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SEA LEVEL RISE (SLR)
3.1 Concept
Mean sea level can be measured at both a global and local scale. Local
‘mean sea level’ (LMSL) is defined as “the height of the sea with respect to a
benchmark, averaged over a period of time, such as a month or a year, long enough
that fluctuations caused by waves and tides are largely removed”(Wikipedia
Website 2006). LMSL takes into account the tectonic movements of the earth’s
crust, atmospheric pressure, ocean currents and local ocean temperature changes
that may result in different mean sea levels between localities.
The other and more commonly used measure of sea level at a global scale is
known as global mean sea level (GMSL) and is influenced by ‘eustatic changes’.
Eustatic change or eustasy is defined as “the world wide sea level regime and its
fluctuations, caused by absolute changes in the quantity of sea water” (Warrick et
al 1993:107)1. The key factors that influence GMSL are thermal expansion and
glaciations which are discussed below in further detail.
Global mean sea level (GMSL) is commonly used by academics as a broad
measurement of sea level rise to highlight the issue on a world-wide scale.
However, LMSL provides a more accurate measurement of sea level at a local
scale and therefore allows a greater understanding of specific impacts region and
localities. There are a number of ways that sea level can be measured.
Measurement of sea level in the earth’s history has been based on scientific
research into sediment core samples (in wetlands), ice sheets and geological
surveys (Gehrels et al 2005). The natural environment has many indicators that can
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be used to ascertain historic sea levels2. For example, contours on ice sheets can be
read to determine the age of that ice (and therefore calculate sea level) the same
way tree rings on a tree stump can be read to determine the age of a tree (Gore
2006).
Recent fluctuations of both LMSL and GMSL have been measured with the
use of tidal gauges. Tidal gauges have been instrumental in determining sea level
fluctuations in the recent past but this system is not without its flaws. In particular,
the location of the tidal gauges results in an inaccurate depiction of sea level rise in
oceans around the globe. This is due to the uneven distribution of the 229 tidal
gauges in 21 locations around the world, with only 6 located in the southern
hemisphere (Bird 1993). As a result of this, tide gauges are generally more
accurate at determining local fluctuations in sea level. Tidal gauges are now
generally regarded as back-up indicators to determine sea level fluctuations as
satellite imagery covers more and more of the globe and reveals greater depths of
data.
Predictions of future sea level fluctuations are generally based on satellite
imaging and provided through computer climate modeling, normally Geographic
Climate Modeling’s the level of the ocean's surface. Sea level at a particular
location changes regularly with the tides and irregularly due to conditions such as
wind and currents. Other factors that contribute to such fluctuation include water
temperature and salinity, air pressure, seasonal changes, the amount of stream
runoff, and the amount of water that is stored as ice or snow. The reference point
used as a standard for determining terrestrial and atmospheric elevation or ocean
depths is called the mean sea level and is calculated as the average of hourly tide
levels measured by mechanical tide gauges over extended periods of time. The so-
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called greenhouse effect or global warming may cause a Sea Level Rise, which
will have a great impact on the long-term coastal morphology. The possible and
gradual Sea Level Rise will cause a general shoreline retreat and an increased
flooding risk and has to be handled according to the local conditions 3.
Local mean sea level (LMSL) is defined as the height of the sea with respect
to a land benchmark, averaged over a period of time (such as a month or a year)
long enough that fluctuations caused by waves and tides are smoothed out. One
must adjust perceived changes in LMSL to account for vertical movements of the
land, which can be of the same order (mm/yr) as sea level changes. Some land
movements occur because of isostatic adjustment of the mantle to the melting of
ice sheets at the end of the last ice age. The weight of the ice sheet depresses the
underlying land, and when the ice melts away the rebounds. Atmospheric, ocean
and local ocean temperature changes also can affect LMSL “Eustatic” change (as
opposed to local change) results in an alteration to the global sea levels, such as
changes in the volume of water in the world oceans or changes in the volume of an
ocean basin.
Various factors affect the volume or mass of the ocean, leading to long-term
changes in eustatic sea level. The two primary influences are temperature (because
the density of water depends on temperature), and the mass of water locked up on
land and sea as fresh water in rivers, lakes, glaciers, ice caps, and sea ice. Over
much longer geological timescales, changes in the shape of oceanic basins and in
land–sea distribution affect sea level. Observational and modeling studies of mass
loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2–
0.4 mm/yr, averaged over the 20th century. Sedimentary deposits follow cyclic
patterns. Prevailing theories hold that this cyclic primarily represents the response
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of depositional processes to the rise and fall of sea level. The rock record indicates
that in earlier eras, sea level was both much lower than today and much higher than
today. Such anomalies often appear worldwide. For instance, during the depths of
the last ice age 18,000 years ago when hundreds of thousands of cubic miles of ice
were stacked up on the continents as glaciers, sea level was 120 metres (390 ft)
lower, locations that today support coral reefs were left high and dry, and
coastlines were miles farther outward4. During this time of very low sea level there
was a dry land connection between Asia and Alaska over which humans are
believed to have migrated to North America (Bering Land Bridge).
For the past 6,000 years, the world's sea level gradually approached the
current level. During the previous interglacial about 120,000 years ago, sea level
was for a short time about 6 metres (20 ft) higher than today, as evidenced by
wave-cut notches along cliffs in the Bahamas. There are also Pleistocene coral
reefs left stranded about 3 metres above today's sea level along the southwestern
coastline of West Caicos Island in the West Indies. These once-submerged reefs
and nearby paleo-beach deposits indicate that sea level spent enough time at that
higher level to allow reefs to grow (exactly where this extra sea water came
from—Antarctica or Greenland—has not yet been determined). Similar evidence
of geologically recent sea level positions is abundant around the world.
Causes of Sea Level Rise
There are two ways in which global warming is causing sea levels to rise
are: (a) thermal expansion and (b) the melting of glaciers, ice caps etc. Global
warming or increases in temperatures (due to increase in the concentrations of
greenhouse gases) cause the oceans to warm and expand in volume inducing a rise
in the sea levels. Furthermore, warmer climate facilitates melting of glaciers, ice
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caps and ice sheets causing further addition of water to the oceans. In fact, the
major cause of SLR is the thermal expansion of the oceans which contributes
substantially in recent time (1993-2003).
