EXOGENOUS HAZARDS

1. EXOGENOUS HAZARDS
Flood:
Flood is a state of high water level along a river channel or on the coast that leads to
inundation of land, which is not usually submerged. Floods may happen gradually and also may
take hours or even happen suddenly without any warning due to breach in the embankment, spill
over, heavy rains etc.
There are different types of floods namely: flash flood, riverine flood, urban flood, etc. Flash
floods can be defined as floods which occur within six hours of the beginning of heavy rainfall,
and are usually associated with cloud bursts, storms and cyclones requiring rapid localized
warnings and immediate response to reduce damage. Wireless network and telephone
connections are used to monitor flood conditions.
Causes for flood:
There are several causes of floods and differ from region to region. The causes may vary from a
rural area to an urban area. Some of the major causes are:
1. Heavy rainfall
2. Heavy siltation of the river bed reduces the water carrying capacity of the rivers/stream.
3. Blockage in the drains lead to flooding of the area.
4. Landslides blocking the flow of the stream.
5. Construction of dams and reservoirs
6. In areas prone to cyclone, strong winds accompanied by heavy down pour along with storm
surge leads to flooding.
Usually, any of the following situations should indicate the possibility of flooding:
1. heavy rainfall in/around the vicinity, especially, if the specific location falls in the
pathway of the water-discharge system from the area receiving heavy rainfall.

2. 2. if there is heavy rainfall/flow of water/accumulation of water, on the other side of a
boundary, e.g. across a dam, side of a river-embankment etc., because, these
boundaries might get breached.
Vulnerability of India to floods:
India is highly vulnerable to floods. Out of the total geographical area of 329 million
hectares (mha), more than 40 mha is flood prone. Floods are a recurrent phenomenon, which
cause huge loss of lives and damage to livelihood systems, property, infrastructure and public
utilities. It is a cause for concern that flood related damages show an increasing trend. The
average annual flood damage in the last 10 years period from 1996 to 2005 was Rs. 4745 crore
as compared to Rs. 1805 crore, the corresponding average for the previous 53 years. This can be
attributed to many reasons including a steep increase in population, rapid urbanization growing
developmental and economic activities in flood plains coupled with global warming. An average
every year, 75 lakh hectares of land is affected, 1600 lives are lost and the damage caused to
crops, houses and public utilities is Rs.1805 crores due to floods. The maximum number of lives
(11,316) was lost in the year 1977. The frequency of major floods is more than once in five
years.
Floods have also occurred in areas, which were earlier not considered flood prone. Eighty
per cent of the precipitation takes place in the monsoon months from June to September. The
rivers a bring heavy sediment load from catchments. These, coupled with inadequate carrying
capacity of rivers are responsible for causing floods, drainage congestion and erosion of river-
banks. Cyclones, cyclonic circulations and cloud bursts cause flash floods and lead to huge
losses. It is a fact that some of the rivers causing damage in India originate in neighboring
countries; adding another complex dimension to the problem. Continuing and large-scale loss of
lives and damage to public and private property due to floods indicate that we are still to develop
an effective response to floods. NDMA's Executive Summary Guidelines have been prepared to
enable the various implementing and stakeholder agencies to effectively address the critical areas
for minimising flood damage.

3. Warning:
Flood forecasting and warning has been highly developed in the past two decades. With
the advancement of technology such as satellite and remote-sensing equipments flood waves can
be tracked as the water level rises. Except for flash floods there is usually a reasonable warning
period. Heavy precipitation will give sufficient warning of the coming river flood. High tides
with high winds may indicate flooding in the coastal areas. Evacuation is possible with suitable
monitoring and warning. Warning is issued by the Central Water Commission (CWC), Irrigation
& Flood Control Department and Water Resources Department. CWC maintains close liaison
with the administrative and state engineering agencies, local civil authorities to communicate
advance warning for appropriate mitigation and preparedness measures.
Urban Floods
Urban flooding is significantly different from rural flooding as urbanization leads to
developed catchments, which increases the flood peaks from 1.8 to 8 times and flood volumes by
up to 6 times. Consequently, flooding occurs very quickly due to faster flow times (in a matter of
minutes). Urban areas are densely populated and people living in vulnerable areas suffer due to
flooding, sometimes resulting in loss of life. It is not only the event of flooding but the secondary
effect of exposure to infection also has its toll in terms of human suffering, loss of livelihood
and, in extreme cases, loss of life. Urban areas are also centres of economic activities with vital
infrastructure which needs to be protected 24x7. In most of the cities, damage to vital
infrastructure has a bearing not only for the state and the country but it could even have global
implications. Major cities in India have witnessed loss of life and property, disruption in
transport and power and incidence of epidemics. Therefore, management of urban flooding has
to be accorded top priority. Increasing trend of urban flooding is a universal phenomenon and
poses a great challenge to urban planners the world over. Problems associated with urban floods
range from relatively localized incidents to major incidents, resulting in cities being inundated
from hours to several days. Therefore, the impact can also be widespread, including temporary
relocation of people, damage to civic amenities, deterioration of water quality and risk of
epidemics.

4. The areas which are vulnerable to flood risk are,
A. places, which have a history of flooding (most important)
B. area receiving heavy rainfall, with not much naturally sloping landscape
C. areas at the lower levels of naturally sloping landscape – where, the higher areas
are receiving heavy rainfall
D. areas around sea-coasts, or, river banks
E. areas downstream of dams etc. As water level upstream of dams might rise, the
dam authorities might be forced to release water (to safeguard the dam) – which
might cause flooding of downstream areas
F. areas on the other side of levies (in case, the levy gets breached)
G. low-lying areas (say: foot of an overbridge etc.)
The most common kinds of loss that are caused during flooding include:
a. Lack of water: Its an irony, that a disaster which mean water everywhere, results in lack
of water to drink and sanitation. Lack of proper drinking water and sanitation causes
widespread outbreak of diseases.
b. Lack of food: Most of the food items get damaged, causing a severe shortage of food.
This shortage could be for the food to be consumed in the near future, or, even standing
crops could be damaged, causing long-term food shortage.
c. Lack of utilities: Utility services might have to be turned off, for the fear of electrocution,
as, there is water everywhere.
d. Widespread damage to structure
e. Drowning: People, livestock, goods etc. might get drowned.
f. Snakes and other creatures: Some of the dangerous creatures which usually stay
underground would be forced to come up, as their natural habitat becomes unlivable.
These could prove dangerous to human beings and cattle.
g. Submerging of vehicles and other equipments: Vehicles and other equipments might get
permanently damaged – as they remain submerged under water – for prolonged duration.

