Bangladesh is a Tropical monsoon country of South Asia with geographical location 20°34’N to 26°38’N and 88°01’E to 92°41’ E bordering the Bay of Bengal in the south with a coast line 734 km, Myanmar for 193 km in the south east and India for 4,053 km in the west, north and east. It is a part of humid tropics, with the Himalayas in the north and the funnel shaped coast touching the Bay of Bengal in the south. The country comprises of an area of 147,570 km² with a population 165 million. The terrain is mostly flat; hilly in southeast. Natural hazards include droughts, cyclones; much of the country routinely flooded during the summer monsoon season. Many people are being forced to live on flood-prone land with limited access to potable water. Water-borne diseases are prevalent. Water pollution is a big problem as a result of commercial uses of pesticides. In addition, there are intermittent water shortages, soil degradation, deforestation, and severe overpopulation.
Urbanization is the major demographic development which is occurring very fast and with larger magnitude in Bangladesh and other developing countries. In the most cases, urbanization is driven unplanned and bottom up process, which transforming the existing landscape without considering the possible consequences and requirement for environmental sustainability. These urban growths have profound adverse effects on the water resources, particularly in the humid tropical region. In the tropical region where monsoon causes huge rainfall during some part of the year are naturally drained by the gravity drainage through stream-river networks, and wetlands works as natural retention storage. Unplanned urbanization hampered this natural state of drainage, and hence causes sudden inundation and water-logging. However, hydrological consideration during urban planning can reduce the adverse effects through conservation of wetlands and stream-networks to be used as retention ponds and canals or designing such elements to drain-store-drain the modified landscape.
Dhaka with an aerial extent of 298 km², bounded by the Buriganga River in the south, the Balu River in the east, the Tongi Khal in the north and the Turag River in the west. These rivers are connected to the Ganges-Brahmaputra River system and also include the Old Brahmaputra River flowing towards southeast from the all sides of the bigger neighboring region. The bigger area is closely dissected by number of rivers and Khals which are hydrologically connected to these major rivers.
The area represents mostly flat land with slight undulations and stands few meters higher than the surrounding area. A large part of this city is covered by low-lying depressions. The area slopes towards southeast, east and west, but general slope is from the north to southeast where the ground surface merges gently with the floodplains of the Buriganga River. The elevation of the surrounding floodplains of the area is variable. The average elevation of the Buriganga and the Lakhya River floodplains are about 3 m above.
Dhaka is situated in the central part of the country and has the fastest urban growth rates among the developing countries with a present population more than 12 million which will rise at 20 million by 2020 and will be world’s 4th highest populated country. The elevated Pleistocene terrace Madhupur Tract landform that stands higher than the neighboring floodplain and low-lying marshlands is being modified extensively by the progressive urbanization for development of infrastructure and services, including road-network, water supply, sanitation, sewerage and drainage services and hence expansion of the city towards the surrounding floodplain and low-lying areas. The huge building structures may be considered as hills or hillocks with a great raised surface area which do not hold surface runoff rather instantly discharge to the ground during the rains. As Dhaka city has very poor surface drainage system and the low-lying water bodies have been filled by land and wastes dumping this rain water floods the most part of the city and the roads, lanes and streets become waterways very frequently.
The wetlands of Dhaka city has been squeezed to 5% only, resulting acute shortage of surface water supply. Pollution has become a great threat for the existence of aquatic lives. Lakes, channels and Khals, which are visible on the images, have been identified by the satellite. The inland water bodies on the aerial photo of 1968 are more prominent than 2001. Analysis and observation for inland water body on 1968 image show that the Gulshan Lake, Dhanmondi Lake and Ramna Lake are highly visible. Some channels and/or Khals are also identified in different parts of the city. Channels are located in the northeastern, eastern, southwestern, southern and north-western corner of the city. The total areas of inland water body are measured 5.1 km². Analysis of satellite image of 2001for inland water body shows that the areas of lakes (Gulshan and Dhanmondi) have shrunken and narrowed down. Some Khals and channels are not identifiable or missing in the southwestern Muhammadpur and southern Motijheel area of the city and the total area is measured 1.8 km² in 2001. In most part of the city open water bodies were acting as a single water body and were well in connection with the surrounding rivers via streams and Khals. It is seen that water bodies have become more sporadic and patchy in 2001 in comparison of 1968 in many parts of the city. Water body compartmentalization, specifically, occurred in the north central, southeast and western part of the city.