Figure 3.1 Causes to sea level change/rise
Source: http://www.lenntech.com/images/sealevelrise.jpg
The main cause for rising sea levels is the expansion of water due to an
increase in water temperature and is thus a mere physical phenomenon. Additional
factors are the melting of mountain glaciers and the ice crust in Greenland, caused
by an increase in temperature of the earth’s atmosphere. Yet, an increase in
rainfalls and the subsequently growing Antarctic ice cover can also cause the sea
levels to fall. The influence of the Antarctic, however, is small in relation to other
factors, resulting in an overall rise of the sea level. Sea levels do not rise identically
in every geographical region. Therefore, in some regions sea levels are expected to
rise slightly more than in others, as the increase in temperature within the different
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(vertical) layers of water takes place in different stages. Independent of global
warming, changes in regional sea levels can also result from continental drifts. For
example, land in some river deltas subside by several millimeters per year because
sediments collapse. In these cases, a rising sea level intensifies the existing
regional effects. In other regions, a rise in sea level remains unnoticed because the
land is rising to the same extent or even more than the sea level itself. In the past,
the rise of sea level was measured solely by fixed measuring positions ashore. As
measuring positions did and do not exist at every point along the coast, the web of
data collected was rather wide-meshed. Since the 1980s, satellite technology has
facilitated the collection of more comprehensive data6.
Sea level, throughout the earth’s history has fluctuated in accordance with
temperature changes in the atmosphere. It is through natural process of climate
change in the past that current mean sea levels have been determined. Natural sea
level fluctuations have been predominately influenced by two main natural factors:
glaciations and thermal expansion of the ocean. Both concepts are defined and
discussed below Glaciations refer to the process of the accumulation of ice on land
to form glaciers (Strahler and Strahler 1999). Glaciers are created by a buildup of
snow which, when on land, freezes into ice and accumulates. Glaciers at any scale
are frozen water reservoirs storing water that would have otherwise run-off land
and flown into rivers and the ocean. The state of a glacier is determined by the
surrounding temperature of the earth’s atmosphere. For example, if temperatures
are high then glaciers will be melting, and if temperatures are low then glaciers
will be accumulating. This is a simplistic example provides a general
understanding of the intimate relationship between glaciers and the temperature of
the earth’s atmosphere.
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The accumulation and ablation of glaciers, as a result of temperature
changes throughout the earth’s history has influenced global sea level fluctuations
with sea level falling in periods of accumulation (i.e. with less water running off
into the ocean) and rising in periods of ablation (i.e. through an increase of water
being released into the ocean). In periods of low temperature glaciers have
dominated the earth’s surface. These periods are known as Ice Ages. Glaciations is
an important process in influencing sea levels around the world however its
influence is minor in comparison to another process; thermal expansion of the
oceans. Walsh et al states that “thermal expansion is the most important component
of global sea level rise” (2004:588)7. Thermal expansion refers to the heating of the
ocean’s water as a result of increases of temperature in the atmosphere. When the
water heats it expands, increasing the overall volume of the ocean and therefore
raising global sea level. The massive size of the ocean and the volume of water
contained within means that sea level fluctuations from thermal expansion will be
experienced at a delay from temperature changes in the atmosphere. This delay is
known as a ‘thermal lag’ and is said to be in the order of around 30 years2 (Walsh
et al 2004, Flannery 2005). Both glaciations and thermal expansion have been
important in determining sea fluctuations throughout the earth’s history. Each
process and therefore the level of sea level resulting from it, is critically dependent
on the temperature of the earth’s atmosphere. The intimate relationship between
the earth’s atmosphere and the sea levels has been evident throughout time “with
ocean levels always fluctuating with changes in global temperatures”8
This timeframe (30 years) refers to the absorption of heat from the
atmosphere into the ocean, not the entire warming of the ocean which takes about
1000 years or more. This period of 30 years is when serious impacts of sea level
rise due to temperature changes will start to be experienced (Flannery 2005). In
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addition to glaciations (ablution and accumulation) and thermal expansion there
are other more minor factors that have been responsible for fluctuations in sea
level. These additional factors for sea level fluctuation include the Greenland and
Antarctic Ice Sheets (melting and accumulation of ground ice), surface and ground
water storage, and global tectonic effects (Walsh et al 2004). The input of these
factors is considered relatively minor in comparison to the process of thermal
expansion and glaciations .Fluctuations in sea level are intimately linked to
fluctuations in global temperature, with changes in temperature impacting on
glaciers and the thermal expansion of the ocean.
This relationship has been witnessed throughout the earth’s history. An
example of this relationship is shown in the period that preceded the last Ice Age;
approximately 120,000 years ago the global average temperature was slightly
warmer than that of today. This resulted in a global sea level five to six metres
higher than it is today (Houghton 2004). This is in contrast to 18,000 years ago
where the world was in an Ice Age (hence temperatures where substantially colder
than today) resulting in a sea level of approximately 120 metres lower than the
current level9. Sea level fluctuations have been a key factor in the formation and
separation of continents and islands over the earth’s history. This was illustrated
clearly between 18,000 - 12,000 years ago when sea level rise was so significant
that it separated
This dramatic rise in sea level was a direct result of a periodical increase in
temperature by 50C. This temperature rise is known as “the fastest rise recorded in
recent earth history” (Flannery 2005).After the dramatic sea level rise commencing
at the end of the last Ice Age (18,000 years ago), sea level rise has been relatively
consistent. For the past 6,000 years sea level has consistently risen 5 to 10 metres
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to reach current global sea levels (Aubrey & Emery 1993). The average rate of sea
level rise has been between 0.14–0.1cm/year. This period of consistent sea level
rise has been a result of the earth’s atmospheric temperature remaining relatively
stable in this period. In the last century, the sea level has risen 10 to 25 centimeters
(i.e. an average of 0.1- 0.25cm/year) (Titus 1990). An estimate of the contribution
that each factor of sea level fluctuation (discussed above) has on current global sea
level in the past century is shown in Figure. These figures reinforce the dominance
of both thermal expansion and glaciers as key factors in the fluctuation of sea level.