5. Typical Adverse Effects:
The most important consequence of floods is the loss of life and property. Structures like
houses, bridges; roads etc. get damaged by the gushing water, landslides triggered on account of
water getting saturated, boats and fishing nets get damaged. There is huge loss to life and
livestock caused by drowning. Lack of proper drinking water facilities, contamination of water
(well, ground water, piped water supply) leads to outbreak of epidemics, diarrhoea, viral
infection, malaria and many other infectious diseases. Flooding also leads to a large area of
agricultural land getting inundated as a result there is a huge crop loss. This results in shortage of
food, and animal fodder. Floods may also affect the soil characteristics. The land may be
rendered infertile due to erosion of top layer or may turn saline if sea water floods the area.
Most of the flood affected areas lie in the Ganga basin, Brahmaputra basin (comprising of
Barak, Tista, Torsa, Subansiri, Sankosh, Dihang and Luhit), the northwestern river basin
(comprising Jhelum, Chenab, Ravi, Sutlej, Beas and the Ghagra), peninsular river basin (Tapti,
Narmada, Mahanadi, Baitarani, Godavari, krishna, Pennar and the Kaveri) and the coastal
regions of Andhra Pradesh, Tamilnadu, Orissa and Kerela. Assam, Uttar Pradesh, Bihar and
Orissa are some of the states who have been severely prone to floods.
Our country receives an annual rainfall of 1200 mm, 85% of which is concentrated in 3-4
months i.e June to September. Due to the intense and periodic rain, most of the rivers of the
country are fed with huge quantity of water, much beyond their carrying capacity.
Possible Risk Reduction Measures:
1. Mapping of the flood prone areas is a primary step involved in reducing the risk of the
region. Historical records give the indication of the flood inundation areas and the period
of occurrence and the extent of the coverage. Warning can be issued looking into the
earlier marked heights of the water levels in case of potential threat. In the coastal areas
the tide levels and the land characteristics will determine the submergence areas. Flood
hazard mapping will give the proper indication of water flow during floods.
2. The first step is to keep the drainage system clean. This allows water to be carried down
very fast. Choked drains cause a significant reduction in the ability and speed of the water

6. to be drained away. In most situations of urban flooding – this is a major cause. The
drains might get choked due to throwing of solid-wastes inside storm drains. These solid-
wastes might include construction material, plastics, paper etc. This is a clear example,
how human activity can amplify the process of flooding. Drains might also get choked
due to falling tree-leaves etc.
3. General clean-up of streets is also important. As rain-water falls down the street, it rushes
into the storm drains. if the streets are not clean, the rain water trying to go into the drain
carries solid wastes into the drain with itself, which then obstructs the flow of water by
the drainage system.
4. Rain water harvesting system: As more rain-water tries to flow down the drains, it puts
that much more stress on the drainage system. Instead, if there are several rain-water
harvesting systems, the rainfall falling in that much area would try to go to the sub-soil of
the region locally, rather than straining the drainage system. Lower is the amount of
water trying to go through the drainage system, the easier it is for the drainage system to
drain off the water.
5. Desilting: The drains should be desilted before the onset of the rainy season. This
prevents the drains from getting choked. And, it also inceases the holding capacity of the
drain, as, accumulated silt prevents that much more water from being accumulated in the
drains.
6. Inspection and repair of dams, levees, embankments etc: Before the onset of seasons
causing accumulation and/or carrying of heavy volume of water (such as rainy season),
these structures should be thoroughly inspected for possible weak-spots, and, these
should be repaired.
7. Afforestation: Forestation helps in binding the loose soil. The most major impact of this
is, as flood-water races through, it might take loose soil with it. This loose soil will now
choke the drains, as well as water-harvesting systems, thus, rendering both of these as
ineffective. On the other hand, trees will prevent soil to flow with the water, as, the roots
of the trees will act as binding force. Another major impact that afforestation provides is
by reducing the impact of flowing water. This has impact on large-scale flooding, such as
overflowing river. As water charges forward, its speed is reduced to some extent due to
resistance offered by trees. This can reduce the force of the charging water – thereby,

7. reducing structural damage – due to weakening in the force with which water hits various
structures.
8. Local lowlands (say: foot of an overbridge) should have storm drains, so that water does
not get accumulated there. These drains should have some kind of mesh covering, so that
only water can flow in. Leaves and other solid debris should not go in these drains.
9. Local embankments around low-lying houses etc: Lets say, for some reason, your house
is at a level lower than its vicinity (e.g. road-level). This can happen, because, say: you
have constructed a basement – which is obviously lower than the road-level, or, over a
period of years, the road-level has risen due to repeated tarring etc. In such cases, you
should create a “local” embankment between the street/road and your property, so that
water can not flow “down” from the street/road inside your house. These embankment
might be permanent – in the form of concrete structure.

8. Flood hazard map of India

9. Drought
Drought is either absence or deficiency of rainfall from its normal pattern in a region for
an extended period of time leading to general suffering in the society. It is interplay between
demand that people place on natural supply of water and natural event that provides the water in
a given geographical region. Falling rainfall levels, falling groundwater levels, drying wells,
rivers and reservoirs, and poor agricultural production warn the onset of drought. According to
the Indian Meteorological Department, the country is said to be drought affected when the
overall rainfall deficiency is more than 10 per cent of the long period average and more than 20
per cent of the country area is affected by such drought conditions.
General characteristics of drought:
1. It is a slow on-set disaster and it is difficult to demarcate the time of its onset and the end.
2. Any unusual dry period which results in a shortage of useful water
3. Drought is a normal, recurrent feature of climate. Climate is expected to show some
aberrations and drought is just a part of it.
4. Drought can occur by improper distribution of rain in time and space, and not just by its
amount.
In India, the occurrence and conditions of drought are influenced by a number of factors.
Rainfallandcroppingpatternsaredifferentacrossmanygeographicalregions.Itisnot just the
deficiency of rainfall, but also the uneven distribution of rainfall across the season,
duration of rainfall deficiency and its impact on different regions of the country that
characterize drought conditions
Causes of Drought:
Though drought is basically caused by deficit rainfall, which is a meteorological
phenomenon, some of the factors are human induced. Though drought is a natural disaster, its
effects are made worst in developing countries by over population, over grazing, deforestation,
soil erosion, excessive use of ground and surface water for growing crops, loss of biodiversity.

10. Vulnerability factors to drought:
1. Low soil moisture holding capacity
2. Absence of irrigation facilities
3. Livestock without adequate fodder storage facilities
4. Poor water management
5. Deforestation
6. Water consuming cropping patterns
7. Soil erosion
8. Population growth and urbanization
9. Industrialization
10. Global warming
Seasonal Characteristics and Intra-Seasonal Variability:
Indiareceivesmostofitsrainfall(73%)fromthesouth-westor“summer”monsoon(the rainfall
received between June and September). The performance of the Indian economy is vitally
linked with the rainfall that occurs during these months. The summer monsoon sets in during the
first week of June in the south-east corner of India and gradually proceeds towards the north-
west region covering the entire country by the second week ofJuly.Monsoon starts itswithdrawal
during the first week of September from the west and north and gradually recedes from the entire
country.
Due to this pattern of onset and withdrawal, the north-west region receives less than a
month of rainy season due to late arrival and early cessation of monsoon conditions.
Conversely, Kerala and north-eastern parts of India receive more than 4 months of rainfall due to
theearlyarrivalandlaterwithdrawalofthemonsoon.
Coastal areas of peninsular India and Tamil Nadu, in particular, also receive rains from
October to December,primarily due to periodic cyclonic disturbances in the Bay ofBengal(north-
eastmonsoonorpost-monsoon).

