This document discusses hydrological extremes such as floods and droughts. It begins by defining different types of floods such as flash floods, river floods, and coastal floods. It then discusses the common causes of floods including heavy rainfall, capacity of soil infiltration, increased discharge in streams, and deforestation. The document also covers types of drought including meteorological, agricultural, hydrological, and socio-economic drought. It provides details on flood and drought management through various control measures such as storage reservoirs and channel improvement.

1. UNIT III
HYDROLOGICAL EXTREMES

2. HYDROLOGICAL EXTREMES
Natural Disasters - Frequency analysis - Flood estimation - Flood
management - Definitions of drought: Meteorological, Hydrological,
Agricultural and Integrated - IMD method - NDVI analysis - Drought
Prone Area Programme (DPAP).

3. Natural disasters floods
• Floods are the most frequent type of
natural disaster and occur when an
overflow of water submerges land that
is usually dry. Floods are often caused
by heavy rainfall, rapid snowmelt or a
storm surge from a tropical cyclone or
tsunami in coastal areas.

5. Natural disasters floods
• Floods can cause widespread
devastation, resulting in loss of life and
damages to personal property and
critical public health infrastructure.
Between 1998-2017, floods affected
more than 2 billion people worldwide.
People who live in floodplains or non-
resistant buildings, or lack warning
systems and awareness of flooding
hazard, are most vulnerable to floods.

7. Natural disasters floods
There are 3 common types of floods:
• Flash floods are caused by rapid and excessive rainfall that raises
water heights quickly, and rivers, streams, channels or roads may
be overtaken.
• River floods are caused when consistent rain or snow melt forces a
river to exceed capacity.
• Coastal floods are caused by storm surges associated with tropical
cyclones and tsunami.

8. Natural disasters floods
Between 80-90% of all documented disasters from natural hazards
during the past 10 years have resulted from floods, droughts,
tropical cyclones, heat waves and severe storms. Floods are also
increasing in frequency and intensity, and the frequency and
intensity of extreme precipitation is expected to continue to increase
due to climate change.

9. How are the Floods Caused?
• There are various natural as well as anthropological causes of flood. Yet,
the most common one is prolonged heavy rain. Now, let’s consider the
primary reasons for the overflow of water to the land area for a longer
period.
Heavy Prolonged Rainfall
• The major cause of floods includes Rainfall. However, Rainfall is not that
much impactful if two factors like the intensity of Rainfall and duration
of Rainfall, remain under control.
• India is prone to such natural disasters every year. The tropical regions
of India affected mostly include Assam, Tripura, West Bengal, Bihar,
Odisha, Karnataka, Kerala, Gujarat, Chattisgarh, Madhya Pradesh, and
Uttar Pradesh.

10. How are the Floods Caused?
• Monsoon is the main time in the year when more than 70% of
prolonged heavy Rainfall causes flooding.
• The intensity of Rainfall refers to the rate of Precipitation per a
particular duration of the period of continuous rain.
• Intense Rainfall is one of the major reasons for flooding. Rivers
contain low water flow throughout the year. However, heavy or
high intensity of rain causes a sudden overflow of water resulting
in flood, and there are so many effects of it.

11. How are the Floods Caused?
Capacity of infiltration
• The second major cause of flooding includes the infiltration
capacity. How readily the soil can absorb water is referred to as
infiltration.
• The continuous heavy Rainfall highly decreases the infiltration rate
as the soil is completely saturated. Thus, the water table increases
significantly, resulting in flooding.
• Increased vegetation helps in the process of infiltration by plants
and crops. However, deforestation prevents infiltration and grows
the runoff to streams.

13. How are the Floods Caused?
Large amounts of discharge
• The rate of water passing is the discharge of a stream. Due to
continued Rainfall and highly saturated soil, water runoff to the
streams and the amount drastically increased.
• The high amount of water in that water body causes high discharge
by rising velocity and cross-sectional area. That time the river
banks are called flood or full bank stage.
• Further, increased discharge leads to a rise in depth, width, and
velocity of the stream. This causes overtopping the nearby land
and causes a flood.

14. How are the Floods Caused?
Hydrograph – a graphical representation of fluctuations in discharge
over time – shows the presence of a flood. A steep rise and steep fall
within a short duration in a hydrograph represent a more disastrous
consequence. Also, the overstep river flow rate raises the chances of
flooding.

15. How are the Floods Caused?
Basin Shape
• The rate of discharge of water is regulated by the shape of the basin
after a heavy Precipitation. Thus, in order to predict a flood, it is
essential to determine the basin shape.
• This calculation is done by elongation and circularity ratio. The diameter
of a circle to the length of the basin is known as the elongation ratio of a
drainage basin.
• Further, the circularity ratio refers to the ratio of the basin area to the
area of a circle with the same perimeter as a drainage basin.
• The elongation ratio usually ranges from 0.6 to 0.1 and depends on the
climatic and geological factors. However, the circularity ratio shows the
drainage basin outline and is a dimensionless index.

