Flood, history, types, impacts

1. Flood

2. Natural disasters
Hydrological disasters
• Floods
• Limnic eruptions
• Tsunami
Geological disasters
• Avalanches and landslides
• Earthquakes
• Sinkholes
• Volcanic eruptions
Wildfires
Meteorological disasters
• Blizzards
• Cyclonic storms
• Droughts
• Thunderstorms
• Hailstorms
• Heat waves
• Tornado
Space disasters
• Impact events and airburst
• Solar flare

3. Floods
• A flood is an overflow of water that 'submerges' land
• The EU Floods Directive defines : flood as a temporary
phenomena covering the land with water which is usually not
covered by water
• In the sense of 'flowing water’:
• Flooding may result from the volume of water within a body of
water, which overflows, causing some of the water to escape its
usual boundaries
• While the size of a lake or other body of water will vary with
seasonal changes in precipitation and snow melt, it is not a
significant flood unless the water covers land used by man, like a
village, city or other inhabited area, roads, expanses of farmland,
etc.

4. • 75% of rainfall is concentrated over four months of monsoon (June -
September) and as a result almost all the rivers carry heavy discharge
during this period. ( South west monsoon)
• Post monsoon season ( North East mansoon) – tamil nadu receives rain
during the months of October and November.

5. Recent floods in india
• June 2013 North Indian floods: Heavy rain due to cloudburst caused severe floods and
landslides on the North Indian states, mainly Uttarakhand and nearby states. More than
5,700 people were presumed dead
• June 2015 Gujarat flood: Heavy rain in June 2015 resulted in widespread flood in
Saurashtra region of Gujarat resulting in more than 70 deaths. The wild life of Gir Forest
National Park and adjoining area was also affected.
• 2015 South Indian floods: Heavy rain in Nov-Dec 2015 resulted in flooding of Adyar,
Cooum rivers in Chennai, Tamil Nadu resulting in financial loss and human lives.
• 2016 Assam floods: Heavy rains in July–August resulted in floods affecting 1.8 million
people and flooding the Kaziranga National Park killing around 200 wild animals.
• 2017 Gujarat flood: Following heavy rain in July 2017, Gujarat state of India was affected
by the severe flood resulting in more than 200 deaths.
• August 2017 Nepal and India floods
• August 2018 Kerala Flood: Following high rain in late July 2018 and heavy Monsoon
rainfall from August 8, 2018, severe flooding affected the Indian state of Kerala resulting
over 445 deaths

6. most common causes of flooding
1. Heavy Rains
When it rains heavily, systems gets overwhelmed, and that water doesn’t
drain nearly as quickly as it needs to.
In short, the drainage systems back up, and the water rises
2. Overflowing Rivers
You do not necessarily need to have heavy rains to experience flooding in
your area. For example, if you live along a river and areas upstream from you
experience heavy rains, it could lead to a serious overflow where you live.
Most larger rivers include a series of dams to help manage large amounts of
rainfall, and most river systems are managed by government authorities.

7. 3. Broken Dams
 Aging Dams ( dams running after the design period)
 When heavy rains come, and water levels rise, aging dams can fail and
unleash torrents of water on unsuspecting households.
This is part of what happened after Hurricane Katrina hit New Orleans in 2005.
Levees failed and made the flooding far worse than it would have been otherwise.
4. Urban Drainage Basins
 Many of our cities are made of mostly concrete and other impermeable
material.
 When you have an urban drainage basin that is made of concrete, there
is no ground for water to sink into. So, when those drainage basins fill
up, it is going to mean flooding for low-lying areas.
This is mostly the case in large urban areas —like, Houston and Los Angeles.
When heavy rains strike, the basins used to drain them cannot always handle
the load.

8. 5. Storm Surges and Tsunamis
6. Channels with Steep Sides
Flooding often occurs when there is fast runoff into lakes, rivers and
other reservoirs
7. A Lack of Vegetation
Vegetation can help slow runoff and prevent flooding. When there is a
lack of vegetation, however, there is little to stop water from running
off. This can be a bit of a conundrum after a drought.
While area residents likely welcome the rain, the lack of vegetation after
the drought can cause flash flooding. This does not always happen given
that basins and reservoirs are close to empty, but it can occur in cases of
extreme rains following long periods of drought

9. 8. Melting Snow and Ice
A winter of heavy snow and other precipitation can lead to a spring of
flooding. After all, that snow and ice have to go somewhere when they
melt.
Most mountainous areas experience relatively consistent snowfall totals
from year to year, but an unusually heavy winter of precipitation can spell
bad news for low-lying areas around the mountains when spring hits.