11. Types of drought:
1. Meteorological drought is defined as the deficiency of precipitation from expected or
normal levels over an extended period of time. Meteorological drought usually precedes
other kinds of drought. According to the scientists, meteorological drought is said to
occur when the seasonal rainfall received over an area is less than 25% of its long-term
average value. It is further classified as moderate drought if the rainfall deficit is 26–50%
and severe drought when the deficit exceeds 50% of the normal.
2. Hydrological drought is defined as deficiencies in surface and subsurface water supplies
leading to a lack of water for normal and specific needs. Such conditions arise, even in
times of average (or above average) precipitation when increased usage of water
diminishes the reserves.
3. Agricultural drought, usually triggered by meteorological and hydrological droughts,
occurs when soil moisture and rainfall are inadequate during the crop growing season
causing extreme crop stress and wilting. Plant water demand depends on prevailing
weather conditions, biological characteristics of the specific plant, its stage of growth and
the physical and biological properties of the soil. Agricultural drought thus arises from
variable susceptibility of crops during different stages of crop development, from
emergence to maturity. In India, it is defined as a period of four consecutive weeks (of
severe meteorological drought) with a rainfall deficiency of more than 50% of the long-
term average (LTA) or with a weekly rainfall of 5 cm or less from mid-May to mid-
October (the kharif season) when 80% of India’s total crop is planted or six such
consecutive weeks during the rest of the year (NRSC, Decision Support Centre).
Elements at Risk:
In general, all those elements that are primarily dependent on water are most affected. It
affects the rainfed crops and then slowly creeps into the irrigated crops. Areas with minimum of
alternative water sources to rainfall (ground and canal water supplies), areas subjected to drastic
environmental degradation such as denuded forest lands and altered ecosystems, and areas where
livelihoods alternative to agriculture are least developed are most vulnerable to drought. The
herdsman, landless laborers, subsistence farmers, women, children and farm animals are the most
vulnerable groups.

12. The following criteria have been set by the Indian Meteorological Division (IMD) for
identifying the drought.
1. Onset of drought: Deficiency of a particular year’s rainfall exceeding 25 per cent of
normal.
2. Moderate drought: Deficit of rainfall between 26-50 per cent of normal.
3. Severe drought: Deficit of rainfall more than 50 per cent of normal.
Drought conditions in India:
The major drought years in India were 1877, 1899, 1918, 1972, 1987 and 2002. Large
parts of the country perennially reel under recurring drought. In India, around 68% of the country
is prone to drought in varying degrees. Of the entire area, 35% of the area, which receives
rainfall between 750 mm and 1,125 mm, is considered drought-prone, while another 33%, which
receives less than 750 mm of rainfall, is called chronically drought-prone. A further
classification of India's regions into arid (19.6%), semi-arid (37%), and sub-humid areas (21%).
Out of about 6 million villages of India, about 2,31,000 are called ‘problem villages’. In
these ‘problem villages’, water is not available within a 1.6 km radius. Rain fall distribution
grossly varies in more than 35 meteorological subdivisions of India. For example, Cherrapunji
receives about 118.70 cm of rainfall in comparison to about 10 mm or less rain received in the
western part of Rajasthan. The most drought-prone regions is located in West Rajasthan, Gujarat,
Saurashtra and Kutch, Maharashtra, Telengana, Rayalaseema, Bihar and some parts of Orissa.
Among the drought years, the 1987 drought was one of the worst droughts of the century,
with an overall rainfall deficiency of 19%. It affected 59–60% of the crop area and a population
of 285 million. In 2002 too, the overall rainfall deficiency for the country as a whole was 19%.
Over 300 million people spread over 18 States were affected by drought in varying degrees.
Around 150 million cattle were affected due to lack of fodder and water. Food grains production
registered the steepest fall of 29 million tonnes. No other drought in the past had caused
reduction in food grain production to this extent.
The drought-prone countries in the Asia region are Afghanistan, Iran, Myanmar, Pakistan,
Nepal, India, Sri Lanka and parts of Bangladesh. In India, about 33% of the arable (agricultural)
land is considered to be drought-prone (i.e. about 14% of the total land area of the country) and a
further 35% can also be affected if rainfall is exceptionally low for extended periods.

13. Cropped area falling under various ranges of Rainfall in India
Si.No. Rainfall ranges classification Percentage
1 Less than 750mm Low rainfall 33%
2 750mm to 1125mm Medium rainfall 35%
3 1126mm to 2000mm High rainfall 24%
4 Above 2000mm Very high rainfall 8%
Impacts of Drought in India:
Drought produces both direct and indirect impacts. Direct impacts or primary impacts are
usually physical / material and include reduced agricultural production; increased fire hazard;
depleted water levels; higher livestock and wildlife mortality rates; and damage to wildlife and
fish habitats. When direct impacts have multiplier effects through the economy and society, they
are referred to as indirect impacts. These include a reduction in agricultural production that may
result in reduced income for farmers and agribusiness, increased prices for food and timber,
unemployment, reduced purchasing capacity and demand for consumption, default on
agricultural loans, rural unrest, and reduction in agricultural employment leading to migration
and drought relief programmes.
Economic impacts refer to production losses in agriculture and related sectors, especially
forestry and fisheries, because these sectors rely on surface and subsurface water supplies. It
causes a loss of income and purchasing power, particularly among farmers and rural population
dependent on agriculture. All industries dependent upon the primary sector for their raw
materials would suffer losses due to reduced supply or increased prices. Drought thus has a
multiplier effect throughout the economy, which has a dampening impact on employment, flow
of credit and tax collections.
Environmental impacts, such as lower water levels in reservoirs, lakes and ponds as well as
reduced flows from springs and streams would reduce the availability of feed and drinking water
and adversely affect fish and wildlife habitat. It may also cause loss of forest cover, migration of
wildlife. A prolonged drought may also result in increased stress among endangered species and
cause loss of biodiversity.
Reduced stream flow and loss of wetlands may cause changes in the levels of salinity.
Increased groundwater depletion, land subsidence, and reduced recharge may damage aquifers

14. and adversely affect the quality of water (e.g., salt concentration, increased water temperature,
acidity, dissolved oxygen, turbidity). The degradation of landscape quality, including increased
soil erosion, may lead to a more permanent loss of biological productivity of the landscape.
Social impacts arise from lack of income causing out migration of the population from the
drought-affected areas. People in India seek to cope with drought in several ways which affect
their sense of well-being: they withdraw their children from schools and sell their assets such as
landorcattle.Inaddition to economic hardships,it causes a loss of social status and dignity,which
people find hard to accept. Inadequate food intake may lead to malnutrition, and in some extreme
cases, causestarvation. Access anduseof scarcewater resources generatesituations of conflict, which
could be socially very disruptive. Inequities in the distribution of drought impacts and relief may
exacerbatethesesocialtensionsfurther.
Drought makes a very perceptible impact on populations that are largely dependent upon
agriculture and related occupations for their livelihood. As crops are adversely affected,
agricultural income shrinks and causes loss of employment in the agriculture sector. It also has
an indirect impact on the other sectors of economy. On the supply side, drought causes a
shortage of raw material supplies for agro-based industries and on the demand side, it reduces the
demand for industrial products due to diminished purchasing capacity of the rural consumers.
Most major droughts in India were followed by recession. Though drought makes its impact over
time, it poses a serious challenge for human well being.
Typical adverse effects
Drought, different from any other natural disaster, does not cause any structural damages. As
the meteorological drought turns into hydrological drought, the impacts start appearing first in
agriculture which is most dependant on the soil moisture. Irrigated areas are affected much later
than the rainfed areas. However, regions surrounding perennial rivers tend to continue normal
life even when drought conditions are prevailing around. The impacts slowly spread into social
fabric as the availability of drinking water diminishes, reduction in energy production, ground
water depletion, food shortage, health reduction and loss of life, increased poverty, reduced
quality of life and social unrest leading to migration.