In particular, these figures show that thermal expansion has been the highest
contributor to sea level rise, with glaciers and ice caps also having significant
contributions. Together, thermal expansion and glaciers and ice caps resulted in a
positive increase in sea level of 4 centimeters by the year 199010. This is in contrast
to the contribution of the Greenland ice sheet which is relatively minor, and the
contribution of the Antarctic ice sheet which has reduced sea level and somewhat
offset rises from other sources as shown in both figures. Scientific information is
inconclusive at the moment to whether sea level rises over the last century have
been as a result of natural or human induced climate change (i.e. brought about by
global warming). For example, studies were undertaken by Woodworth, Gornitz,
Solow and Douglas separately to determine whether sea level rise has been a result
of natural or human induced influences. From these studies” no author found
conclusive evidence of a global acceleration of sea level, especially compared to
what is predicted to accompany future global warming” (Douglas 2001:61)
Sea level rise Indian and world scenario
Many scientists consider global warming-forced climatic change as the most
serious environmental threat facing the world today (IPCC 2007). Global warming
has the potential to affect many humans dramatically and adversely as a
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consequence of both natural and anthropogenic changes to temperature,
precipitation, sea level, storms, air quality, and other climatic conditions.
Figure 3.2 : IPCC estimated contributions to sea level rise over twentieth century
(in cm)
Source: IPCC 2001:2006
Sea-level rise (SLR) poses a particularly ominous threat because 10% of the
world’s population (634 million people) lives in low-lying coastal regions within
10 m elevation of sea level (McGrananhan et al.2007). Much of this population
resides in portions of 17 of the world’s 30 largest cities, including Bombay, India;
Shanghai, China; Jakarta, Indonesia; Bangkok, Thailand, London and New York.
The population of many of the Asian cities will likely continue to increase as ports
and work forces expand to keep pace with economic globalization and increasing
shipping traffic (McGrananhan et al, 2007). Between 1980 and 2003, the
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population of 672 coastal counties in United States increased from 120 million to
153 million people (to 53% of the total population) and this number is expected to
rise to 160 million people by 2008 (Crossett et al. 2004). In 2003, coastal counties
in the United States accounted for 23 of the 25 most densely populated counties. In
addition to inundating low-lying coastal areas, rising sea level increases the
vulnerability of coastal regions to flooding caused by storm surges, tsunamis, and
extreme astronomic tides. As sea level rises, storms of a given magnitude reach
higher elevations and produce more extensive areas of inundation11.
Likewise, storm surges of a given height have greater recurrence intervals.
Rising sea level causes more frequent accidence of natural thresholds that, in turn,
leads to greater occurrences of waves breaking over seawalls, flood waters
overtopping levees, and storm surges over washing and breaching barriers. In areas
affected by tropical storms, warmer ocean surface temperatures may exacerbate
these conditions by increasing the magnitude of storms (Webster et al. 2005). The
recent loss of life and destruction of property in the northern Gulf of Mexico due to
Hurricanes Katrina and Rita in 2005 underscore the vulnerability of coastal regions
to storm surges and flooding12. The potential loss of life in low-lying areas is even
more graphically illustrated by the 1970 Bhola cyclone that traveled northward
through the Bay of Bengal producing a 12m high wall of water that drowned a half
million people in East Pakistan (now Bangladesh) (Garrison 2005).
The long-term association of population centers with lowland coastal regions
dates back to early civilizations when people congregated at river mouths and
estuaries because of abundant and accessible food sources (Stanley and Warne
1997, Kennett and Kennett 2006). A recent theory now ties the emergence and
rapid expansion of the first complex societies to a slowing rate of SLR (Horton et
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al.2006, Day et al. 2007). These authors suggest that the rapid growth of complex
societies did not occur until rising sea level began decelerating approximately
7,000 yrs BP following deglaciation. They argue that prior to that time, sea level
rose too quickly (1 m /century; Fairbanks 1989) to permit communities to become
permanently established and prosper. Prior to 7000 yrs BP, shorelines along lowgradient coastal zones, such as delta plains, retreated at a rate of about 1
km/century (Day et al. 2007). The commonality in response of early civilizations
and recent inhabitants of coastal regions such as the rapidly subsiding Louisiana
lower delta plain, the disappearing islands in Chesapeake Bay, and other
abandoned lands to the encroaching sea emphasizes the degree to which SLR has
and continues to influence human populations13.
Figure 3.3:
source: IPCC 2007
It is important to note that many of the present ills associated with rising sea
level represent the cumulative effects of processes that have been ongoing for
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many decades and perhaps centuries and that these effects may be related to other
natural and anthropogenic factors in addition to SLR, such as reduced or exhausted
sediment supplies and human actions. Despite the possible influence of these other
factors, sea-level rise may still have served as the major forcing agent in causing
erosion of coasts worldwide (Leatherman et al. 2000, Pilkey and Cooper 2004). As
acknowledged in the recent IPCC (2007) report, a growing number of tide gage
and field studies demonstrate that the rate of SLR began increasing between the
mid-19th and mid-20th centuries, (Nydick et al. 1995, Gehrels 1999, Donnelly and
Bertness 2004, Donnelly 2006) and recent tide gage data suggest that since 1993,
the rate of SLR has increased to 3 mm/yr (Church and White 2006). Thus, many of
the impacts of accelerating SLR can be generalized as worsening existing longterm conditions14. For example, flooding lowlands, beach erosion, saltwater
intrusion, and wetland loss are all processes that have been ongoing along coasts
for centuries and have been widely recognized for many years (Bird 1993,
Leatherman 2001).
