Bangladesh is a country that has been intrinsically associated with natural disaster and vulnerability. Bangladesh has a highly irregular deltaic coastline of about 580 kilometers towards south along the Bay of Bengal. This area is fissured by many rivers and streams flowing into the sea. The territorial waters of Bangladesh extend 12 nautical miles, and the exclusive economic zone of the country is 200 nautical miles. This region is exposed to threat of natural calamities like floods, tropical cyclones, tornadoes, destructive waves and tsunami from time immemorial. Besides, increasing salinity and scouring adversely affects any crop production. Periodic natural disasters in this region increase the vulnerability and in recent years it has been serious threat to the overall development of the country. During the period 1978-2016, Bangladesh was hit by 33 tropical cyclones, causing enormous loss of life and property. Our unique geographic location is also responsible for so frequent disasters. The funnel-shaped northern portion of the Bay of Bengal amplifies the tropical cyclones and storm surge. Any low-pressure if developed over Bay of Bengal, is likely to intensify into a cyclonic storm. Knowing the reasons of these disasters will ensure better preparedness which in turn will contribute to national development.

The Bay of Bengal is a triangle-shaped water body, which is an extension of the Indian Ocean to the north. It is the largest bay in the world and around 500 million people live on the coastal rim that surrounds it. Out of the 36 most deadly tropical cyclones in recorded history, 26 have been over the Bay of Bengal. The most devastating cyclone in Bengal so far was Bhola in 1970 where five lakh people were killed.

Cyclones generally occur in early summer (April-May) or late rainy season (October-November) in Bangladesh. The Bay of Bengal has lesser salinity because most of the important rivers like Ganga and Brahmaputra add huge volumes of Fresh water. Since freshwater is lighter than the salty waters in the Bay, it forms a thin layer on the surface. This thin layer of fresh water is heated more easily than the salty waters. These rivers also deposit the soil into Bay of Bengal to make it shallow near the coast. Less depth, comparatively small water body and presence of land on three sides are reasons for less heat circulation. Interestingly, adjacent Andaman Sea is warmer than the Bay of Bengal. Internal tide energy dissipation and associated vertical mixing are mainly responsible for this. So, occasionally vast low pressure created by the warm water of the ocean. Gusty cold air rushes towards that low pressure zone and gets concentrated as a storm. The Bay of Bengal shaped like a trough that makes it more hospitable for storms to gain force. Moreover, lack of landmass between the Pacific Ocean and the Bay of Bengal tend cyclonic winds to move into the coastal areas causing heavy rainfall. The absence of air movements from north-western India towards the Bay in the post-monsoon phase is also another reason for the chances of cyclones in the Bay of Bengal. Cyclone brings "multi-hazard" like strong winds, physical damage, tidal waves, heavy rains and flooding.

Storm surge and tidal surge are abnormal rises in sea levels formed due to the change in water circulation pattern and storm. During September to November the coastal current is more pronounced and it diverges towards the near shore region and form counterclockwise eddies with high surge. If depression activities takes place these waves intensifies and become destructive. Meteorologists believe that when strong winds of a tropical cyclone pushes water through a shallow concave bay it gets concentrated or funneled as the storm moves up. It can certainly devastate communities and homes long after bad weather has passed. The Bay of Bengal is the best example of this type of geography. The tidal amplitude at the Bay of Bengal along the Bangladesh coast varies from 2 to 7 meter. At the time of landfall of cyclone, the tidal phase has a significant impact on surge height, arrival time and duration. Bay of Bengal has lower pressure leading to the more circulation of air and eventually formation of higher tides and cyclones. Violent tides are also responsible for scouring of land.

Tides in Bangladesh coast originate in the Indian Ocean. It enters the Bay of Bengal through the two submarine canyons, the 'Swatch of No Ground' and the 'Burma Trench' and thus arrives very near to the Hiron point and Cox's Bazar respectively at about the same time. A semidiurnal tidal cycle is a cycle with two nearly equal high tides and low tides every lunar day. The relative distances and positions of the sun, moon and Earth all affect the size and magnitude of the Earth's routine two tidal bulges. During the new Moon and full Moon time high tidal surges occur due to diurnal inequality. When low-pressure systems that contribute to cloudy, rainy conditions prevail it will cause tides much higher than predicted. The geometrical configuration of the coastline, in addition to the wide continental shelf, could contribute to the amplification of both semi-diurnal and diurnal constituents.