15. Distribution Pattern
1. Around 68 per cent of India’s total area is drought prone to drought.
2. 315 out of a total of 725 Talukas in 99 districts are drought prone.
3. 50 million people are annually affected by drought.
4. In 2001 more than eight states suffered the impact of severe drought.
5. In 2003 most parts of Rajasthan experienced the fourth consecutive year of drought.
Possible Risk Reduction Measures:
There are various mitigation strategies to cope up with drought.
1. Public Awareness and education: If the community is aware of the do’s and don’ts, then half
of the problem is solved. This includes awareness on the availability of safe drinking water,
water conservation techniques, agricultural drought management strategies like crop contingency
plans, construction of rain water harvesting structure. Awareness can be generated by the print,
electronic and folk media.
2. Drought Monitoring: It is continuous observation of the rainfall situation, availability of
water in the reservoirs, lakes, rivers etc and comparing with the existing water needs in various
sectors of the society.
3. Water supply augmentation and conservation through rainwater harvesting in houses and
farmers’ fields increases the content of water available. Water harvesting by either allowing the
runoff water from all the fields to a common point (e.g. Farm ponds, see the picture) or allowing
it to infiltrate into the soil where it has fallen (in situ) (e.g. contour bunds, contour cultivation,
raised bed planting etc) helps increase water availability for sustained agricultural production.
4. Expansion of irrigation facilities reduces the drought vulnerability. Land use based on its
capability helps in optimum use of land and water and can avoid the undue demand created due
to their misuse.
5. Livelihood planning identifies those livelihoods which are least affected by the drought.
Some of such livelihoods include increased off-farm employment opportunities, collection of
non-timber forest produce from the community forests, raising goats, carpentry etc.
6. Drought planning: the basic goal of drought planning is to improve the effectiveness of
preparedness and response efforts by enhancing monitoring, mitigation and response measures.
7. Planning would help in effective coordination among state and national agencies in dealing
with the drought. Components of drought plan include establishing drought taskforce which is a

16. team of specialists who can advise the government in taking decision to deal with drought
situation, establishing coordination mechanism among various agencies which deal with the
droughts, providing crop insurance schemes to the farmers to cope with the drought related crop
losses, and public awareness generation.

17. Cyclone
Cyclone is a region of low atmospheric pressure surrounded by high atmospheric
pressure resulting in swirling atmospheric disturbance accompanied by powerful winds blowing
in anticlockwise direction in the Northern Hemisphere and in the clockwise direction in the
Southern Hemisphere. They occur mainly in the tropical and temperate regions of the world. The
word "Cyclone" is derived from the Greek, word "Cyclos" or “Kyclos” meaning the coils of a
snake. A full-grown cyclone is a violent whirl in the atmosphere 150 to 1000 km across and 10
to 15 km high. Cyclones vary in diameter from 100 to 1,000 km but their effect dominates over
thousands of square kilometres over the ocean as well as along the coast. The powerhouse is
located within a 100 km radius of the eye of the cyclone where very strong winds, sometimes
more than 250 km per hour, can be generated in a narrow zone beyond the eye diameter.
Cyclones are known by different names in different parts of the world:
1. Typhoons in the Northwest Pacific Ocean west of the dateline
2. Hurricanes in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline,
or the South Pacific Ocean.
3. Tropical cyclones - the Southwest Pacific Ocean and Southeast and Southwest Indian
Ocean.
4. Willie-Willie in Australia
5. Tornado in South America

18. Cyclones in India are moderate in nature. Some of the general characteristics of a cyclone are:
1. Strong winds: the highest wind speed ever recorded in case of a Tropical cyclone in the world
is 317 km/h. Tropical cyclones are of moderate intensity, in most of the cases wind speeds have
not exceeded 185 km/h. the estimated highest wind speed in AP is 270 km/h in 1977 cyclone
which is so far the severest storm recorded in the north Indian ocean.
2. Exceptional rain: Floods caused by cyclone rainfall are more destructive than winds. The
rainfall is the heaviest around the wall cloud zones. Rainfall of the order of 20 to 30cm per day is
very common. Associated with the tropical cyclone in 24 hours there will be occurrence of
100cm rainfall and hence it leads to floods in that region.
3. Storm surge: It is a coastal phenomena, is the inherent catastrophic feature of cyclones the
world over. The degree of disaster potential depends on the storm surge amplitude associated
with the cyclone at the time of landfall, characteristics of the coast, phases of the tides and
vulnerability of the area and community. The world’s highest recorded storm tide was about 12.5
m (about 41 ft) and it was associated with the Backergunj cyclone in 1876 in Bangladesh.
Cyclones are generally accompanied by strong winds which cause a lot of destruction. In
some cases it is accompanied by heavy downpour and also the rise in the sea which intrudes
inland thereby causing floods.
The development of a cyclone covers three stages namely,
a) Formation and initial development state: Four atmospheric/ oceanic conditions are necessary
for the formation of a cyclone namely,
1. A warm sea temperature in excess of 26 degree centigrade, to a depth of 60 meters,
which provides abundant water vapor in the air by evaporation.
2. High relative humidity (degree to which the air is saturated by water vapor) of the
atmosphere to a height of about 7000 meters, facilitates condensation of water vapor
into droplets and clouds, releases heat energy and induces drop in pressure.
3. Atmospheric instability (an above average decrease of temperature with altitude)
encourages considerable vertical cumulus cloud convection when condensation of rising
air occurs.

19. 4. A location of at least 4-5 latitude degrees from the Equator allow the influence of the
force due to the earth’s rotation (Coriolis force) to take effect in inducing cyclonic wind
circulation around low pressure centers.
b) Fully matured Tropical cyclone:
The main feature of a fully mature tropical cyclone is a spiral pattern of highly turbulent
giant cumulus thundercloud bands. These bands spiral inwards and form a dense highly active
central cloud core which raps around a relatively calm zone. This is called the “eye” of a
cyclone. The eye looks like a black hole or a dot surrounded by thick clouds. The outer
circumference of the thick cloud is called the ‘eye wall’. The diameter of the eye is about 40-
50km surrounded by the thunderstorms.
c) Weakening or decay of Cyclone:
A tropical cyclone begins to weaken as soon as its source of warm moist air is abruptly
cut off. This is possible when the cyclone hits the land, on the cyclone moves to a higher altitude
or when there is the interference of another low pressure.
Depending on their track on the warm tropical sea and proximity to land a cyclone may
last for less than 24 hours to more than 3 weeks. On an average the life cycle of a cyclone (a