In addition to increased flooding and greater storm impacts to coastal
communities in many low-lying regions, accelerated SLR will dramatically affect
sandy beaches and barrier island coasts. These impacts go beyond simple
inundation caused by rising ocean waters, and involve the permanent or long-term
loss of sand from beaches. The loss results from complex, feedback-dependent
processes that operate within the littoral zone including onshore coastal elements
(e.g., the, near shore, beach face, dunes, tidal inlets, tidal flats, marshes and
lagoons). Sediment budget analyses have shown that near shore, tidal deltas, capes,
and the inner continental shelf can serve as sediment reservoirs (Komar 1998).
Long-term beach erosion may increase due to accelerated SLR and may eventually
lead to the deterioration of barrier chains such as those along U.S. East and Gulf
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coasts (Williams et al. 1992, FitzGerald et al.2007), Friesian Islands in the North
Sea, and the Algarve coast in southern Portugal. Barriers protect highly productive
and ecologically sensitive back barrier wetlands as well as the adjacent mainland
coast from direct storm impacts and erosion. Moreover, barriers support residential
communities and a thriving tourist industry15. It is estimated that $3 Trillion are
invested in real estate and infrastructure on the barriers and mainland beaches
along the East Coast of the U.S. (Evans 2004). A single 7 km long barrier in North
Carolina, Figure Eight Island, has a tax base of more than $2 Billion (W. Cleary
pers. comm.). In many developing countries tourism is a major part of their
economy and the success of this industry is dependent on the vitality of its beaches.
Determining the socio-economic impacts of sea-level rise on coastal areas
comprises one of this century’s greatest challenges (Titus and Barth 1984, Gornitz
1990, Titus et al. 1991, Nicholls and Leatherman 1996, Gornitz et al. 2002). This
challenge, in turn, depends on accurate determinations of the effect of accelerated
sea-level rise on the natural (physical and ecological) environment. In fact, the
National Assessment of Coastal Vulnerability to Future Sea-Level Rise (USGS
2000) states that determining the physical response of the coast to SLR constitutes
“one of the most important problems in applied coastal geology today.”
Consequently, studies have used various sea-level rise scenarios to explore the
socio-economic, physical and ecological impacts on coasts in the U.S. and
throughout the world16.
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Table 3.1 : projected global average surface warming and sea level rise
case
constant year 2000
concentrations
B1 scenario
A1 scenario
B2 scenario
A1B scenario
A2 scenario3.4
A1F1 scenario
temperature change
(0 C
at
2090-2099
relative to 1980-1999)
best
likely range
estimate
sea level rise(m at 2090-2099
relative to 1980-1999)
0.6
0.3-0.9
not available
1.8
2.4
2.4
2.8
2.0-6.4
4.0
1.1-2.0
1.4-3.8
1.4-3.8
1.7-4.4
0.26-0.54
2.4-6.4
1.18-0.38
0.20-0.45
0.20-0.43
0.21-0.48
0.23-0.51
0.26-0.59
madel based range
Excluding future rapid dynamical
changes in ice flow.
Source:IPCC synthesis report 2007
Rising sea level is affecting coastlines throughout the world; the magnitude
and types of impacts are related to the geologic setting and physical and ecological
processes operating in that environment. Unlike infrequent large-magnitude storms
that can change the complexion of coast in a few hours (e.g. Mississippi coast due
to Katrina, Katrina/photo comparisons/ mainmississippi.html), impacts attributed
solely to SLR are usually slow, repetitive, and cumulative. This paper reviews the
state of knowledge concerning the response of coasts to SLR, and concentrates on
coastal plain settings including beaches and barrier chains and associated tidal
inlets and back barrier wetlands.
Although eustatic sea level is presently rising only a few millimeters per
year, this condition has widespread influences on physical and ecological processes
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on coasts (IPCC 2007). The rate of SLR determines how quickly areas will be
inundated given their slope, the rate at which wetlands, such as salt marshes, must
accrete vertically to maintain their surpratidal and intertidal extent, the rate of
erosion and shoreline recession, and the rate of sand exchange between the beach
and the near shore. Sea level is a function of the ocean surface, which is a
controlled, by, the: volume of ocean water, volume of the ocean basins, and
distribution of the water, and the land surface, which is affected by crustal
deformation and sediment compaction. Although sea level is influenced by many
elements that operate globally and locally over a wide range of time scales
including days to weeks (tides, storms), seasonal (steric changes, weather), 100 –
104 years (climate, tectonic), and up to millions of years (ocean basin evolution),
the two primary factors dictating the present rate of SLR are thermal expansion17.
due to heat uptake by ocean surface waters and water input caused by the transfer
of water from the land to the oceans (IPCC 2007).
Earth’s warming since the early 1900’s corresponds well with retreating
mountain glaciers, decreasing snow cover in the Northern Hemisphere, reduction
of Arctic ice and with other more subtle proxies, such as migration patterns of
birds and butterflies, and early growth season of certain plants (IPCC 2007). The
record shows that from 1850 to 1915 average global temperatures fluctuated but
with no significant net change. During the past 100 years temperatures have risen
by 0.74°C with most of that increase taking place since the 1970’s (0.55°C) (Fig.
3; IPCC 2007).
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Figure 3.4: Vulnerable Coastal Districts of India
Source: Indian government organization
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Warming of the atmosphere has translated to a warming of the ocean as
documented in three independent studies of temperatures data (Fig. 4; Willis et al.
2004, Levites et al. 2005, Ishii et al. 2006). From 1961 to 2003 the world’s ocean
increased in temperature by 0.1°C. For the period between 1955 and 2003 the heat
content change in the upper 700 m is 0.14 +/- 0.04 W m-2, whereas the during the
restricted 1993 to 2003 period the value is considerably higher at 0.5 +/- 0.18 W m
(IPCC 2007).
Sea level rose approximately 120 m since the last glacial maximum
approximately 20,000 years ago (Fairbanks 1989) and reached a near still stand
2,000 to 3,000 years ago when the rate of sea level rise slowed to 0.1 to 0.2 mm/yr
(Lambeck and Bard 2000). Global warming during the last 100 years has lead to
thermal expansion of the ocean and a net influx of water from melting glaciers 18.
