Md. Ashaduzzaman
Dept. of Urban and Regional Planning
Introduction:
Bangladesh
currently ranks as one of the world’s foremost disaster prone country. The
situation is aggravated, all the more by its being the most densely populated
country in the world (Choudhury, 2007). Every year, natural calamities upset people's
lives in some parts of the country.
The extreme natural events are termed disasters when they adversely affect the whole environment, including human beings, their shelters, or the resources essential for their livelihood. The geographical setting of Bangladesh makes the country vulnerable to natural disasters (MoEF, 2005). The mountains and hills bordering almost three-fourths of the country, along with the funnel shaped Bay of Bengal in the south, have made the country a meeting place of life-giving monsoon rains, but also make it subjected to the catastrophic ravages of natural disasters. Its physiographic setting and river morphology also contribute to recurring disasters. The major disasters that are concerned here are the occurrences of flood, cyclone and storm surge, flash flood, drought, tornado, riverbank erosion, and landslide (Hossain, 2008). Among these disasters, Strom surge is considered as the major and most devastating disaster to the human habitation of this country.
The extreme natural events are termed disasters when they adversely affect the whole environment, including human beings, their shelters, or the resources essential for their livelihood. The geographical setting of Bangladesh makes the country vulnerable to natural disasters (MoEF, 2005). The mountains and hills bordering almost three-fourths of the country, along with the funnel shaped Bay of Bengal in the south, have made the country a meeting place of life-giving monsoon rains, but also make it subjected to the catastrophic ravages of natural disasters. Its physiographic setting and river morphology also contribute to recurring disasters. The major disasters that are concerned here are the occurrences of flood, cyclone and storm surge, flash flood, drought, tornado, riverbank erosion, and landslide (Hossain, 2008). Among these disasters, Strom surge is considered as the major and most devastating disaster to the human habitation of this country.
A storm surge is an offshore rise of
water associated with a low pressure
weather system, typically tropical cyclones
and strong extra-tropical cyclones. Storm surges
are caused primarily by high winds
pushing on the ocean's
surface. The wind causes the water to pile up higher than the ordinary sea level.
Low pressure at the center of a weather system also has a small secondary
effect, as can the bathymetry of the body of water. It is this
combined effect of low pressure and persistent wind over a shallow water body
which is the most common cause of storm surge flooding problem (Wikipedia, 2014).
Storm surges are
oscillations of the water level in a coastal or inland water body in periods
ranging from a few minutes to a few days, resulting from atmospheric forces in
the weather system. A storm surge is partly caused by pressure differences
within a cyclonic storm and partly by high winds acting directly on the water.
This results in a mass of water, a huge wave, moving at the same speed as the
cyclone. In the northern Bay of Bengal, a unique combination of high tides, a funneling
coastal configuration, the low flat coastal terrain and a high population
density have produced some of the highest mortality figures associated with
storm surges (Flierl and Robinson, 1972).
An average of
1-3 severe to moderate cyclonic storms hit Bangladesh each year, with
associated storm surges as much as 13 meters higher than normal in extreme
cases, which can reach as far as 200 km inland (Milliman et al. 1989). Catastrophic
cyclones, which originated in the Bay of Bengal and hit Bangladesh from 1960 to
1997, are presented in the Table 1, along with their dates, maximum wind speed,
and heights of storm surge.
Table 1: Major Cyclones that
hit Bangladesh Coast.
Date
|
Maximum wind
(km/hour)
|
Strom surge
height ( meter)
|
|
30 October
|
1960
|
211
|
4.6-6.1
|
30 May
|
1961
|
160
|
6.1-8.8
|
28 May
|
1963
|
203
|
4.2-5.2
|
11 May
|
1965
|
160
|
6.1-7.6
|
15 December
|
1965
|
211
|
4.6-6.1
|
1 November
|
1966
|
146
|
4.6-9.1
|
23 October
|
1970
|
163
|
3.0-4.9
|
12 November
|
1970
|
224
|
6.1-9.1
|
25 May
|
1985
|
154
|
3.0-4.9
|
29 October
|
1988
|
160
|
3.0-4.0
|
29 April
|
1991
|
225
|
6.0-7.5
|
2 May
|
1994
|
210
|
2.0-3.0
|
25 November
|
1995
|
140
|
2.0-3.0
|
19 May
|
1997
|
220
|
3.1-4.2
|
Source: Chowdhury 1987, 1991 and BBS, 1998
Objective of the study:
·
To
identify the Strom surge prone area in Bangladesh.