20. cyclone to complete these three stages mentioned above) takes six days. The longest cyclone is
typhoon John which lasted for 31 days (August to September, 1994 in the north east and north
west pacific basins).
Indian Cyclones:
Tropical Cyclones:
The major natural disaster that affects the coastal regions of India is Tropical cyclone
and as India has a coastline of about 7500 kms, it is exposed to nearly 10 percent of the world’s
tropical cyclones. About 71 percent of this area is in ten states (Gujarat, Maharashtra, Goa,
Karnataka, Kerala, Tamil Nadu, Puducherry, Andhra Pradesh, Orissa and West Bengal). The
islands of Andaman, Nicobar and Lakshadweep are also prone to cyclones. On an average, about
five or six tropical cyclones form in the Bay of Bengal and Arabian sea and hit the coast every
year. Out of these, two or three are severe.
When a cyclone approaches to coast, a risk of serious loss or damage arises from severe
winds, heavy rainfall, storm surges and river floods. Most cyclones occur in the Bay of Bengal
followed by those in the Arabian Sea and the ratio is approximately 4:1. The incidence of
cyclonic storms, with wind speeds between 65 Km/h and 117 Km/h and severe cyclonic storm
with wind speeds between 119 Km/h and 164Km/h, reaching Tamil Nadu and Andhra Pradesh is
high during the north east monsoon season in October – December, where as the highest annual
number of storms, severe storms occur in the Orissa - West Bengal coast.
The Cyclone hazard map of India may be seen in the Figure 1. gives the vulnerability map of
hazard due to cyclone.
29th October 1999, Super tropical cyclone with wind speed of 260-300 km/hour hit the
140 kilometer coast of Orissa with a storm surge created in the Bay-of-Bengal with water level
9m higher than normal. The super storm travelled more than 250 km inland and within a period
of 36 hrs ravaged more than 200 lakh hectares of land, this is the worst cyclone of the country in
the Orissa region and was responsible for as many as 10,000 deaths, devouring trees and
vegetation, leaving behind a huge trail of destruction. The violent cyclone was merciless and
broke the backbone of Orissa’s economy and killed thousands and devastated millions.

21. The criteria followed by the Meteorological Department of India to classify the low
pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World
Meteorological Organization (WMO) are:
Types of Disturbances Associated wind speed in the Circulation (1
knot = 1.85 km/hr)
1. Low Pressure Area Less than 17 knots ( < 31 kmph)
2. Depression 17 to 27 knots ( 31 to 49 kmph)
3. Deep Depression 28 to 33 knots ( 50 to 61 kmph)
4. Cyclonic Storm 34 to 47 knots ( 62 to 88 kmph)
5. Severe Cyclonic Storm 48 to 63 knots ( 89 to 118 kmph)
6. Very Severe Cyclonic Storm 64 to 119 knots ( 119 to 221 kmph)
7. Super Cyclonic Storm 120 knots and above ( 222 kmph and above)
Warning:
Cyclones warnings are provided through six cyclone warnings centers located at Kolkata,
Bhubaneshwar, Visakhapatnam, Chennai, Mumbai and Ahmedabad. These centers have distinct
responsibilities area wise covering both east and west coast of India and oceanic areas of Bay of
Bengal and the Arabian Sea, including the Andaman and Nicobar Islands.
Low pressure and the development can be detected hours or days before it causes
damage. The satellites track the movement of these cyclones based on which the people are
evacuated from areas lively to be affected. It is difficult to predict the accuracy. Accurate landfall
predictions can give only a few hours’ notice to threatened population.
India has one of the best cyclone warning systems in the world. The India Meteorological
Department (IMD) is the nodal department for wind detection, tracking and forecasting cyclones.
Cyclone tracking is done through geo-stationary INSAT satellites. Powerful cyclone radars with
a wide range of 400km are installed at Kolkata, Vishakhapatnam, Chennai on the east coast and

22. Goa, Mumbai, and Bhuj on the west coast. Cyclone warning is disseminated by several means
such as satellite based disaster warning systems, radio (AIR), television, telephone, public
announcements and News papers. These warnings are disseminated to the general public, the
fishing community especially those in the sea, port authorities, commercial aviation and the
government machinery.
Two stage warning system:
The cyclones warnings are provided in two stages. In the first stage, a ‘Cyclone Alert’ is
issued 48 hours before the commencement of adverse weather along the coast. In the second
stage, a ‘Cyclone warning’ is issued before the anticipated rainfall. The ports and fisheries
warnings start much earlier. Ports are warned day and night with specially designed Port warning
signals.
HAZARD ZONES
The wind zone map illustrates the area vulnerable to high wind speeds. The macro-level
wind speed zones of India have been formulated and published in IS 875 (Part-3) – 1987. There
are six basic wind speeds considered for zoning, namely:
55m/s (198km/hr) - Very High Damage Risk Zone-A
50m/s (180 km/hr) - Very High Damage Risk Zone-B
47m/s (169.2 km/hr) - High Damage Risk Zone
44m/s (158.4 km/hr) - Moderate Damage Risk Zone-A
39m/s (140.4 km/hr) - Moderate Damage Risk Zone-B
33m/s (118.8 km/hr) - Low Damage Risk Zone
The cyclone affected areas of the country are classified in 50 and 55m/s zones. It is
known that in certain events, the wind gusts could appreciably exceed the given basic wind
speeds.

23. For design of structures and classification of vulnerability and risk to buildings, the above
macro-level zoning is considered as sufficient. In surge prone coasts of India, storm surge
heights depend on the intensity of the cyclone, i.e., very high-pressure gradient and consequent
very strong winds and the topography of seabed near the point where a cyclone crosses the coast.
Sea level also rises due to astronomical high tide. Elevation of the total sea level increases when
peak surge occurs at the time of high tide. Vulnerability to storm surges is not uniform along
Indian coasts.
The following segments of the East Coast of India are most vulnerable to high storm
surges:
1. North Orissa, and West Bengal coasts.
2. Andhra Pradesh coast between Ongole and Machilipatnam.
3. Tamil Nadu coast, south of Nagapatnam.
The West Coast of India is less vulnerable to storm surges than the east coast of India in
terms of both the height of storm surge as well as frequency of occurrence. However, the
following segments are vulnerable to significant surges:
1. Maharashtra coast, north of Harnai and adjoining south Gujarat coast and the coastal belt
of Mumbai.
2. The coastal belt around the Gulf of Kutch in Gujarat.
Typical Adverse effects:
First, in a sudden, brief onslaught, high winds cause major damage to infrastructure and
housing, in particular fragile constructions. They are generally followed by heavy rains and
floods and, in flat coastal areas by storm surge riding on tidal waves and inundating the land over
long distances of even up to 15 kilometer inland.
Physical damage – structures will be damaged or destroyed by the wind force, flooding
and storm surge. Light pitched roofs of most structures especially the ones fitted on to industrial
buildings will suffer severe damage.

