural or man-made obstruction and stop the flow of

water.

A lot of water in a little time

Heavy Rain is the main cause of flooding

Large Flooding events since 1980

Global Warming Projections for intense rainfall events

2090-1990

Mean/#rainy days

River Floods

Very high amounts of rain, snowmelt, or

both lead within watershed will lead to

water levels in river rising higher than

the banks, spreading out into the flood

plain – low lying areas along a river.

Coastal Flooding

Normally when dry low-lying land is

flooded by sea water. The extent of

flooding is a function of the land

elevation and topography along the

coast. Caused by severe low pressure

weather systems, or even tsunamis

(linked to earthquakes).

Urban Flooding

Specific related to the lack of

drainage in urban areas. As there

is little open soil that can be used

for water storage, nearly all the

rainfall needs to be transported as

surface water, or via the sewage

system.

Ice Jams

Ice jams can occur near river

bends, mouths of tributaries,

points where the river slope

decreases, downstream of

dams and upstream of

bridges or obstructions. The

water held back by ice jams

can cause flooding upstream.

If the ice jam suddenly

breaks because of the water

pressure, this can result in

flash flooding downstream.

Flash Floods

A rapid flooding that

develops in minutes to

hours. Several factors

contribute to flash flooding.

The two key elements are

rainfall intensity and

duration. Intensity is the

rate of rainfall, and duration

is how long the rain lasts.

Topography, soil

conditions, and ground

cover also play an

important role.

Differences between large river floods, and flash floods

due to rapid development

Flash Flood in Zarvraggia, Switzerland, July 1987

Time difference of 15 minutes between photos

Flash floods occur

with intense rainfall

rates that often exceed

the soil's infiltration

capacity even when

the soil is dry.

When rain is very intense...

Soil Influences

The three critical soil

properties to consider when

assessing the risk of flash

flooding are soil moisture,

soil texture, and soil

profile.

Soil moisture is often

considered the most

important soil factor for rapid

runoff and flash flooding.

There is some validity to this

reasoning in that if the soil is

saturated there is no room

for additional rainfall to

infiltrate, and all rainfall

becomes runoff regardless of

the soil texture.

Soil Influences: Soil Texture

Certain soil textures such

as clay, and to some extent

silt, can be associated with

low infiltration rates and

result in rapid runoff during

intense rainfall. Therefore,

runoff from intense rainfall

is likely to be more rapid

and efficient with clay soils

than with sand.

Soil Influences: Soil Texture

Although sandy soils

permit greater infiltration

of intense rainfall, rapid

runoff can occur if there

is only a thin layer of

soil. For example, if an

impermeable layer of

rock underlies a thin

layer of soil, that soil

layer can saturate

quickly and result in

large amounts of runoff.

Rainfall and Runoff Relationship

Flash floods are rapid-onset

hydrologic events that can be

very difficult to forecast. A

combination of high rainfall

rate with rapid and often

very efficient runoff is

common to most flash flood

events. Ice jam formation

on rivers can also play a role

in rapid-onset flash flooding,

particularly upstream of the

ice jam.

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Early Warning Systems can help…

The soil becomes saturated and overland flow and through flow reach the river and discharge increases. Overland flow arrives first.

The time from peak rainfall to peak discharge is the LAG TIME.

The discharge starts to fall slowly as water is added from through flow and groundwater flows which are much slower.

The base flow supplies the river with water between storms and keeps it flowing in summer. Rainfall is

intercepted or infiltrated into the soil moisture store

Start of the storm there is a slow rise in discharge, as only a small amount of water falls into the channel

HYDROGRAPH

FACTORS WHICH

INFLUENCE STORM

HYDROGRAPHS

VEGETATION COVER This varies seasonally. The type and amount will affect interception and stemflow/throughfall. Overland flow is reduced. Lag time will be increased.

ROCK TYPE Impermeable rocks prevent groundwater flow and encourage through flow and overland flow. These rocks will decrease lag time. Permeable rock will have the opposite effect.

LAKES & RESERVOIRS These will store floodwater and thus increase lag time and control river response to heavy rainfall.

SOIL TYPE & DEPTH Deep soils store more water, pipes in the soil encourage through flow. Soils with small pore spaces will reduce infiltration and increase overland flow.

LAND USE Impermeable surfaces created by urbanisation will reduce infiltration and encourage overland flow. Different types of crops affect interception rates e.g. cereals 7-15%.

RAINFALL INTENSITY & DURATION Intense rain will increase overland flow and reduce lag times. Gentle rain over a longer time will allow more infiltration.

SLOPES Steep slopes will encourage overland flow and gentle slope will slow run off down.

CLIMATE The distribution of rainfall over the year and the temperatures will affect the lag times.

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Wadi Ara, Israel, 1-2 April, 2006 RainRain EvaporationEvaporation

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Simulated vs. Observed River Discharge (location A and B)

Flood Management Measures Flash floods

Reducing Risks of Floods

1993 Mississippi Floods, USA

• Some locations on the Mississippi River flooded for almost 200 days while locations on the Missouri River neared 100 days of flooding.

