A brief presentation on coastal hazard

1. A Presentation
On
Coastal Hazard and Disaster
Prepared By:
S.M.Muhibullah
Md.Shahadat Hossain Biplab
Md.Nazim Uddin Naeem

2. Coastal Hazard
 Coastal hazards are those natural hazards that occur at the interface
between the lake and the shoreline, inclusive of the uplands that
impact the lake throughout the coastal watershed.
 At present, about 1.2 billion people live in coastal areas globally, and
this number is predicted to increase to 1.8–5.2 billion by the 2080s
due to a combination of population growth and coastal migration.

3. • Most coastal hazards can be grouped into five main impacts,
usually with several contributing processes causing an event
or cumulative harm:
- coastal erosion and shoreline stability
- coastal flooding
- elevated groundwater levels in coastal plains
- recreational activities with hazards arising from surf
zone conditions (e.g.rips, wave conditions, long-wave surges)
- maritime activities with navigation hazards arising
from wave/swell conditions, storm tide, strong winds, low tides,
long-wave surges and tsunami.

4. • In recent times, New Zealand has been spared from any major coastal
disaster or damage, but we remember the sinking of the inter-island
ferry Wahine (1968), the February 1936 storm – the most destructive of
the 20th Century – that caused $800M damage across the North Island,
and the 3 to 10-m-high tsunami after the 1855 Wairarapa earthquake.
• However, already the frequency of flooding or wave overtopping is
increasing (e.g. Tamaki Drive - Auckland, south Wellington coast, Avon-
Heathcote estuary, South Dunedin) and will become a common
occurrence (e.g. yearly) in low-lying areas with only a modest rise in sea
level of 30–45 cm (depending on tide range).

5. • The risks (combining consequences and likelihood), from coastal
hazards will rise substantially as sea level continues to rise and
waves and storm surge increase. This increase in risk comes from
both the rising hazard exposure and also the rising value of real
estate and infrastructure and further development around coastal
and estuarine margins.
• All these factors, along with the uncertainty in sea-level rise mean
we have to adaptively respond by planning adaptation pathways for
new and existing development on our coastal margins and for
maritime activities.

6. The major causes of coastal hazards are storm surge, sea-level
rise, erosion, and inlet migration.
Storm Surge
• Storm surge is the temporary elevation (or depression) of the
sea surface caused by changing atmospheric pressure and
strong winds.
• This elevation is superimposed on and is independent of the
tides.
• The threat of coastal flooding and erosion is greatest when a
storm surge is superimposed on spring or perigean high tides.
Causes Of Coastal Hazards

7. Sea-Level Rise
 Historic sea-level rise along the Maine coast has been measured by tide
gauge stations. Sea level in Maine has risen erratically over the last several
decades. Since 1912, when the Portland tide station was established, sea
level has risen at an average rate of 9 inches per century (2.2 mm/yr; Lyles
et al., 1988).
 Over the last millennia sea level (as measured in the geologic record of
coastal Maine) has risen at a rate of about 2 inches per century (0.5
mm/yr, see Maine's History of Sea-Level Changes).
 Future sea-level rise is expected to accelerate as a result of global
warming. The combined effects of coastal sinking and global warming may
cause sea level to rise 0.5 to 1.0 m (20 to 40 in) along the Maine coast by
the year 2090. These estimates will be refined further by scientific studies.

8. Erosion
 Erosion of beaches and dunes is driven by both storm action and long-
term sea-level rise. As ocean levels rise, coastal storm flooding is able to
reach farther inland and overtop low-lying dunes more frequently. Even
without global warming and accelerated sea-level rise, more frequent
flooding of dunes is expected based on the historic record. Erosion and
accretion of beaches tend to be variable due to short-term changes in sea
level and regional storm histories.
 With continuing long-term sea-level rise, erosion should dominate over
accretion in most beach locations. Net loss of sand to the seafloor in
offshore areas may result from coastal erosion. However, some beach and
dune sand may be preserved in the sand dune system if storm waves
overwash the dunes and carry sand in a landward direction. In time, the
erosion and landward deposition of sand may shift the frontal dune
landward, over the back dune environment. In some locations back dunes
may form over adjacent salt or freshwater marshes.
 Consequently, it is important to allow sand-laden flood waters to pass
from the beach into and through the dune system. By keeping dune areas
open to the flow of flood waters, the dunes can naturally migrate and
build upward as sea level rises.

