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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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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.
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.