·
To
identify the cause of Strom surge.
Strom
surge prone area in Bangladesh:
Strom surge is
the tidal flood that occurs during the time of cyclone or hurricane in the
coastal area. Some districts of coastal divisions Khulna, Barisal and
Chittagong are affected by Strom surge.
Khulna division:
In Khulna division Mangrove forest area of Satkhira, Bagerhat and Khulna
districts are heavy Strom surge porn area.
Barisal
division: Patuakhali, Barguna, Bhola are the high risk area.
Chittagong
Division: Noakhali, Laxmipur, Feni, Chittagong, Cox’s Bazar are in the high
Strom surge prone area. S
 |
| Fig: Strom surge prone area of Bangladesh. |
Causes
of Strom surge:
There are some
causes of Strom surge of Bangladesh are given below:
v The phenomenon of recurvature of tropical cyclones
in the Bay of Bengal:
The phenomenon
of recurvature of tropical cyclones in the Bay of Bengal is the single most
cause of the disproportional large impact of storm surges on the Bangladesh
coast. Extra-tropical cyclones, such as those that occur in Canada and Europe,
generally travel from west to east. On the other hand, tropical cyclones such
as those that occur in the Bay of Bengal are expected to travel from east to
west, as would be expected from considerations of the general circulation of
the atmosphere. However, in the Bay of Bengal, tropical cyclones most often do
not travel towards the west or Northwest, but they turn towards the north or
even Northeast. This turning back, referred to as recurvature, If the
phenomenon of recurvature does not happen, then Bangladesh would rarely be
affected by tropical cyclones and the storm surges.
v Shallow continental shelf, especially in the eastern
part of Bangladesh:
The coast line
of Bangladesh is characterized by a wide continental shelf, especially off the
eastern part of Bangladesh. This wide shelf amplifies the storm surges as the
tangential sea-level wind stress field associated with the tropical cyclone
pushes the sea water from the deep water side onto the shelf. Being pushed from
the south by wind stress, the water has no place to go but
Up-wards; which
is the storm surge.
v Triangular shape at the head of the Bay of Bengal:
The triangular
shape at the head of the Bay of Bengal helps to funnel the sea water pushed by
the wind towards the coast and causes further amplification of the surge. This
is basically what happens in the amplification of surges on the Bangladesh
coast.
v Almost sea-level orography of the Bangladesh coastal
land:
There are 70%
land of Bangladesh are lies below 8 meters above the sea level so if cyclone
occurs in the sea then it carry huge amount of water with high wind and the
coastal land are affected by the Strom surge.
Factor
that further modify the Strom surge:
Once storm
surges are generated, a number of local factors contribute to their
modification/amplification. Contributions from some of such factors are
discussed below in the context of Bangladesh.
Shallow Water
Effect:
Storm surges are mostly shallow water phenomena. The amplitude/height of surges
depends inversely on the depth of water. The effect of shallow water on the
modification/amplification has been discussed in the previous section.
Coriolis Effect: The Coriolis
force due to the rotation of the earth has a minor contribution to surge modification/amplification.
This force acts to the right of the direction of water motion in the Northern Hemisphere
and to the left in the Southern Hemisphere. Thus, for example, if surge water
is moving northward in the northern Bay of Bengal, it will be deflected towards
the east, thereby increasing surge height along the east coast.
Convergence
Effect:
Surge height is directly proportional to convergence. Convergence leads to
amplification (Proudman, 1955). Because of the northward converging nature of
the Bay, surge water is funneled towards Bangladesh in the north (depending on
the track of a cyclone) leading to amplification of surge height. Amplification
by convergence takes place somewhat like amplification due to shallow water.
Tidal Effect: Tidal range in
Bangladesh shows a gradual increase from west to east to reach a maximum at the
Meghna estuary and then it decreases south eastward. Tidal amplitude varies in
different seasons along the coast of Bangladesh. During the monsoon time a
large volume of water discharges through the Meghna estuary and other
distributary channels. This non-saline water when mixed with the saline water
of the Bay, causes an increase of the volume of the water; and consequently the
tidal amplitudes are increased. (Pattalo et al., 1989) estimated an increase of
tidal height above the normal height between 60 cm to 100 cm during the monsoon
period. When this high tide level (during the months of October and November) coincide
with the storm surge, the impact becomes severe, especially around the estuary.