9. Inlet Migration
 Inlets where streams and rivers pass through the dunes and enter the sea are
perhaps the most dynamic portion of the sand dune system. Dunes and low
energy beaches along the margins of tidal inlets are affected by changes in
channel position.
 The complex tidal mixing of freshwater with saltwater in estuarine channels
and the geometry of constricted inlets between dunes result in a complex
series of sand bars adjacent to the dunes. These sand bodies form a dynamic
network of channels and shoals.
 As sand bars and channels shift position, erosion and accretion of adjacent
dunes and beaches may take place rapidly. Over time there is exchange of
sand between the dunes, beach and channel bars.
 The dynamic nature of inlet margins makes projection of future shoreline
positions difficult.
 In summary, the changes forced upon the beach and dunes by storms,
sea-level rise, erosion and inlet migration are natural and necessary for
the future continuation of these environments. The dynamic nature of
the sand dune system strongly contrasts with the fixed nature of human
development on dunes and can lead to the creation of coastal hazards.

10. Coastal Hazards In Bangladesh
• Bangladesh has the eighth largest population of the world
This high population density (800/km 2 ) increases the
susceptibility to storm surge disasters.
• An other factor which increases the susceptibility is the
migration of workers to the southern parts of the country in
the months of April, May, October and November in quest of
job.
• The coastal areas and off-shore islands of Bangladesh are low
lying and very flat. The height above mean sea level of the
coastal zone is less than 3m.

11. Figure 1:Monthly distribution of 35 Major Cyclones in Bangladesh (after M.M. Hoque, 1991).

12. • The range of astronomical tide plays a great role behind coastal
hazards.
• The normal tidal range is about 3m near the Indian border in the
west and becomes higher to the east (central coastal part) to about
5m near Sandwip Island at the mouth of the Meghna estuary (Figure
2).
• The tidal range in the southeastern part is about 3.5 to 4m. If storm
surges are superimposed on high tides, the situation becomes
disastrous.
• A funneling coast line reduces the width of storm induced waves
and increases the height.

13. Figure 2. Average tidal amplitudes in the three coastal zones of Bangladesh.

14. Types of Coastal Hazards in Bangladesh
• In Bangladesh the risks of atmospheric and exogenic hazards
are high whereas risk from endogenic hazards are relatively
low
1. CYCLONES
- Typhoons are tropical revolving storms. They are called `Cyclones'
in English, when they occur in the Indian Ocean area.
- The coastal regions of Bangladesh are subject to damaging
cyclones almost every year.
- They generally occur in early summer (April-May) or late rainy
season (October-November). Cyclones originate from low
atmospheric pressures over the Bay of Bengal.

15. Storm Surge Disasters in Bangladesh
• 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.
• 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 funnelling 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).

16. Storm Surge and Flooding
Flooding by storm surge in Bangladesh can be classified as follows:
a) Normal coastal flooding - no damage on crops,
b) Moderate coastal flooding - very limited damage on crops,
c) Moderately high coastal flooding - high damage on crops but relatively low damage on
properties and lives,
d) High coastal flooding - large scale damage on crops, properties and lives,
e) Severe coastal flooding - severe damage on crops, properties and lives.
Storm surge and tidal bore
• Tidal bore destruction includes the widespread demolition of houses, uprooting of
trees, damage of CROPS, roads, buildings and structures, and death to human and
loss of livestock.
• Tidal bore is also known as ban in Bangla. In Bangladesh, tidal bore is observed in
the Meghna estuary and other southern coastal areas in the months of April-May
and also between September-December.

17. • In the Meghna estuary, the 1970 Cyclone (Nov 12-13) with cyclonic surge of
3.05m to 10.6m high with wind speed of 222 km/h occurred during high tide
causing most appalling natural disaster claiming 0.3 million human lives.
• On the 29 April 1991 a devastating cyclone hit Chittagong, Cox's Bazar, Barisal,
Noakhali, Patuakhali, Barguna and Khulna along with tidal bore of 5-8m high with
wind speed of 240 km/h which killed 150,000 human beings, 70,000 cattlehead,
and the total loss was about Tk 60 billion. [HS Mozaddad Faruque]
Flood
• Every year near about one-fifth of Bangladesh undergoes flood during the
monsoon season.
• The geographic position, structural set-up, PHYSIOGRAPHY and geomorphology are
jointly responsible for flooding in the country.
• A flood season in Bangladesh may start as early as May and can continue until
November.
• Five floods during the last fifty years are extensive and devastating and these are
the floods of 1955, 1974, 1987, 1988, and 1998.

18. Riverbank erosion
Every year millions of people of the country are affected by riverbank erosion that
destroys standing crops, farmland and homestead land. It is estimated that about 5%
of the total FLOODPLAIN of Bangladesh is directly affected by riverbank erosion.
Riverbank erosion is taking place in about 94 upazilas out of 496 upazilas/thanas of
the country. Over 25,000 families were rendered homeless in June 1993 by riverbank
erosion in 16 districts.