The relief controlled landforms of the area were efficiently drained via streams and canals or ‘Khals’ to the floodplain and low-lying area and ultimately to the downstream via large rivers. These canals, wetlands and depressions have been filled up by new urbanization, both in and around the built-up city area. These unplanned Buy Xenical Online Without Prescription urbanizations have been destroying the water-bodies and flow-paths causing rainfall-flooding and drainage congestion in many locations in the city. Filling activities, embankments and roads are being compartmentalized the wetlands and water bodies and hence obstructed the natural drainage. The wetlands and other water bodies in Dhaka city is greatly reduced over the decades due to progressive urbanization. There were number of studies mentioned about the land filling activities and their effects on the drainage congestion and water-logging. The city at the moment, particularly, during the heavy downfall in the wet-season gets water-logged. The scenario would be worsening with time as erratic and intense rainfall events with increasing frequencies due to climate change. Cloud outburst of 28th of July 2009 intense rainfall: 333 mm rainfall over 9 hours and 104 mm over 3 hours on the 4th of August 2004 caused severe flooding which led to a city-wide disruption, and took several days to recede the water.
Moreover, rivers around the city get the liquid waste and also liquid waste from the houses and industries. These liquids and other pollutants have been polluting the river water. In the dry season, this problem reaches at its extreme; one can hardly use this polluted water nor is it recyclable. Additionally, the huge quantity polluted water with city wastes of biological and chemicals origin is polluting the downstream wide-rivers, estuaries and the Bay of Bengal causing endless destruction of biodiversity especially the fresh-, brackish- and marine-water fishes, animals and plants. On the other hand, withdrawal of a great volume of ground water is leading the city towards high risk of earthquake.
In some areas, roads are developed and raised from the adjacent settlements, markets and shopping malls and the residential and marketing areas suffer from water-logging during the wet season, every year. To counter this, residential, marketing or industrial areas are also uplifted to escape from flood water or surface runoff but they seldom consider for wider and smooth running clean drainage system. Thus the water-logging and transportation problems persist year after year and sufferings have been aggravated.So, wider and smooth running drainage system, considering the volume of water to be discharged, should immediately be planned and implemented.
In the recent decades, the city is being expanding to the eastern part on low-lying areas. These urban expansions are mostly occurring on to the wetlands in the eastern part, which used to act as retention basin for urban drained water. It is evident from the record that the floodplains and low-lying areas were not under any sort of urban infrastructure during 1960s and urban growth in the area boosted during the 1990s. However, these wetlands of the city could play a significant role in reducing the pollutants loads if the wetlands had been designed as retention basin for the urban drainage to the rivers. Plants and soils in wetlands play a significant role in purifying water, removing high levels of nitrogen and phosphorus, and in some cases, removing toxic chemicals through biogeochemical cycling and storage. The storm runoff could accumulate in the low-lying areas, flow through Khals and local rivers and ultimately discharge to the major rivers. These lowlands and wetlands would have perform important drainage function by storing storm water and keep the relatively higher lands free from rainfall flooding.
The preservation of existing low-lying wetlands and revival of the pre-urban wetlands must have to included in Detailed Area Plan (DAP) of Dhaka Mega City as per Ramsar Convention which has defined wetlands as “areas of marsh, fen, peat land or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters”. Wetland holds water for a significant duration sufficient to support organism adapted to life in inundated or saturated soil condition and consists of wide variety of types ranging from lakes, rivers and coastal forest to deepwater paddy fields and ponds. The built-up area of the city is traversed and surrounded by wetlands of different types. Preservation and revival of wet lands will restore and rehabilitate of the flora and fauna of the wetlands, the acute water crisis of the city will mitigate, recharge ground water and revival of the wetland ecosystem, the city will be free from water stagnancy, flooding and will facilitate the communication system both waterways and road transportation system. Above all, the wetlands will create employment, recreation, stabilize the ecological balance.
This needs urgent attention for wetland preservation and reviving them for keeping Dhaka city livable.
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