24. Casualties and public heath – caused by flooding and flying elements, contamination of
water supplies may lead to viral outbreaks, diarrhea, and malaria.
Water supplies – Ground and pipe water supply may get contaminated by flood waters.
Crops and food supplies – high winds and rains ruin the standing crop and food stock
lying in low lying areas. Plantation type crops such as banana and coconut are extremely
vulnerable. Salt from the sea water may get deposited on the agricultural land and increase the
salinity and that land will not be suitable for growing crops. The loss of the crop may lead to
acute food shortage.
Communication – severe disruption in the communication links as the wind may bring
down the electricity and communication towers, telephone poles, telephone lines, antennas and
satellite disk and broadcasting services. Transport lines (road and rail) may be curtailed, Lack of
proper communication affects effective distribution of relief materials.
Destruction caused by Cyclones:
There are three elements associated with a cyclone, which cause destruction. They are explained
in the following points,
1. Cyclones are associated with high-pressure gradients and consequent strong winds.
These, in turn, generate storm surges. A storm surge is an abnormal rise of sea level near the
coast caused by a severe tropical cyclone; as a result, sea water inundates low lying areas of
coastal regions drowning human beings and livestock, eroding beaches and embankments,
destroying vegetation and reducing soil fertility.
2. Very strong winds may damage installations, dwellings, communication systems, trees
etc. resulting in loss of life and property.
3. Heavy and prolonged rains due to cyclones may cause river floods and submergence of
low lying areas by rain causing loss of life and property. Floods and coastal inundation due to
storm surges pollute drinking water sources+ causing outbreak of epidemics.

25. Possible Risk Reduction Measures:
Coastal belt plantation - green belt plantation along the coastal line in a scientific
interweaving pattern can reduce the effect of the hazard. Providing a cover through green belt
sustains less damage. Forests act as a wide buffer zone against strong winds and flash floods.
Without the forest the cyclone travel freely inland. The lack of protective forest cover allows
water to inundate large areas and cause destruction. The Orissa calamity has also highlighted the
need for urgent measures like shelterbelt plantation along cyclone-prone coastal areas.
Hazard mapping – Meteorological records of the wind speed and the directions give the
probability of the winds in the region. Cyclones can be predicted several days in advance. The
onset is extensive and often very destructive. Past records and paths can give the pattern of
occurrence for particular wind speeds. A hazard map will illustrate the areas vulnerable to
cyclone in any given year. It will be useful to estimate the severity of the cyclone and various
damage intensities in the region.
Land use control: designed so that least critical activities are placed in vulnerable areas.
Location of settlements in the flood plains is at utmost risk. Siting of key facilities must be
marked in the land use. Policies should be in place to regulate land use and building codes should
be enforced.
Engineered structures – structures need to be built to withstand wind forces. Good site
selection is also important. Majority of the buildings in coastal areas are built with locally
available materials and have no engineering inputs.
Good construction practice should be adopted such as: -
1. Cyclonic wind storms inundate the coastal areas. It is advised to construct on stilts or
on earth mound.
2. Houses can be strengthened to resist wind and flood damage. All elements holding
the structures need to be properly anchored to resist the uplift or flying off of the
objects. For example, avoid large overhangs of roofs, and the projections should be
tied down.

26. 3. A row of planted trees will act as a shield. It reduces the energy.
4. Buildings should be wind and water resistant.
5. Buildings storing food supplies must be protected against the winds and water.
6. Protect river embankments. Communication lines should be installed underground.
7. Provide strong halls for community shelter in vulnerable locations.
Flood management – Torrential rains, strong wind and storm range leads to flooding in
the cyclone affected areas. There are possibilities of landslides too. Flood mitigation measures
could be incorporated.
Improving vegetation cover – The roots of the plants and trees keep the soil intact and
prevent erosion and slow runoff to prevent. The use of tree planted in rows will act as a
windbreak. Coastal shelterbelt plantations can be developed to break severe wind speeds. It
minimizes devastating effects.. Species chosen for this purpose should not only be able to
withstand the impact of strong cyclonic winds, but also check soil erosion.
Community based mitigation:
Construction of cyclone resistant houses and strengthening of existing houses can be
done through community participation. Local engineers and masons can take part in the
construction of the buildings in their area and demonstrate to the people about disaster resistant
construction methods. Construction of multipurpose cyclone shelters in the vulnerable locations
are desirable. During normal time these buildings can be used as schools or as community
centres. In case of cyclones or floods, community can take shelter in these designed buildings.
The local communities will be responsible for the maintenance and management of these
community shelters. Protection measures need to be taken for the livestock, the boats, fishing
nets, household items and other possessions.
Other activities that can be taken up as part of the community based mitigation are
construction of saline embankments for protection against sea water ingress, reforestation,
conservation of green belt areas and participating in coastal shelterbelt plantation programme.
Cyclone Disaster Prevention and Preparedness
In 1969, the Govt. of India suggested to the governments of the coastal states to set up
"Cyclone Distress Mitigation Committee" (CDMC) in the respective states with the objective of
preventing loss of life and minimizing damage to properties. CDMCs are to plan the
communication systems in the state for quick dissemination of meteorological warnings and

27. prevention measures. Prevention measures include construction of storm shelters, connecting
roads for evacuation of people, construction of wind breaks, dykes, bunds, flood storage
reservoirs, afforestation along the coastal belts and improvement of drainage facilities. An
advance warning will not be effective unless the public is enlightened about the destructive
features and the actions to be taken by them to avoid sufferings. CDMCs have also, therefore,
programmes for generating public awareness through information pamphlets, brochures,
audiovisual materials, cyclone preparedness meetings, talks and discussions over the radio and
television.

28. Fig.1 Cyclone Hazard Map of India

29. Soil erosion
Importance of Soil
The soil covering the surface of the earth has taken millions of years to form and we must learn
to respect it. Soil is formed at a rate of only 1 cm every 100 to 400 years and it takes 3 000 to 12
000 years to build enough soil to form productive land. This means that soil is a nonrenewable
resource and once destroyed it is gone forever.
If we disregard this, a time will come when there would not be enough soil left to sustain life on
earth, because the soil is a necessary growth medium for plants, a home for certain insects and
animals, as well as a medium from which we get minerals, such as gold. It is important therefore
to treat soil, especially topsoil, as a living entity.
Soil is the most precious gift of nature, Prime resource-for food, fodder etc. In India, more than
100 million hectares of soil is degraded and eroded. About 17 tones/ha soil detached annually out
of which 20% of soil is transported by river to sea and 10% deposited in reservoir results 1 to 2%
loss off storage capacity.
Soil erosion is a naturally occurring process that affects all landforms. In agriculture, soil
erosion refers to the wearing away of a field’s topsoil by the natural physical forces of water and
wind.
Soil erosion occurs when soil is removed through the action of wind and water at a greater rate
than it is formed, When a raindrop hits soil that is not protected by a cover of vegetation and
where there are no roots to bind the soil, then soil particles are loosened, washed down the slope
of the land and either end up in the valley or are washed away out to sea by streams and rivers.
Erosion removes the topsoil first. Without soil and plants the land becomes desert like and
unable to support life.
Erosion, whether it is by water, wind or tillage, involves three distinct actions — soil
detachment, movement and deposition. Topsoil, which is high in organic matter, fertility and soil
life, is relocated elsewhere “on-site” where it builds up over time or is carried “off-site” where it

30. fills in drainage channels. Soil erosion reduces cropland productivity and contributes to the
pollution of adjacent watercourses, wetlands and lakes.
Important terms:
Soil texture: Soil texture is the size distribution of soil particles. The size of particles never
changes. A sandy soil, therefore, remains sandy and a clayey soil remains clayey. The three main
particles are sand, silt and clay. The more sandy a soil the easier it will erode.
Soil structure: The term soil structure means the grouping or arrangement of soil particles. Over
cultivation and compaction cause the soil to lose its structure and cohesion (ability to stick
together) and it erodes more easily.
The vulnerability of a field to soil erosion is dependent on a number of factors:
 The climatic conditions of the area
 the proportion of sand, silt and clay sized particles in a particular soil
 the organic matter level
 the water permeability of the soil
 the length and slope of the field
 amount of crop rotation
 direction of cultivation
Water erosion: It is defined as wearing away of a field’s topsoil by the natural physical forces
of water.
Figure 1. The erosive force of water from concentrated surface water runoff.