• Approximately 10,000 homes were destroyed as a result of the flooding.

• 15 million acres (60,000 km²) of farmland was inundated.

• The floods cost thirty two lives officially; however, a more likely number is suspected to be around fifty people.

• An estimated total 20 billion dollars in damages across all areas.

• Floodwater removed more than 600 billion tons of topsoil from the heavily agriculturally developed Midwest, and deposited untold amounts of silt and sand in their place.

• Barge traffic on both rivers was completely stopped for two months.

• Numerous water and sewage treatment plants across the affected area were flooded and deactivated or destroyed. The water treatment facility in Des Moines, Iowa, for instance, had its levies fail, and caused a complete shutdown of service of fresh water for the entire downtown and southern end of the city of ~350,000 people for 18 days, and running water of any sort for 11 days.

Total Damages:

Mozambique Flood 2000 Physical Causes

Continuous heavy rain for more

than 5 weeks in Jan/Feb over

Southern Africa

A total of 1163 mm fell compared

to an average of 177 mm !

In late February hurricane Eline

struck bringing more torrential

rain

Human Causes

Destruction of grasslands in the

high plateaus which normally act

to soak up rainfall and release it

slowly into the rivers.

Draining of the wetlands along the

rivers for farming –these marshy

areas normally store floodwaters.

Huge growth of urban area

creating lots impermeable areas.

Thousands died. Around 1 million

people needed shelter, food, and

medical care.

Disease and malnutrition were

common in the months after the

flood due to lack of medical

supplies, clean water and food.

Contaminated water lead to an

outbreak of cholera.

The few bridges and roads that were in

the country were washed away –

economic and social problem

Worst flooding in 50 years

Impacts of Mozambique Floods

Pakistan Floods – July, August 2010

Floods covered at least 37,280 square kilometers of Pakistan at some time between July 28 and September 16, 2010. Relief agencies used maps derived from satellite data to direct aid to many of the victims and to plan recovery efforts.

http://www.unitar.org/unosat/maps/PAK

Relentless Rain (Monsoon)

The 2010 floods in Pakistan were caused by extremely high rainfall in the Indus River watershed during July and August. These maps show the satellite estimates of the difference in rainfall between 2010 and the long-term average for the region.

A long-lived high-pressure system north of the Black Sea trapped hot air over Russia in 2010, and triggered heavy rainfall over Pakistan. This image shows water vapor in the atmosphere (left) and thermal infrared emissions of the Earth (right). Water is bright in the left image; on the right, dark areas are hot (desert in mid-day) and cold areas (cloud tops) are white.

Animation shows the interaction between high-level flow of water vapor and the dynamics of clouds.

Human Factors made things worse

Construction—including dams, roads, and canals—can divert water from its natural path. This can exacerbate flooding, or cause water to pool in areas without an outlet, sometimes for months.

Some parts of Pakistan remained under water for months after the rains subsided. These false-color satellite images show flood water (blue) in western Sindh province in September 2010, November 2010, and January 2011. It is apparent that roads and other infrastructure constrained the flow of flood water.

Floods in India, 1-2 December 2015

Chennai, India 2015

More than 400 people have died and over 1.8 million people have been displaced. With estimates of damages and losses ranging from over US$3 billion to US$15 billion the floods are among the costliest natural disaster of the year

Public Health Risks from Flooding

Immediate and short-term dangers Drowning • Most deaths from flash floods are people who drown while trying to swim

away or while trapped in flooded buildings. • Many people drown when driving through floodwater, misjudging how deep

the water is and how quickly the current is moving. For example, over 57% of deaths from floods in the United States are associated with motor vehicles.

Injuries • Injuries happen during and after floods when people return to clean and

repair their homes. • During floods, heavy objects move quickly through floodwater and can hit

people. • Flooded buildings can have damaged wiring and electrical appliances that

could electrocute people or cause fires. • In any season most floodwater is well below normal human body

temperature and can cause hypothermia.

Related and longer-term dangers Diseases spread through water contamination and sewage backup • Heavy rain can overwhelm drainage, water treatment and sewage systems and

contaminate drinking water. • Floodwater can mix with pollutants such as agricultural waste, chemicals, raw

sewage or metal and can contaminate local waterways that supply drinking water.

• Combined sewer systems (those that carry both raw sewage and storm water) can overflow and contaminate waterways. Combined sewer systems can also back up into household plumbing and increase the risk of contaminating food and water.

• Flood-contaminated water can carry diseases that can impact human health. Diseases spread through food contamination • Food contamination and related illness can occur following flooding and power

outages given that temperatures required to keep food safe may not have been maintained or that food may have come in contact with contaminated water.

Mental health • Floods can lead to physical

health problems, personal loss and financial difficulties which can result in common mental health disorders such as anxiety, depression.

• Long term health issues, such as depression and post-traumatic stress disorder, can affect people for a long time after a flood. For example, after Hurricane Katrina most adults with post-traumatic stress disorder still had not recovered two years after the event.

Homework

Ahern, M., et al., 2005: Global health impacts of floods: Epidemiologic Evidence, Epidemiologic Reviews, 27, 36-46.