Tide and surges are not linearly additive but interactive. This means that if,
for example, a 5m surge is superimposed on a 5m tide, the total height will not
be 10m. Tides modify the surge amplification.
River Effect: A river system
can have a number of effects on surges and tides. Firstly, the presence of
rivers has a negative effect on surge amplification. If there were no rivers,
then the surges in the coast would be higher (Sinha et al., 1985). Secondly,
fresh water discharge through rivers will modify the sea surface elevation.
Thirdly, the presence of a number of waterways allows a deep inland penetration
of surges originating in the Bay. A consequence of this is an inland flood
hazard and saline water intrusion, which may extend over several hundreds of Kilometers.
Lastly, the surge and tidal water has a back water effect on river discharges.
This is particularly important during flood periods. The back water effect
slows down the discharge rates, thus making the flood situation more
disastrous.
Island Effect: The presence
of offshore islands plays an important role in surge modification. Some of
these may be identified as follows:
i)
The
channels in between the islands confine the water within them and compel it to
pass through them (until the island becomes inundated), thereby causing surge
amplification,
ii)
Islands
act as barriers to impending surge water which may lead to surge amplification,
and
iii)
Islands
may also retard the outflow of surge water back to the Bay after the cyclone
dissipation.
Track Effect: Surge height,
particularly in Bangladesh, is strongly dependent on the cyclone track. Storm surges
are usually highest to the right (looking down the direction of the cyclone
motion) of the track. This is principally because of the occurrence of the
maximum wind speed to the right of the cyclone where the forward motion of the
cyclone is superimposed on the wind speed. As a cyclone approaches the land to
the north, this wind tends to pile up water against the coast. This tendency is
reinforced by the shallow water and convergence effects. The consequence of all
these factors is to produce even a higher positive surge to the right of the cyclone
path or point of landfall. Wind on the left (being predominantly in the
off-shore direction) drives water away from the coast and produces a negative
surge.
Conclusion:
Every year, many people in the
coastal belt of the country lose their valuable properties and lives due to
Strom surge. This affects the social and economic condition of the whole
country. When Strom surge occurs the people lose their crops and fishing instruments.
They become jobless and have to depend on relief. On the other hand, the
country faces more complicity because it has to manage relief, provide
medicine, develop social condition, etc. To deal with these problems the
country has to face economic suffering. It is impossible to prevent the natural
disasters, like cyclone but coping with cyclone is easily possible. So the
mitigation measures and preventive measures (if possible) should have to take
by the concern local and national authority.
References:
1.
Proudman, J., 1955. The Propagation of tide and
surge in an estuary- Proc. R. Soc.
Lond. A, Vol. 231, 8-24.
2. Sinha,
P.C., Dube, S.K. and Roy, G.D., 1985. Influence of river on the storm surges in
the Bay of Bengal- Proceedings, International Workshop on
Operational Applications of Mathematical Models (Surface Water) in Developing
Countries, 26 Feb - 1 May, 1985, New Delhi.
3.
Patullo, J., Munk. W. H., Revelle, R. and
Strong, E., 1955. The seasonal oscillation in sea level. Journal of Marine Research, 14, 88-156.
4.
Flierl, G.R., and A.R. Robinson, 1972. Deadly
surges in the Bay of Bengal: Dynamics and storm tide tables. Nature,
239, 213-215.
5.
Choudhury,
A. M. 2007. “Natural Disasters, Adaptation and the People of Bangladesh” in
Biswas, S. K. (eds), Bangladesh Quarterly, Vol. 27 (No.4), Srijoni
Printers, Dhaka, Bangladesh, p. 38.
6.
Hossain,
S. M. N. 2008. Addressing the Problems of Flood Management at Sirajganj
District: A Planning Perspective on Regional – Local Context. An
Unpublished BURP thesis, Department of Urban and Regional Planning,
Jahangirnagar University, Savar, Dhaka.
7.
MoEF
2005. National Adaptation Program of Action for Environmental Management.
Ministry of Environment and Forest, Govt. of People’s Republic of Bangladesh,
November 2005.
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