Tide gage records and other sea level proxy indicate that from 1870 to 2004 sea
level rose by 195 mm with an average rate of rise of 1.7 +/- 0.3 mm/yr and an
acceleration of 0.013 +/- 0.006 mm/yr (Church and White 2006). Based on 177
tide gage stations for the 1948 to 2002 period, Holgate and Woodworth (2004)
estimated a SLR rate of 1.7 +/- 0.9 mm/yr. The most recent records of sea level
change consist of altimetry data from TOPEX/Poseidon and Jason satellites
(Nerem and Mitchum 2001). For the 10-year period between 1993 and 2003
satellite altimetry data show a SLR rate of 3.1 +/- 0.7 mm/yr (Cazenave and Nerem
2004, Leuliette et al. 2004). Estimates of the various contributions of the present
and past rates of SLT are presented (IPCC 2007). More than half of the present
SLR trend (1993-2003) is attributed to thermal expansion (1.6 +/- 0.5 mm/yr)
caused by warming to a depth of 3,000 m, while the influx of water by melting
glaciers is about half that value (0.77 +/- 0.22 mm/yr). Comparatively, lesser
amounts of water come from the Greenland and Antarctic ice sheets, which
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combined store enough water to raise sea level by 63.9 m (Bamber et al. 2001,
Lythe et al. 2001). The IPCC (2007) estimates that sea level will rise from 0.18 to
0.59 m relative to the 1980-1999 position by the end of this century (Table 2). This
range is based on different Atmospheric-Ocean Global Circulation Models using
various warming scenarios.
The rise in sea level by 1.29 millimeter every year along the Indian coastline
has not caused any major erosion, science and technology and earth sciences
minister informed the Rajya Sabha.He said during Question Hour that the ministry
of environment was conducting a study on the impact of the rising sea levels.
Referring to question related with Gujarat coast, Deshmukh said that as part of
long-term monintoring of sea level, the Survey of India has established four tide
gauges (Okha, Veraval, Porbander and Kandla) for continuous measurements of
sea level along the state's coast19.
He said all these gauge stations were transmitting data in real time to the
Indian National Centre for Ocean Information Services (INCOIS), Hyderabad.
Imagine a Chennai city where well-known, low-lying residential areas Velachery,
Madipakkam and Kotturpuram may permanently be submerged by sea water. The
Napier bridge may be seen rising directly out of the sea, while the mouth of the
river Cooum is pushed inland to open into the sea before the Napier Bridge. Island
Grounds may cease to exist. This is a picture painted by environmental experts on
the grim scenario that might confront Chennai if the sea level rises by one metre.
With over 70% of the population living along the coast, the displacement of human
population may be massive. Says Professor J S Mani of the Ocean Engineering
department in IIT Madras: ''The only solution is that the government must plan
well in advance and decongest the coastal areas”. There is a general consensus
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among ocean scientists that the sea level may raise by as much as one metre in the
next 50 years. While the reports vary in their projections of the speed of the rise, it
is commonly concluded that a significant rise in the sea level would occur during
the later years of the 21st century20.
Experts point out that Chennai, as a low-lying area with an average height of
2 metres above mean sea level, is likely to face several changes. The Coromandel
Coast comprises a series of sand dunes along the shoreline after the beach. This
area is of higher elevation. The elevation decreases further inland. Goa's massive
biodiversity and topographic attributes, including a long coastline, make it
vulnerable to climate change, notably sea level rise, said eminent climate scientist
and Nobel laureate R K Pachauri .Delivering the keynote address at the fourth
Vasudeva VSinai Dempo memorial lecture on 'Restoring respect for nature in
economic development', Pachauri pointed to the flash floods in Canacona in 2009
as an implication of climate change. "Goa is sensitive to sea level rise; one doesn't
have to wait for total submergence to realize the gravity of the situation.
Groundwater contamination by salt water intrusion in Goa can be one of the
dangers in the state," said Pachauri. He stressed that Goa should start a model to
sustain its ecology. "Goa is a fragile coastal state subject to tourism, mining,
shipping, which leads to negative externalities and has its tangible and visible
results. Even a small hotel has its impacts on a large scale and is not just confined
to the area at which it is situated21.
For this zoning laws must be enforced strictly," he said. He suggested that
consumption of water be regulated, buildings be given permission only if solar
water heaters are installed and building materials are energy intensive. Spread the
message of climate change through school children, he said. A sudden rise in the
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sea level at high tide on Tuesday on certain North Goa beach stretches especially
Baga and Morjim-startled locals, shack owners, and tourists and sparked rumours
that a Tsunami was fast approaching. Allaying fears, a National Institute of
Oceanography (NIO) scientist said it was purely a local and wind-driven
phenomenon. Jenny Madeira described the huge wave that swept past her shack
Golden Eagle on Morjim beach, the deck chairs and beach beds on Tuesday noon
as "scary". This is a very unusual scene for this low-lying beach," said Madeira,
who has been running the shack along with her husband for almost a decade.
"We have never seen anything like it, at least during the day," she added.
Worried tourists who had come all the way from Baga to Morjim, decided to give
lunch a miss22. "They gulped down their drinks and left," Madeira said, hoping the
scene did not recur at night. The Madeira’s shack built on private property is
located on higher ground. Fishermen's boats, anchored closer to the shore,
experienced more strongly the impact of the rising sea. "We had to push our boats
further out of the water's reach," a fisherman said. At Baga, the scene was no
different, as shack owners and tourists had to run for cover as the waves swept the
higher end of the narrow beach. "This is the first time I have seen the sea rise so
high during this part of the year.
Waves surged in at around 12-12.30pm and continued to do so until 2pm.
People panicked and there were even rumors that a tsunami may hit the coast, said
general secretary of the Shack owners' Welfare Society. Lobo,who owns Good
Luck shack at Baga beach, explained, "While water entered a few shacks at the tip
of Baga beach, it almost approached my shack which is around 50m away from the
water."
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NIO scientist Anthony Joseph said the shape and topography of the beach
coupled with gusty winds, may have given rise to some "local phenomenon". "The
Verem sea-level gauge showed that the water level was normal on Tuesday,"
Joseph said. Verem is barely about 20 km from Morjim or 10 km from Baga.
