Global Climate Change & Bangladesh....

Water Resource Vulnerability to Climate Change

Introduction
Bangladesh climate change country study considered vulnerability of water resources from two perspectives, viz.,

i) changes in water level which would reflect the change in depth of inundation.
ii) salinity intrusion due to sea level rise and low water flow from upstream during the winter season.
 

Table 1 : Temperature, precipitation and evaporation and their changes

Temperature changes: The results for Bangladesh are presented in Table-1. The details of the methodology may be found in the Final Report on Vulnerability. The observed average monthly temperature values for December, January and February (DJF) and June, July and August (JJA) gave average winter and average monsoon temperatures for the base year, respectively. Similar average winter and monsoon temperatures for the projection years 2030 and 2075 are estimated. The seasonal differences in temperatures for the two years are also estimated. The relative change in temperatures by comparing projection year average winter and monsoon temperatures with the base year data have also been calculated as presented in Table.

Table shows that the average increase of temperature for Bangladesh may be 1.3°C and 2.6°C for the year 2030 and 2075, respectively. For the purpose of analysis these figures are rounded up as 1.5 and 3.0, respectively. There would, however, be a seasonal difference in that winter temperature (average of temperature of December, January and February) would change more (1.3°C ) compared to the monsoon time (averaged considering temperature of June, July and August) temperature (0.7°C) for the year 2030. The respective temperature changes for the year 2075 would be 2.1°C  and 1. 7°C for the two seasons.

The GCM results indicate that there is a general increasing trend of temperature. In 2030, the increase is much pronounced in winter months, although the maximum change is observed for post-winter months, i.e., April, May and June. However, in 2075, the increase in temperature during April and May is much higher; about 4.0°C.

Precipitation: Precipitation changes are shown in Table-1. The results indicate that the winter precipitation in 2030 may increase slightly in winter and moderately in monsoon. In 2075, however, the change is much pronounced in monsoon (about 530 mm/month), while there would not be any appreciable change in winter precipitation.

Evaporation: The changes in evaporation were computed by using Lang's index of aridity. For details see the Final Report on Vulnerability and Adaptation. The results are presented in Table-1. From the results it appears that the average evaporation would be almost similar in 2030 and slightly higher in 2075 compared to that in 1990. But in winter months (December to March) evaporation in 2075 would be much higher owing to less precipitation, which would decrease moisture availability in dry months. Even in monsoon months evaporation would be much higher in 2075.

Coastal Morphology of Bangladesh

Bangladesh is drained by a large network of rivers centering one of the largest river systems in the world: the Ganges, the Brahmaputra and the Meghna (GBM). The GBM carries about 2.4 billion tons of sediments per year (Holeman, 1968) into the Bay of Bengal. These sediments interact with dynamic processes in the Bay leading to accretion in one place and erosion in the other. A huge amount of sediments are also thought to be carried by under currents into the deeper Bay of Bengal and the Indian Ocean.

The bottom topography of the Bay of Bengal plays a dominant role in the dynamic processes in the north Bay and Bangladesh coast. This topography is characterised by the Ninety East Ridge to the west of which lies the Swatch of No Ground (a submarine canyon) and the Bengal Deep Sea Fan. To the east are the Myanmar trench and the Nicobar Fan. In most of the northern Bay, particularly in and near the Meghna estuary, the water is shallow which helps amplification of waves like storm surges and tides. The bottom topography greatly influences the circulation pattern in the Bay.

The main features of this circulation: a western boundary current (WBC) runs northward and almost in parallel to the west coast of the Bay and it deflects eastward at the head Bay somewhere near 1 8-200N latitudes and breaks into cyclonic and anticyclonic gyres. These gyres may be partly responsible for balancing the northward mass transport by the WBC by a compensating southward transport through the eastern part of the Bay of Bengal (Ali, 1991). There may be some under currents also being generated/influenced by the Swatch of No Ground (Curray and Moore, 1974). A persistent warm water zone is also observed (Huh et al., 1985) along the Myanmar-Chittagong coast. The Bay dynamics is also controlled, particularly at the head Bay, by the river discharge through the GBM river system. This discharge apparently breaks (as is apparent in satellite imagery) into two branches: one moves south-westward and seems to be governed by the Swatch as well as the Coriolis force (Au, 1995) while the other component moves southward along the Chittagong coast and it may be joined by the gyres.

According to Ali (1991) the salient and major features of coastal morphology of Bangladesh are

• Low coastal bottom topography

• Low coastal land topography

• A large network of rivers, canals and streams

• A huge discharge of river water heavily laden with sediments

• A deep submarine canyon called Swatch of No Ground apparently controlling to a great extent the flow dynamics

• A funnel shaped Bay converging northward and meeting the Bangladesh coast

• High wind and tidal actions

• Frequently occurring tropical cyclones and storm surges

• A vast tract of mangrove forests influencing the flow dynamics

• A large continental shelf particularly near the Meghna estuary.

Under the given conditions of geomorphological features, circulation dynamics, topographical settings, coastal configuration, hydrological regimes/features, etc., the coast of Bangladesh has been divided into three distinct regions - the eastern, central and western regions (Pramanik, 1983; UN ESCAP 1987). Figure-1 shows the three distinct coastal zones and the general region-wise descriptions are given below.
 

EASTERN REGION

This region runs from Big Feni river to Badar Mokam (southern tip of the mainland Bangladesh) along Chittagong - Cox’s Bazar coast. The east coast is more or less regular and unbroken and protected from the sea by mud flats and sandy shores. A long sandy beach of about 145 km runs from Cox’s Bazar to Badar Mokam. This region, particularly its southern part, is less vulnerable to sea disasters like storm surges. The noteworthy rivers that cut across the coast are Karnaphuli, Sangu, Matamuhuri and Naaf. The Myanmar Trench, a submarine canyon, which protrudes northward and which is an extension of the Sundra Trench, plays a significant role in the dynamics of this region. The present study concentrates on the erosion phenomenon due to sea level rise in this region.

CENTRAL REGION

The central region lies between the Tetulia river and the Big Feni river, including the Meghna estuary. The region is characterized by heavy sediment load and fresh water discharge, accretion and erosion, highly broken coastline, a series of small and big islands and a number of channels and rivers. The area has a large and extended continental shelf with shallow depth. This is the most active coastal region of Bangladesh and here tropical cyclones and storm surges bring about most catastrophic ravages.

WESTERN REGION

This region covers the area west of Tetulia river and upto the international boundary with India. This region is relatively stable and covered by a large tract of mangrove forests. To the south of the western region lies the submarine canyon Swatch of No Ground which largely controls the flow dynamics in the northern Bay of Bengal. This area has less erosion activity.

Changes in the Coastal Zone

In the foreseeable future, a looming disaster is likely to inflict casualties in Bangladesh. This is the rise in sea level which is a consequence of global warming due to the increase of the concentration of greenhouse gases in the earth’s atmosphere. In addition to its direct effect, sea level rise (SLR) is likely to have multiple effects on the already occurring natural disasters in the country, thus further aggravating the disastrous situation prevailing in the country. The impact of an SLR apart from the inundation shall also manifest through the erosion of the beaches and increased salinity both resulting in major dislocations in the society.

Source : Global Climate Change : Bangladesh Episode, 1997, DOE, MoEF-GOB.