16. How are the Floods Caused?
Slope or Gradient
• The slope of a stream is the variation in altitude across a particular
horizontal distance.
• This progressively reduces with the length of a stream channel. Slope
estimates the extent of a river falls in elevation after traveling a
distance. The higher the slope or Gradient, the more will be the velocity
of water flow.
• The measurement of Gradient is done in meters/kilometers or feet/mile.
When a stream flows down a mountain slope, we refer to it as a steep
Gradient with a narrow channel.
• Whereas a stream flowing across a flat plain contains a gentle gradient.
A gentle slope can lower the extent of floodwater by preventing the
easy passing.

17. How are the Floods Caused?
Deforestation
• Deforestation is one of the major man-made causes of floods,
which has devastating effects on our environment. When the
destruction of the forest in the upper catchment of the rivers
occurs, flood causes.
• As we discussed, the infiltration rate decreases when the number
of trees reduced in the flood-prone area. This is because of the
decrease rate of absorption of rainwater by the soil. Trees improve
the absorptivity of rainwater through their roots into the deep soil
and decrease the water flow to the river channels.

18. How are the Floods Caused?
Increase of urbanization
• Other anthropogenic causes of floods include increased urbanization.
Rising population leads to an increase in urbanization, which ultimately
accelerates deforestation and expands build-up areas.
• Concrete lands diminish the water absorption capacity of the soil and
thus more water runoff to the rivers.
• Further, dense culverts and pits make the surplus water travel through
the overland or subsurface to reach the water bodies.
• Therefore, a high amount of rainwater passes directly to the rivers and
streams, causing flood in the nearby regions. This is due to the
decreased capacity of soil to hold water and quick runoff.

19. Consequences of Flood
• Several causes and effects of floods impact the vast majority of
people. Broadly, we can classify the impacts of flooding into
primary, secondary, and tertiary types, which cause social,
ecological as well as economic effects. However, the major
consequences include the following;

20. Consequences of Flood
Damage of agricultural lands
• Damage or destruction of agricultural lands and crops affects the
entire population. Yet, it majorly affects agricultural laborers and
cultivators.
• This also leads to a high price of food products in the market.
Further, this leads to economic loss. A large amount of soil erosion
also takes place due to the intense flow of water.
• Sometimes, subsequent floods result in scarcity of crop materials,
and drought conditions arise.

21. Consequences of Flood
Destruction of human habitats
• One of the most tragic impacts of the flood is the damage of habitats.
The high velocity of water brings large particles, including rocks. These
cause erosion of buildings, bridges, levees, and lead them to collapse.
• Floods also destroy several households like furniture, stored food
materials, and electronic devices. Floor and walls, moreover ruined.
• Flood also carried away automobiles, vehicles, castles, and also humans
in their way. Lots of people lose their family members due to such
incidents.
• High amounts of sediments cause the deposition of thick layers of mud
over things and building interiors. Irrigation is further affected by the
submerged pump houses.

22. Consequences of Flood
Health hazards
• The deposition of a high amount of toxins or pollutants near
human habitats causes the spread of several water-borne diseases.
• Further, immersed sewage plants give rise to high amounts of fecal
pathogens by the mixture of floodwater with untreated
wastewater or sewage.
• Moreover, flood water drags pesticides and fuel oils, which are
poisonous for animals as well as humans. Large standing water
helps in the breeding of several pathogen carriers like mosquitoes.

23. Consequences of Flood
Disruption of many essential services
• Disruption of many essential services comes under the secondary
effects of the flood. Flooding results in the destruction of electric
poles, water tanks, had stations, telephone lines, and such crucial
services and influenced the regular life.
• Some areas remain in the dark and without water supply for
several months. The damage of roadways also influences the
transport system.

24. Control Measures
• About each sector, including the housing sector, livestock sector,
agriculture or business sector, water and sanitation sector, health care
sector, and also the education sector, become affected due to flooding.
• Thus, preventive measures after a flood, should be taken by the
government as soon as possible.
We should take the following measures in order to tackle several causes
and effects of floods.
• The government should convince communities suffering the
consequences of floods every year to relocate permanently.
• The government should provide support to settle in the new location.

25. Control Measures
• About each sector, including the housing sector, livestock sector,
agriculture or business sector, water and sanitation sector, health care
sector, and also the education sector, become affected due to flooding.
• Thus, preventive measures after a flood, should be taken by the
government as soon as possible.
We should take the following measures in order to tackle several causes
and effects of floods.
• The government should convince communities suffering the
consequences of floods every year to relocate permanently.
• The government should provide support to settle in the new location.

26. Storage Reservoirs

32. Channel improvement

34. Drought
• The National Weather Service office of
the NOAA defines drought as "a
deficiency of moisture that results in
adverse impacts on people, animals, or
vegetation over a sizeable area".
Drought is a complex phenomenon −
relating to the absence of water − which
is difficult to monitor and define.

35. Drought
• A drought is defined as drier than normal
conditions. This means that a drought is
"a moisture deficit relative to the average
water availability at a given location and
season". A drought can last for days,
months or years.

36. Types of droughts
•
Meteorological Drought – referring to
lack of precipitation.
• Agricultural drought – referring to lack of
moisture in the soil where crops grow.
• Hydrological drought – referring to low
levels of water in reservoirs.
• Socio-economic drought – referring to
water shortages affecting people in
society, which impacts availability of
foodgrains, fodder, etc.