However, the waves are driven by winds and the intensity may not be the same
everywhere. "The wind is not steady, but very strong and comes in pulses (jets) in
some areas," Joseph added. Explaining the topographical effect, he said that even
the impact of a Tsunami may be felt more in certain areas, depending on shoreline
conditions. "It may not happen all over Goa's coastline, but more so near rivulets
and estuaries," Oceanographic experts advise that human settlements should be at
least 500 metres to one km away from the seashore in India because besides
tsunamis, rise in sea level also poses a threat. Or else what happened to Dwarka,
which mythology says was founded by Lord Krishna, and now submerged in the
Bay of Kutch, may happen to present settlements on shorelines23.
These predictions are not based on any mythological statement, but on
scientific facts derived after studying 15,000-year-old microfossils found in
oceans. Was it a tsunami which hit the Dwarka? "No", said Rajiv Nigam, who is
head of the geological oceanography division, National Institute of Oceanography,
Goa. "Evidences are that a tsunami hit the western coast of India about 8000 years
before present (BP), but Dwarka submerged due to rise in the sea level. Nigam has
done extensive research on depth fluctuations along Indian coasts. The sea level
has always been changing, he said, due to various climatic factors. Studies show
that sea level was 100 meters below the present sea level 15,000 years BP, rose
steadily and was 60-70 metres below present sea level 10,000 years BP. In the next
1500 years it reached to 40 metres below present sea level, came at par with the
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present sea level around 7000 years BP, then rose by five metres in the next 1000
years.
Thereafter, there was a gradual fall and sea level came down to 20-30 metres
below present sea level about 3500 years BP when Dwarka is presumed to have
been constructed. Again levels rose to above 5 metres present sea level about 1000
years BP. In the last 1000 years, the sea level has come down to the present
position and is now stationary, but may rise in future. Nigam explained that
mythology says that Lord Krishna constructed Dwarka after snatching land from
the sea. But as per science constructions can be on a land reclaimed from sea.
However, when sea level rises, these settlements are first to get submerged.
Dwarka submerged due to rise in sea level 3500 years BP24.
He has also used marine archaeological studies to resolve a three-decade
long controversy of Harappan settlement in Lothal, Gujrat. He recovered
foraminiferal micro fossils, around 4500 years old from the site, which established
that sea water once flowed in Lothal and there also existed a dockyard. The rate of
Sea level rise in the Indian Ocean is much larger than the global average ,Total
mean sea level rise in the Indian Ocean is contributed from both steric and water
mass changes The changing atmospheric circulation could be a cause for the
spatial patterns In the Northern Indian Ocean the trends are mostly due to the
trends in net heat gain In the Southern Ocean, the decadal oscillations contribute to
the observed trends in sea level
A new study in Nature Geosciences finds that Indian Ocean sea levels are
rising unevenly and threatening residents in some densely populated coastal areas,
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particularly those along the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra,
and Java. This image shows the key player in the process, the Indo-Pacific warm
pool, in bright orange. This enormous, bathtub-shaped area spans a region of the
tropical oceans from the east coast of Africa to the International Date Line in the
Pacific. The warm pool has heated by about 1 degree Fahrenheit, or 0.5 degrees
Celsius, in the past 50 years, primarily because of human-generated emissions of
greenhouses gases. Kerala's tranquil stretches of emerald green backwaters and
Mumbai are among several locales on the western and eastern coasts facing threat
from the rising sea level due to climate change.
Deltas of the Ganga, Krishna, Godavari, Cauvery and Mahanadi on the east
coast may also be threatened along with irrigated land and adjoining settlements,
according to a Government report. "It is estimated that sea level rise by 3.5 to 34.6
inches between 1990 and 2100 would result in saline coastal groundwater,
endangering wetlands and inundating valuable land and coastal communities. The
most vulnerable stretches along the western Indian coast are Khambat and Kutch in
Gujarat, Mumbai and parts of the Konkan coast and south Kerala," says the report
submitted to the UN25.
The report India's Second National Communication to the United Nations
Framework Convention on Climate Change- was prepared by multi-disciplinary
teams and other stakeholders comprising more than 220 scientists belonging to
over 120 institutions. "The loss of these important economic and cultural regions
could have a considerable impact in some states," it says. The experts who
prepared the report visited some vulnerable areas, including the 2004 tsunami-hit
Nagapattinam in Tamil Nadu, backwaters surrounding Kochi in Kerala and Paradip
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in Odisha, in order to make a detailed impact study of the rise in sea level. The
study, using digital elevation model data (90m resolution), digital image
processing and GIS software, showed that estimated inundation areas are 4.2 sq km
and 42.5 sq km in case where the sea level rise is 1.0 m and 2.0 m respectively in
the region surrounding Nagapattinam. "But for the same sea level conditions, 169
sq km and 599 sq km will be inundated in the coastal region surrounding Kochi,"
The coastal zone is an important and critical region for India. It is densely
populated and stretches over 7,500 km with the Arabian Sea in the West and Bay
of Bengal in the East. The total area occupied by coastal districts is around 379,610
square km, with an average population density of 455 persons per square km,
which is about 1.5 times the national average of 324 persons per square km. Under
the present climate, it has been observed that the sea-level rise (0.4-2.0 mm/ year)
along the Gulf of Kutchh and the coast of West Bengal is the highest. Along the
Karnataka coast, however, there is a relative decrease in the sea level. Future
climate change in the coastal zones is likely to be manifested through worsening of
some of the existing coastal zone problems. Some of the main climate-related
concerns in the context of the Indian coastal zones are erosion, flooding,
submergence and deterioration of coastal ecosystems, such as mangroves and
salinization. In many cases, these problems are either caused by, or exacerbated by,
sea level rise and tropical cyclones. The key climate related risks in the coastal
zone include tropical cyclones, sea-level rise, and changes in temperature and
precipitation. A rise in sea level is likely to have significant implications on the
coastal population and agricultural performance of India26. A one-metre sea level
re projected to displace approximately 7.1 million people in India and about 5,764
square kilometers of land area will be lost, along with 4,200 km of roads.