10.  Flood severity is controlled by the
watershed characteristics
• Basin size (area)
• Topography
• Drainage dentistry (length of streams per area)
• Vegetation type and distributions
• Geology
• Soil type and thickness
• Runoff processes

11. Floods in India:
The peculiar nature of India’s climate, dominated by monsoons, causes situations
where drought and floods may affect different pockets at the same time of year. The
main reasons for floods in India are:
1. Heavy concentrated rainfall
2. Cyclone and strong winds
3. Inadequate drainage
• Indiscriminate deforestation in catchment areas and upper reaches leads to soil
erosion.
• This in turn causes silting of river courses downstream.
• A thinned soil cover also results in reduction of infiltration and consequent
increase in runoff of large volumes of water.
• Overgrazing, especially in the foothills, leaves the soil without cover and therefore
vulnerable to erosion.
• Unscientific farming practices like shifting cultivation result in loss of vegetation
cover and consequent soil erosion.

12. Types of flood
1. Coastal Flood
2. Fluvial Flood
3. Pluvial Flood

13. Coastal (Surge Flood)
A coastal flood, occurs in areas that lie on the coast of a sea,
ocean, or other large body of open water.
 It is typically the result of extreme tidal conditions caused by
severe weather.
Storm surge — produced when high winds from hurricanes and
other storms push water onshore — is the leading cause of coastal
flooding and often the greatest threat associated with a tropical
storm.
 In this type of flood, water overwhelms low-lying land and often
causes devastating loss of life and property.

14. Coastal flooding is categorized in three levels:
Minor: A slight amount of beach erosion will occur but no major
damage is expected.
Moderate: A fair amount of beach erosion will occur as well as
damage to some homes and businesses.
Major: Serious threat to life and property.
Large-scale beach erosion will occur, numerous roads will be
flooded, and many structures will be damaged. Citizens should review
safety precautions and prepare to evacuate if necessary.

15. The severity of a coastal flood is determined by several factors,
including the strength,
size,
speed,
and direction of the storm.
• The onshore and offshore topography also plays an important role.
• To determine the probability and magnitude of a storm surge, coastal
flood models consider this information in addition to data from
historical storms that have affected the area, as well as the density of
nearby development.

17. Fluvial (River Flood)
Riverine flooding, occurs when excessive rainfall over an extended
period of time causes a river to exceed its capacity.
It can also be caused by heavy snow melt and ice jams.
 The damage from a river flood can be widespread as the overflow
affects smaller rivers downstream, often causing dams and dikes to
break and swamp nearby areas.

18. There are two main types of riverine flooding:
1. Overbank flooding
occurs when water rises overflows over the edges of a river or
stream.
This is the most common and can occur in any size channel —
from small streams to huge rivers.
2. Flash flooding
 characterized by an intense, high velocity torrent of water that
occurs in an existing river channel with little to no notice.
Flash floods are very dangerous and destructive not only because
of the force of the water, but also the hurtling debris that is often
swept up in the flow.

19. The severity of a river flood is determined by
the amount of precipitation in an area,
how long it takes for precipitation to accumulate,
previous saturation of local soils,
 and the terrain surrounding the river system.
In flatter areas, floodwater tends to rise more slowly and be
more shallow, and it often remains for days.
In hilly or mountainous areas, floods can occur within minutes
after a heavy rain.
To determine the probability of river flooding, models consider
past precipitation, forecasted precipitation, current river levels,
and temperatures.

21. Pluvial (Surface Flood)
• A pluvial, or surface water flood, is caused when heavy
rainfall creates a flood event independent of an overflowing
water body.
• One of the most common misconceptions about flood risk is
that one must be located near a body of water to be at risk.
• Pluvial flooding debunks that myth, as it can happen in any
urban area — even higher elevation areas that lie above
coastal and river floodplains.

22. There are two common types of pluvial flooding:
• Intense rain saturates an urban drainage system. The system
becomes overwhelmed and water flows out into streets and
nearby structures.
• Run-off or flowing water from rain falling on hillsides that are
unable to absorb the water. Hillsides with recent forest fires are
notorious sources of pluvial floods, as are suburban communities
on hillsides.
• Pluvial flooding often occurs in combination with coastal and fluvial
flooding, and although typically only a few centimeters deep, a pluvial
flood can cause significant property damage.