31. Causes of soil erosion:
The rate and magnitude of soil erosion by water is controlled by the following factors:
1. Rainfall Intensity and Runoff
Both rainfall and runoff factors must be considered in assessing a water erosion problem. The
impact of raindrops on the soil surface can break down soil aggregates and disperse the
aggregate material. Lighter aggregate materials such as very fine sand, silt, clay and organic
matter can be easily removed by the raindrop splash and runoff water; greater raindrop energy or
runoff amounts might be required to move the larger sand and gravel particles.
Soil movement by rainfall (raindrop splash) is usually greatest and most noticeable during short
duration, high-intensity thunderstorms. Runoff from the agricultural land may be greatest during
spring months when the soils are usually saturated, snow is melting and vegetative cover is
minimal.
2. Soil Erodibility
Soil erodibility is an estimate of the ability of soils to resist erosion, based on the physical
characteristics of each soil. Texture is the principal characteristic affecting erodibility, but
structure, organic matter and permeability also contribute. Generally, soils with faster infiltration
rates, higher levels of organic matter and improved soil structure have a greater resistance to
erosion.
Sand, sandy loam and loam-textured soils tend to be less erodible than silt, very fine sand and
certain clay-textured soils. Tillage and cropping practices that reduce soil organic matter levels,
cause poor soil structure, or result in soil compaction, contribute to increases in soil erodibility.
3. Slope Gradient and Length
The steeper and longer the slope of a field, the higher the risk for erosion. Soil erosion by water
increases as the slope length increases due to the greater accumulation of runoff. Consolidation
of small fields into larger ones often results in longer slope lengths with increased erosion
potential, due to increased velocity of water, which permits a greater degree of scouring
(carrying capacity for sediment).

32. 4. Cropping and Vegetation
The potential for soil erosion increases if the soil has no or very little vegetative cover of plants
and/ or crop residues. Plant and residue cover protects the soil from raindrop impact and splash,
tends to slow down the movement of runoff water and allows excess surface water to infiltrate.
The erosion-reducing effectiveness of plant and/or crop residues depends on the type, extent and
quantity of cover. Vegetation and residue combinations that completely cover the soil and
intercept all falling raindrops at and close to the surface are the most efficient in controlling soil
erosion (e.g., forests, permanent grasses). Partially incorporated residues and residual roots are
also important as these provide channels that allow surface water to move into the soil.
5. Tillage Practices
The potential for soil erosion by water is affected by tillage operations, depending on the depth,
direction and timing of plowing, the type of tillage equipment and the number of passes.
Generally, the less the disturbance of vegetation or residue cover at or near the surface, the more
effective the tillage practice in reducing water erosion. Minimum till or no-till practices are
effective in reducing soil erosion by water.
Tillage and other practices performed up and down field slopes creates pathways for surface
water runoff and can accelerate the soil erosion process. Cross-slope cultivation and contour
farming techniques discourage the concentration of surface water runoff and limit soil
movement.
FORMS OF WATER EROSION
1. Sheet Erosion
Sheet erosion is the movement of soil from raindrop splash and runoff water. It typically occurs
evenly over a uniform slope and goes unnoticed until most of the productive topsoil has been
lost. Deposition of the eroded soil occurs at the bottom of the slope (Figure 3) or in low areas.
Lighter-coloured soils on knolls, changes in soil horizon thickness and low crop yields on
shoulder slopes and knolls are other indicators.

33. Fig 3. The accumulation of soil and crop debris at the lower end of this field is an indicator of
sheet erosion.
2. Rill Erosion : Rill erosion results when surface water runoff concentrates, forming small
yet well-defined channels (Figure 4). These distinct channels where the soil has been
washed away are called rills when they are small enough to not interfere with field
machinery operations. In many cases, rills are filled in each year as part of tillage
operations.
Figure 4. The distinct path where the soil has been washed away by surface water runoff is an
indicator of rill erosion.

34. 3. Gully Erosion
Gully erosion is an advanced stage of rill erosion where surface channels are eroded to the
point where they become a nuisance factor in normal tillage operations (Figure 5). There are
farms in Ontario that are losing large quantities of topsoil and subsoil each year due to gully
erosion. Surface water runoff, causing gully formation or the enlarging of existing gullies, is
usually the result of improper outlet design for local surface and subsurface drainage systems.
The soil instability of gully banks, usually associated with seepage of groundwater, leads to
sloughing and slumping (caving-in) of bank slopes. Such failures usually occur during spring
months when the soil water conditions are most conducive to the problem.
Figure 5. Gully erosion may develop in locations where rill erosion has not been managed.
Bank Erosion : Natural streams and constructed drainage channels act as outlets for surface
water runoff and subsurface drainage systems. Bank erosion is the progressive undercutting,
scouring and slumping of these drainageways (Figure 6). Poor construction practices, inadequate
maintenance, uncontrolled livestock access and cropping too close can all lead to bank erosion
problems.

35. Figure 6. Bank erosion involves the undercutting and scouring of natural stream and drainage
channel banks.
EFFECTS OF WATER EROSION:
On-Site
The implications of soil erosion by water extend beyond the removal of valuable topsoil.
Crop emergence, growth and yield are directly affected by the loss of natural nutrients and
applied fertilizers. Seeds and plants can be disturbed or completely removed by the erosion.
Organic matter from the soil, residues and any applied manure, is relatively lightweight and can
be readily transported off the field, particularly during spring thaw conditions. Pesticides may
also be carried off the site with the eroded soil.
Off-Site
The off-site impacts of soil erosion by water are not always as apparent as the on-site
effects. Eroded soil can accumulate on down-slope properties and contribute to road damage.
Sediment that reaches streams or watercourses can accelerate bank erosion, obstruct stream and
drainage channels, fill in reservoirs, damage fish habitat and degrade downstream water quality.
Wind erosion: It is defined as wearing away of a field’s topsoil by the natural physical forces of
wind.

36. Figure 2. The erosive force of wind on an open field.
Types of soil movement by wind
1. Saltation - Fine particles lifted from surface and following specific path w.r.t wind and
gravity
2. Suspension - floating of small particles
3. Surface creep - rolling or sliding of large soil particles along soil surface.
The rate and magnitude of soil erosion by wind is controlled by the following factors:
1. Soil Erodibility
Very fine soil particles are carried high into the air by the wind and transported great distances
(suspension). Fine-to-medium size soil particles are lifted a short distance into the air and drop
back to the soil surface, damaging crops and dislodging more soil (saltation). Larger-sized soil
particles that are too large to be lifted off the ground are dislodged by the wind and roll along the
soil surface (surface creep). The abrasion that results from windblown particles breaks down
stable surface aggregates and further increases the soil erodibility.
2. Soil Surface Roughness
Soil surfaces that are not rough offer little resistance to the wind. However, ridges left from
tillage can dry out more quickly in a wind event, resulting in more loose, dry soil available to
blow. Over time, soil surfaces become filled in, and the roughness is broken down by abrasion.