Chapter -3
The diverse impacts expected as a result of sea-level rise include land loss
and population displacement, increased flooding of low-lying coastal areas, loss of
yield and employment resulting from inundation, and salinization. Damage to
coastal infrastructure, aquaculture and coastal tourism, due to the erosion of sandy
beaches, is also likely. The extent of vulnerability, however, depends not just on
the physical exposure to sea-level rise and the population affected, but also on the
extent of economic activity of the areas and capacity to cope with impact
Sea level rise in study area: an analysis
Data analysis that indicates the impact of sea level rise based on sea level
meter scenarios is needed to project the types of impact than can occur, and the
cost of damages compared to adaptive mechanisms. This chapter analyzes
topographic map contours that show the extent of inundation associated with sea
level rise occurring under three different scenarios: a 2 meter rise, a 4 meter rise,
and a 6 meter rise. With more insight on location of impacts, next steps of
determining impacts would allow more cost effective recommendations on how to
protect West Bengal Coast from the “siege” of sea level rise as a result of climate
change. Hence, this chapter will determine the locations of SLR impact on West
Bengal Coast under the impending scenarios.
Chapter -3
Figure.3.5 Projection of Sea Level Rise
Source: IPCC Synthesis Report 2007
Topographic Analysis of SLR at the West Bengal Coast
A study of the locations of SLR impact aims to designate planning
approaches and potential adaptation mechanisms where they are needed most
while discovering what facilities surround the most vulnerable areas. Additionally,
an analysis of the impacts due to sea level rise is necessary to make
recommendations on how to adapt to the possibility of up to 6 meters sea level rise,
which is possible if we continue with present day emissions of greenhouse gas
(IPCC, 2007). However, the abatement of greenhouse gases does not guarantee
that SLR will not occur since there are other mechanisms taking place in the ice
flow in Greenland and there are other causes that may lead the West Antarctic Ice
Sheet to collapse (Tol et al., 2006). Hence, it is clear that SLR will have an impact
on the West Bengal Coast. Addressing the problem of sea level rise at the West
Bengal Coast compliments West Bengal Coast’s regional transportation objective
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of implementing infrastructure needs for the safe, environmentally friendly, and
efficient transport of goods throughout the region.
Figure 3.6 : Current sea level
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Figure 3.7: 2 Meter Sea Level Rise
In order to define the most vulnerable areas that would be affected by sea level
rise, Scenario A models a 2 meter sea level rise using topographic maps provided
by West Bengal Coast for the purpose of this study. The previous study on tsunami
impacts on West Bengal Coast could not conclude that more damage would occur
further into the Coast.
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Figure 3.8 : 4 Meter Sea Level Rise
Although it was not determined in the previous study on tsunami impacts at
West Bengal Coast that more damage at 4 meters above sea level would occur,
Figure show that further damage would occur at the sunderbans and surrounding
areas including an island just east of the sagar , as well as coastal areas to the west
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Figure 3.9: 6 Meter Sea Level Rise
A 6 meter sea level rise would inundate the area below plateaus, the lower
ganga plain, and the Hooghly river. A 4 SLR would also cause significant
inundation. If there were a 6 sea level rise, approximately 1,567 acres of West
Bengal coast’s 4,300 acres of land would be under water. Figure show
approximately 32 percent of West Bengal Coast under water and gives a dramatic
visualization of the economic implications if such a scenario were to occur. In
summary, all three scenarios, a 2m SLR, a 4m SLR, and a 6m SLR, have an impact
on facilities at West Bengal Coast. Figure provides a map to indicate where the
facilities’ inundation would occur in the various scenarios. Figure allows the
specification of where the inundated facilities are located. Two types of facilities
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would be inundated at a 2m SLR. They include community and industrial facilities
located throughout the west coast, central and east coast as mapped in Figure.
Supply Yard, while the other facilities can be relocated elsewhere. Three types of
facilities would be inundated in a 4m SLR. They are community, commercial, and
municipal facilities mostly located along the Main coastal region, as mapped in
Figure . The facilities inundated in a 4m SLR are not critical to port operations and
they can be relocated. The level of impact becomes most severe in a 4m SLR as
four types of facilities would be inundated. Community, commercial, municipal
and federal facilities would be inundated throughout the entire coast. Twice as
many facilities would be impacted in the 6m SLR scenario than in any of the other
scenarios. Hence, the magnitude of the impact is greatest in a 6m SLR as 1,567
acres meaning 32% of the west bengal would be under water.
IUNEP (1989) has identified India among the 27 countries that are most
vulnerable to sea level rise. India has a coastline that stretches about 5422 kms on
the mainland and exhibits most of the known geomorphologic features of coastal
zones. We attempt to find the time trend of the rising mean sea level, measured at
the nine tidal gauge stations spread in six states along the Indian coast. The worst
hits are the Calcutta, diamond harbour in West Bengal; Kandla in Gujarat then
again Haldia in west Bengal. One of the interesting results is the study of
Mangalore station in Karnataka. Prior to 1980 the sea level falls and a negative
time trend is estimated, after the year 1980 a significant positive time trend is
estimated. For one of the monitoring station Sager, we have observed the negative
time trend this contrasting physical nature of the West Bengal is also reflected in
the coastal resources east and west of Hugli estuary. Climate change and sea level
rise are now a reality. The West Bengal coast represents a typical deltaic strip
with almost a flat terrain.
Chapter -3
The Hooghly and its distributaries form the most conspicuous drainage
network and form an estuarine system. The Sundarbanss with coverage of about
1,430 square km is one of the largest single blocks of the halophytic mangrove
systems of the world. The major geomorphic features are mudflats, bars, shoals,
beach ridges, estuaries, extensive network of creeks, paleomudflats, coastal dunes,
large number of islands like Sagar and salt pans. Rising water levels have eroded,
by some accounts, 40 to 50 per cent of the landmass on the Ghoramara island in
Sundarbanss."Sea level rise in the Sundarbanss area is higher than the global
average of 2.5 mm per year – here it is 3.1 mm per year that is the rise in the sea
level," Wildlife Protection Society of India Member S R Banerjee. Ghoramara
island is part of the Sundarbanss – the Sundarbanss delta. It is home to the largest
mangrove forest in the world. With their thick roots these mangroves hold together
the shifting sands. Ghoramara was home to more than twelve thousand people
about two decades ago according to local officials. But half of the population has
fled – today just over five thousand live on the island."If the whole island goes in
the water – only then we will go," said Sheikh Jiyan. That's the sentiment across
this island it's a fragile ecosystem – but people have built their lives and homes on
the island and are reluctant to leave.