24. A “100-year flood” is a flood that has a 1% chance of occurring
in any given year
• How to determine the discharge of a “100-year flood?

29. Impacts
• Moving water has remarkable destructive power.
• When a river overflows its banks or the sea drives inland, structures
poorly equipped to withstand the water’s strength are no match.
• Bridges, houses, trees, and cars can be swept away.
• The erosive force of moving water can drag and carry away the
material constituting the foundation of a building, causing it to
crack and tumble.
The problem of
Sediment deposition
Drainage congestion
Synchronization of river floods compound the flood hazard
with sea tides in the coastal plains

30. Social Impact
Loss of lives and property:
Immediate impacts of flooding include loss of human life, damage to property,
destruction of crops, loss of livestock, non-functioning of infrastructure
facilities and deterioration of health condition owing to waterborne diseases.
Flash floods, with little or no warning time, cause more deaths than slow-
rising riverine floods.
Loss of livelihoods:
• communication links and infrastructure such as power plants, roads and
bridges are damaged and disrupted, economic activities come to a standstill
• Similarly, the direct effect on production assets, be it in agriculture or industry,
can inhibit regularly activity and lead to loss of livelihoods
• The spill over effects in adjacent non-flooded areas.

31. • Decreased purchasing and production power:
• Damage to infrastructure causes long-term impacts, such as disruptions to clean water
and electricity, transport, communication, education and health care.
• reduction in purchasing power and loss of land value in the flood plains lead to
increased vulnerabilities of communities living in the area.
• The additional cost of rehabilitation, relocation of people and removal of property
from flood-affected areas can divert the capital required for maintaining production.
• Mass migration:
• Frequent flooding, resulting in loss of livelihoods, production and other prolonged
economic impacts and types of suffering can trigger mass migration or population
displacement.
• Migration to developed urban areas contributes to the overcrowding in the cities.
• Selective out-migration of the workforce sometimes creates complex social problems.
• Psychosocial effects:
• The loss of loved ones can generate deep impacts, especially on children.
• Displacement from one’s home, loss of property and livelihoods and disruption to
business and social affairs can cause continuing stress.
• The stress of overcoming these losses can be overwhelming and produce lasting
psychological impacts.

32. Hindering economic growth and development:
• The high cost of relief and recovery may adversely impact investment in
infrastructure and other development activities in the area
• Recurrent flooding in a region may discourage long-term investments by the
government and private sector alike.
• Lack of livelihoods, combined with migration of skilled labour and inflation may
have a negative impact on a region’s economic growth.
• Loss of resources can lead to high costs of goods and services, delaying its
development programmes.
Political implications:
• Ineffective response to relief operations during major flood events may lead to
public discontent or loss of trust in the authorities
• Lack of development in flood-prone areas may cause social inequity and even
social unrest posing threat to peace and stability in the region.

33. Before the floods...
• Know about your local relief centers and evacuation routes.
• Keep emergency numbers and important information handy, as well as emergency supplies, kits,
first aid items. These may include water, canned food, can opener, battery-operated radio,
flashlight and protective clothing.
• Fold and roll up anything onto higher ground (or upper floors of your home), including
chemicals and medicines.
• Make sure everything that is of importance is secured (jewelry, documents, pets, and other
valuables).
• Plant trees and shrubs and keep a lot of vegetation in your compound if you are in a low-lying
area as that can control erosion and help soften the
speed of the
flowing water.

34.  During the floods...
• Flash floods occur in a short spate of time. As soon as they start, be quick,
keep safe and ensure that children and elderly are safe by leaving the house
to a higher ground.
• Turn off all electrical appliance, gas, heating and the like if there is a bit of
time.
• Leave the area before it gets too late. Do not drive through the water as
moving water can sweep you away.
• Stay away from power lines or broken power transmission cables.
• Try to keep away from flood water as it may contain chemicals or other
hazardous materials.

35.  After flood
• Make sure you have permission from emergency officers to get back
inside your house.
• Keep all power and electrical appliance off until the house is cleaned
up properly and an electrical personnel has confirmed that it is OK to
put them on.
• Make sure you have photographs, or a record of all the damage, as it
may be needed for insurance claims.
• Clean the entire home, together with all the objects in it very well
before you use them again. They may be contaminated.
• Wear appropriate gear (mask and gloves) before cleaning begins.