37. This results in a smoother surface susceptible to the wind. Excess tillage can contribute to soil
structure breakdown and increased erosion.
3. Climate
The speed and duration of the wind have a direct relationship to the extent of soil erosion.
Soil moisture levels are very low at the surface of excessively drained soils or during periods of
drought, thus releasing the particles for transport by wind. This effect also occurs in freeze-
drying of the soil surface during winter months.
4. Unsheltered Distance
A lack of windbreaks (trees, shrubs, crop residue, etc.) allows the wind to put soil particles into
motion for greater distances, thus increasing abrasion and soil erosion. Knolls and hilltops are
usually exposed and suffer the most.
5. Vegetative Cover
The lack of permanent vegetative cover in certain locations results in extensive wind erosion.
Loose, dry, bare soil is the most susceptible; however, crops that produce low levels of residue
(e.g., soybeans and many vegetable crops) may not provide enough resistance. In severe cases,
even crops that produce a lot of residue may not protect the soil.
The most effective protective vegetative cover consists of a cover crop with an adequate network
of living windbreaks in combination with good tillage, residue management and crop selection.
EFFECTS OF WIND EROSION:
Wind erosion damages crops through sandblasting of young seedlings or transplants,
burial of plants or seed, and exposure of seed. Crops are ruined, resulting in costly delays and
making reseeding necessary. Plants damaged by sandblasting are vulnerable to the entry of
disease with a resulting decrease in yield, loss of quality and market value. Also, wind erosion
can create adverse operating conditions, preventing timely field activities.

38. Soil drifting is a fertility-depleting process that can lead to poor crop growth and yield reductions
in areas of fields where wind erosion is a recurring problem. Loss of fine sand, silt, clay and
organic particles from sandy soils serves to lower the moisture-holding capacity of the soil. This
increases the erodibility of the soil and compounds the problem.
In India almost 55 percent of total land is affected; especially Himalayan, and lower Himalayan
regions highly affected by soil erosion. And as far as major reservoirs are concerned more than
25 percent of the reservoir capacity as lost due to the soil erosion problems.
TILLAGE EROSION:
Tillage erosion is the redistribution of soil through the action of tillage and gravity
(Figure 8). It results in the progressive down-slope movement of soil, causing severe soil loss on
upper-slope positions and accumulation in lower-slope positions. This form of erosion is a major
delivery mechanism for water erosion. Tillage action moves soil to convergent areas of a field
where surface water runoff concentrates. Also, exposed subsoil is highly erodible to the forces of
water and wind. Tillage erosion has the greatest potential for the “on-site” movement of soil and
in many cases can cause more erosion than water or wind.
Figure 8. Tillage erosion involves the progressive down-slope movement of soil.
CONSERVATION MEASURES:
The adoption of various soil conservation measures reduces soil erosion by water, wind and
tillage. Tillage and cropping practices, as well as land management practices, directly affect the

39. overall soil erosion problem and solutions on a farm. When crop rotations or changing tillage
practices are not enough to control erosion on a field, a combination of approaches or more
extreme measures might be necessary. For example, contour plowing, strip-cropping or terracing
may be considered. In more serious cases where concentrated runoff occurs, it is necessary to
include structural controls as part of the overall solution — grassed waterways, drop pipe and
grade control structures, rock chutes, and water and sediment control basins.
Some of the following measures can be implemented to prevent soil erosion:
1. The use of contour ploughing and windbreaks
2. Leave unploughed grass strips between ploughed lands (strip cropping)
3. Avoid overgrazing
4. Conserve wetlands
5. Minimum or no tillage
6. There are a number of other conservation practices which can be used by farmers. Any
single conservation practice can significantly decrease soil erosion rates. Combining a
number of soil conservation practices is often more effective.
7. The ideal goal would be to achieve a soil loss rate of 6.7 tonnes/ha/year. This is roughly
the rate at which soil can rejuvenate itself.
8. Making sure there are always plants growing on the soil and that the soil is rich in
organic matter are two key methods in prevention. Organic matter binds soil particles
together which reduces erosion.
9. Organic matter in soil can be increased with crop rotation or by incorporating organic
fertilizers. Crop rotation is also effective at enhancing soil structure.
10. Mulching is one example. It involves spreading hay or straw over a field as a substitute
for a cover crop. It is vegetation that keeps soil from eroding. This is because soil is
usually covered with shrubs and trees, by dead and decaying matter or by a thick mat of
grass. The root systems of plants are able to hold the soil together. Plants slow down
water as it flows over the land and it allows much of the rain to soak into the ground.
11. Plants also break the impact of a raindrop before it hits the soil. This reduces water
erosion. When this covering is stripped away through deforestation, over-grazing,

40. ploughing and fire, soil erosion is greatly accelerated. Over-cultivation and compaction
cause the soil to lose its structure and cohesion and it becomes more easily eroded.
12. Soils with high clay content are more cohesive and allow soil particles to stick together.
Soil with more clay are less vulnerable to erosion than soil with high sand or silt content.
Vulnerability profile of India to soil erosion:
In India a total of 17,50,000 km2
out of the total land area of 32,80,000 km2
is prone to
soil erosion. Thus about 53% of the total land area of India is prone to erosion. Areas affected by
soil erosion in India can be broadly grouped into two categories, representing, firstly, the
Himalayan and Lower Himalayan region and, secondly, other regions. The other regions of India
affected by severe erosion include the severely eroded gullied lands along the banks of the rivers
Yamuna, Chambal, Mahi and other west flowing rivers in Gujarat state and the southern rivers
namely the Cauvery and the Godavari river systems. Sheet and rill erosion is the most severe
problem in the catchments of these rivers.
As a result, agricultural production is greatly affected on the red soils, which cover an area of
7,20,000 km2
in the basins of the Chambal and Godavari. The depth of these soils is limited to
200 mm, in most of these areas.
Most parts of the Himalayas, particularly the Shiwaliks which represent the foothills of the
Himalayas in the northern and eastern Indian states, are comprised of sandstone, grits and
conglomerates with the characteristics of fluvial deposits and with deep soils.These formations
are geologically weak, unstable and hence highly prone to erosion.
Accelerated erosion has occurred in this region due to intensive deforestation, large Scale road
construction, mining and cultivation on steep slopes.
Approximately 30 000 km2 have been severely eroded in the northeastern Himalayas due to
shifting Cultivation Deforestation and associated soil erosion has caused desertification of land
in the Shiwalik hills in the Hoshiyarpur district of the Punjab state. The extent of degraded land
in this area 2000 km2
in 1939, while it increased to 20,000 km2
in 1981.
In addition the Himalayan and lower Himalayan regions have been greatly affected by
soil erosion due to intensive deforestation, large scale road construction, mining and cultivation
on steep slopes. Surveys of existing large and medium-sized Indian reservoirs have indicated that
at least six large reservoirs (storage > 100 Mm3) and three medium-sized reservoirs (storage 20-
100 Mm3) have already lost more than 25% of their capacities.