19. • During monsoon, extensive overbank spills, bank erosion and bankline shifts have
become typical. The unpredictable shifting behavior of the RIVERS and their
encroachments not only affect the rural floodplain population but also the urban
growth centres and infrastructures.
• The rivers that are highly susceptible to riverbank erosion are GANGES, JAMUNA, PADMA
and Lower Meghna. Along the right and left banks of the Ganges, erosion rates are 56m
and 20m per year respectively. The rate of widening of the Jamuna within the period
1973 to 2000 is 128m/year. In this period, the average width of Jamuna increased from
9.7 to 11.2 km.
• The maximum bank erosion during 1984-92 occurred at the left-bank, just upstream of
Aricha. The widening of Jamuna in a 28-year period resulted in a loss of floodplain of
70,000 ha over the total 220-km length of the river in Bangladesh.
• Banklines of Padma are very unstable and the widening rate is 159m/year. The Lower
Meghna river eroded both its banks, causing formation of medial bars.

21. Coastal erosion
• The deltaic coastline of Bangladesh can be recognised as
consisting of two basic physiographic units:
1. the inactive or abandoned Ganges tidal plain and
2. the active Meghna deltaic plain.
• The Meghna deltaic plain extends from the Chittagong coast in
the east to Tetulia Channel in the west. Broad map comparisons
indicate that the delta of the Ganges-Brahmaputra rivers has
not grown significantly toward the sea over last two centuries.
• Changes can be noticed in Sandwip and adjacent islands, in
Hatiya island, in Bhola Island, and in the coastline of the
Noakhali mainland.

23. Coastal Subsidence In Bangladesh
• Coastal subsidence, which can be described as the downward displacement of the
land relative to sea level, often occurs in deltaic regions associated with riverine
and estuarine sedimentation.
• Land Subsidence Contributes to Relative Sea-Level Rise:
Fig: Shoreline retreat caused by the combination of Sea level rise and Land Subsidence ( Source: Researchgate)
Fig: Shoreline retreat caused by the combination of Sea level rise and Land Subsidence ( Source: Researchgate)
Fig: Shoreline retreat caused by the combination of Sea level rise and Land Subsidence ( Source: Researchgate)
Fig: Shoreline retreat caused by the combination of Sea level rise and Land Subsidence ( Source: Researchgate)

24. Coastal Subsidence and Sea-Level Rise In Bangladesh
• There is a widespread misconception,
- a rising sea-level with global warming will
overwhelm Bangladesh’s coastal area contour by contour
and will thereby displace as many as 10–30 million
people in the 21st century
- In some accounts, that situation will be
aggravated by high rates of land subsidence (Syvitski et
al., 2009), a recent doubling of the rate of sea level rise
(Smith, 2012) and rapid, on-going rates of coastal
erosion (Vidal, 2013).

25. Causes Of Coastal Subsidence in Bangladesh
Four main mechanisms are apparent in the GBM
delta:
(1) tectonic subsidence/uplift
(2) compaction of sediment or peat
(subsidence);
(3) anthropogenic subsidence, such as fluid
extraction, drainage, embankment building; and
(4) sedimentation (i.e. accretion or
elevation gain) (Hoque and Alam, 1997; Syvitski,
2008

26. Mitigation Measures
• the creation of artificial raised land using material from neighboring
soft-rocked hills should be investigated for planning further
expansion.
• Existing buildings and infrastructure on floodplain land (including
the airports) will need to be protected by reinforcement of existing
embankments plus pump drainage.
• constructing barriers or locks on the Karnaphuli river to protect
Chittagong port.
• In the Meghna estuary, research is needed to develop methods for
bringing new land formations under cultivation more quickly and to
provide fresh water for domestic use.
• Most urgently needed are ways to maintain freshwater flow to
western parts of the Ganges Tidal Floodplain in order to prevent
the salt-water front from moving further inland.

27. Coastal hazards, vulnerabilities, and management efforts
• Out of a total of 147,570 km2 of territory, Bangladesh has 47,201
km2 of coastal area, which is one-third of the country and
accommodates 28% of the country’s population (MoWR 2006;
Islam 2004).
• This large population is relatively more vulnerable than any part of
the country.
• Four key vulnerabilities have been identified in the coastal areas of
Bangladesh. These are i) saline water intrusion, ii) drainage
congestion, iii) extreme events, and iv) changes in coastal
morphology.
• Recognizing these acute impacts, both governmental and NGOs
with the financial and technical support of donors and international
bodies have undertaken diversified programs and projects.

28. Conclusion
• Bangladesh is one of the disaster prone countries of the world with
extremely limited resources. Its real development is not possible
without the integration of disaster mitigation programs. Among
them, cyclone and flood are devastating,
• Bangladesh is striving hard to establish an elaborate and
experienced disaster management system from national to
community level (top down approach) to mitigate the effects of
disasters.
• The most significant measure of Bangladesh‟s success is the
dramatic reduction in lives lost to natural disasters. The
Government of Bangladesh (GoB) has, therefore, taken as a whole
commitment towards reduction of human, economic and
environmental coasts of disasters by enhancing overall disaster
management capacity.