The average elevation of the southwest coastal zone ranges from 1 to 2 meter and in the southeast coastal zone it is 4 to 5 meter. The low elevation, active delta and dynamic morphology play a significant part in its vulnerability to sea level change. Sea level rise affects the coastal zone and its geometry in a number of ways including inundation, erosion and salt water intrusion into the water table. Main reasons are expansion of ocean water due to global warming and addition of ice water on the ocean through the melting of glacier ice sheets. The factors that have regional and local influence are local atmospheric circulation, tectonic movement, land subsidence, soil compaction, sediment contributions and anthropogenic contributions.

The total amount of salinity affected land in Bangladesh was 105.6 million hectares in 2009 and continuing to increase (Reaz Haider, 2019). This scenario is very threatening to the primary production system, coastal biodiversity and human health. There are numerous interacting drivers that influence soil salinity in Bangladesh including irregular rainfall, evaporation, faulty management of coastal polders, back water effect, sea level rise, cyclone and storm surge inundation, salinity of groundwater, and brackish shrimp farming. One of the reasons for low flow condition of the rivers is construction of barrages and dams in the upstream. Delta plain mud deposits of coastal area have permeability sufficiently low to prevent lateral exchange of water and dissolved salts. Moreover, upward or lateral movement of saline ground water takes place during dry season. Again, the coastal zone of the country is still carrying salinity which intruded during cyclone Sidr and Aila.

The coastal zone of the country is subject to different type of floods. The south and south-central coastal zone is dominated by rainfall flood attributed by intense rainfall beyond the drainage capacity, whereas the south-eastern coast is dominated by flash flood endorsed by heavy rainfall in the hilly area. The zone is also subject to tidal flooding dominated by tidal water. Several factors including enhanced glacier melt in the Himalayas, severe monsoon precipitation, and the possibility of an increase in intensity of cyclones are likely to contribute to increased flood risk.

El Nino is characterized by strong anomalous warming in the eastern equatorial Pacific. El Nino has a significant relationship with rainfall, temperature and hydrology at the Bay of Bengal. El Niño may still cause lower than normal rainfall, warmer than normal temperature, drier than normal climate, and higher than normal cyclonic activities.

A tsunami is a wave produced by a disturbance that displaces a large mass of water as a result of geologic activities such as earthquakes, volcanic eruptions, underwater landslides, or if meteor strikes. In the deep ocean, tsunamis can move at a speed of over 800 km/h. The distance between waves is the wavelength. In deep sea its wavelength remains hundreds of miles long and height rarely 3 feet. So, it is barely noticeable in the deep ocean. As a tsunami enters shallow water near land, it slows down, compresses, wavelengths decrease, waves grow in height, and currents intensify. At the shore, most tsunamis slow to the speed of approximately 30 to 50 km/h.

The coast of Bangladesh stands at the edge of an extraordinarily extended continental shelf. This has been built through huge discharges of river sediments along the Brahmaputra and Ganges rivers. As a result of this enormous discharge, another interesting feature of the area is the deep underwater Canyon, connected with the estuaries, running south towards the continental slope. The extended shallow bathymetric profile or underwater topography of the continental shelf plays a key role in flattening the waveform through a defocussing process while the Canyon delays the process. So, it will have moderate impact here. Other locations with no extended shelf experiences significant wave heights in the same configuration.

Many natural reasons highlighted in this paper are beyond our control. Still, lot can be done to address greenhouse gas emission, constructing water regulatory structures, limited land reclamation and controlling tidal inflow. Present government of Bangladesh has approved the Delta Plan 2100 on September 4 of 2018 to secure the future of water resources and mitigate the likely effects of climate change and natural disasters.

Mohammad Mahmudur Rahman Niaz is a civil Engineer and a serving Military Officer. Email: [email protected]

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