What's interesting is that many say they've never heard of global warming or
climate change but many scientists say they're living on the frontlines of it. You
have heard of slums and resettlement colonies but this is the first of its kind. It's a
colony of climate change refugees who came to the Sunderbans from Ghoramara
island. As the sea levels rise, these people are already victims of climate change.
The Nobel Prize winning Inter-governmental Panel on Climate Change predicts
that extreme weather, along with just a 45 cm rise in sea level would submerge 75
per cent of the Sundarbanss. School children gathered at Sagar Island to voice their
Chapter -3
protest against climate change. For them global warming is not a text-book term.
It's a daily reality."The water levels are rising. Sagar island is sinking and there is
terror everywhere," says a school girl. And they are not the only ones who are
worried. Experts like Dr Hazra feel resettlement colonies are not the long-term
answer. He is mobilizing people in the area to plant mangroves, which are saltresistant and perhaps the only natural defence against the rising waters.
"Developed nations have to realize they must cut carbon emissions. It's the people
of Sunderbans who are paying the price for no reason," says Dr Hazra.
But while the rest of the world continues to lag behind, this may be too little
to save the Sunderbans. If you have ever doubted global warming, Sagar Island is a
place on earth is you need to visit. A recent study by Sugata Hazra, an
oceanographer at Jadavpur University in nearby Calcutta, found that during the
past 30 years, about 80 square kilometers, or 30 square miles, of the Sundarbanss
has disappeared. More than 600 families have been displaced, according to the
local government authorities. Fields and ponds have been submerged. Ghoramara
alone has shrunk to less than five square kilometers, about half its size in 1969,
Hazra's study concluded. In the past 20 years, two other islands have vanished
entirely.
The Sundarbanss are among the world's largest collection of river delta
islands. In geological terms, they are young and still under formation, cut by an
intricate network of streams and tributaries that straddle the border between India
and West Bengal. Ever since the British settled the Sundarbanss 150 years ago in
pursuit of timber, the mangroves have been steadily depleted, half of the islands
have lost their forest cover and the population has grown. Today, sea rise and
deforestation threaten the Sundarbanss' most storied inhabitant, the Royal Bengal
Chapter -3
tiger, which drinks these salty waters and has a peculiar appetite for human flesh.
Environmental degradation also threatens the unsung human residents: Four
million people live here on the Indian side of the border alone. Certainly, nature
would have forced these islands to shift size and shape, drowning some, giving rise
to others. But there is little doubt; scientists say that human-induced climate
change has made them particularly vulnerable.
The Intergovernmental Panel on Climate Change predicts that global
warming, spurred by the buildup of heat-trapping gases in the atmosphere, could
raise the ocean's surface as much as 23 inches, or nearly 0.6 of a meter, by 2100.
According to its latest report, made public this month, the ecology and people of
this river delta system are among the most vulnerable in the world.
Incessant rain and high tide with 8-metre tall waves have flooded over 100 coastal
villages in East Midinipure, including Digha, affecting nearly 10,000 people and
rendering thousands homeless. Over 500 mud huts have collapsed since last night,
rendering over 2,000 people homeless. The villagers have been shifted to relief
camps set up by panchayats in Shankarpur, Deshpran, Tajpur, Khejuri and
Kendamari in Nandigram. The Ganga has its confluence with the Bay of Bengal in
Khejuri and Kendamari.District relief officer Sandip Nag said Digha alone had
received 53mm of rain between 10am yesterday and 10am today the depression
crossed the Bengal-West Bengal coast, 100km east of Calcutta, . “The depression
had made the sea turbulent,” a Met official said.
The officials said the sea was rough and fishermen had been asked not to
venture into the high sea. Normally, during high tides, sea waves rise to a height of
around 4 metres in Digha, they added. Ratikanta Jana, 50, a fisherman from the
Chapter -3
coastal village of Tajpur, about 10km from Digha, said the sea began to swell
around 8pm yesterday.“There was chest-deep water in most parts of the village by
11pm yesterday. The winds were so strong that my hut collapsed in front of my
eyes. This morning, we took shelter at a relief camp set up at a primary school in
our village,” Ratikanta said. Around 500 people from our village and neighbouring
areas have taken shelter at the camp, he added. In Digha town, eight-metre high sea
waves last night washed away 30 shops near the coast selling cashew nuts and
artefacts made from sea shells and conch shells. Some eateries and garment stores
were also washed away.“We tried our best to save our wares but lost most of it,”
Netai Jana, 35, a shop owner said. , there were around 5,000 tourists in Digha.
Today being a weekend, the tourist count crossed 8,000. Many tourists lined the
coast to catch a glimpse of the crashing waves. Digha police patrolled the coast
through the day and requested tourists not to go near the water. “Eight tourists
suffered minor injuries after falling on boulders, hit by waves,” said Kaushik
Basak, the officer in charge of Digha police station. Irrigation department officials
said 50km of East Midnapore’s 78km coastline had been lying unrepaired for over
five years. A 4km-long breach had occurred at Shankarpur two years ago but it was
yet to be plugged, the officials said.“As a result, the sea easily flooded the villages.
Ten villages, including Shankarpur, Tajpur, Chandpur, Lachhimpur, JamraShyampur, Jalda and Shaula, are the worst-affected,” said Swapan Pandit, an
executive engineer of the irrigation department. A senior district health official
said a team of doctors would be sent to the relief camps to examine the health of
those who have taken shelter.The sabhadhipati of the Trinamul Congress-run zilla
parishad, Ranajit Mondal, said crops across 10,000 hectares had been damaged.
Chapter -3
End Notes
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