BANGLADESH is the largest delta of the world, created and shaped by the Himalayan orogeny initiated during the Cretaceous Age and the sediments carried and deposited by the numerous river systems. Among the river systems, the Ganges (also known as Padma), the Brahmaputra-Teesta, the Surma-Meghna, and the Karnaphuli are the most notable. These rivers and other numerous small rivers originate from India, China, and Myanmar and, unfortunately, the people of Bangladesh have no control over the river systems. The socio-cultural, economic, and political history of Bangladesh and greater Bengal are actually the history and geomorphology of these rivers. The rivers have been providing the people with food, shelter, transportation, trade, and prosperity by bringing new nutrition-rich sediments and sheltering a myriad of wildlife including fish, birds, aquatic plants, and fruits. The rivers have also cursed the lives of the people of Bangladesh by bringing sudden and catastrophic floods. Floods displaced people, washed away crops, and affected lives in every aspect. Floods also brought new nutrition-rich sediments, and washed away the toxic chemicals accumulated in the land during drier times, which renewed life and society.
The water flow of the Padma at Hardinge Bridge is 2,000,000 cubic feet per second (cfs). The flow of the Brahmaputra at Sirajganj is 2,500,000cfs while flow of the Meghna at Bhairab Bazaar is 420,000cfs. A combined flow of 4,900,000 cubic feet per second of water flows to the Bay of Bengal via these river systems. These rivers carry approximately 1,200,000,000 tonnes of sediments via the delta of Bangladesh. These flow of water and sediments shaped the physiographic, geomorphology of the country and as such influence the societal development and the economic activities. The landscape developed into the largest delta in the world with almost flat slope with a gradient of one centimetre per kilometre. A slight variation is discernable near the bank of the rivers due to the formation of natural levee and the land called Barind Track. The combined flow of Ganges (also known as Padma) and the Brahmaputra at Goalando Ghat is 4,200,000cfs and flow to the south-easterly direction towards Chandpur to cause widespread scour and erosion to the land on the eastern bank of the river. The flow of the Meghna upstream of Chandpur is 720,000cfs and flows south towards the Bay of Bengal. These two flow vectors meet at Chandpur in an angular way and the flow of Meghna diffuse the erosive force of the flow of the Padma to reduce the scour and river shifting. From the diagram presented in Figure 2, it is obvious that south-eastern flow of the Padma is impeded by the landmass east of Chandpur to continue flow towards Comilla-Noakhali. In addition, the flow momentum of the Meghna deflects the trajectory and forces the combined flow to go to the south. Due to their out of synch peak flow, some erosion and significant scour occur at Chandpur. This angular trajectory of the flow momentum has created a quasi-equilibrium condition. Any imbalance in this quasi-equilibrium will cause significant scour to the town of Chandpur and the river course may change its direction.
The rivers and the overall climate of the area have created a society which is bonded together like a family. They are used to farming together, fishing at the same time in same areas, harvesting the crops as a unit mass during harvesting time. Activities such as unique societal bond are as strong as the ionic bonds of metals and chemical compounds and the strong bonds among the people are a gift from the floods of the rivers. This unity swivels the political and morale flow of the country to the right direction like competent navigators pilot his vehicles during endless storms.
The perenial flow of the rivers together with the sediment provided the land with not only with water but also energy in the form of temperature and speed of water and sediments. The nutrients including phosporous, nitrates, microbes, fungus are generally gathered in the sediments, espcially in the finer sediments, are coated due to their electrical charge deficiency created by isomorphous substitution during the heavy and turbulent flow in the upstream. These micronutrients and trace metals are released by the decomposing leaf litters, detritus and dead animals including animal wastes in the upper watersheds of the rivers. During the receding floods and as the flow enters the flat slopes, the velocity of the water decrease to result in these nutrient coated sediments settle in the flood plain providing naturally rich soils for crops to thrive. This particular phenomenon plays significant part in the rivers of Bangladesh, especially the Meghna, as the Barak river flow through dense forests of the Tipara and Naga Hills. The kinematic thrust of the river flow also keep the intrusion of the sea water from getting inland and the groundwater is recharged continuousely.
The lean period, or low flow, provides a higher temperature to accelerate growth of algae, phytoplankton, periphytons and other microbial organisms that provide the basis of nutrients for fish, mammals, and plants. The bank full discharge is the most efficient flow within the river channel which flushes and cleanses the river bed to provide shelter for fish and lay their egg to hatch. The overflow discharge or flood discharge provide nutrient rich sediment to the flood plain. Periodic large flood replenish the entire river basin. Therefore, for the river basin to act as beneficial to society, all these flow events including pulsating floods are essential.
As the population increases in this part of the world, including within the watersheds of our rivers and in our country, stress on the rivers, their productive nature, and their mending power also are affected negatively. This gives more frequent floods, environmental degradation, and ecological downtrends. Unregulated, greedy, and profiteering industries have been discharging pollutants to the rivers starting from the sources, either knowingly or unknowingly. As such, the water quality, and aesthetics of the rivers are plunging downward while sediment toxicity is rising. The massive use of unplanned agro-chemicals, discharge from municipal solid wastes, human and animal remains, and erratic construction within active river channels (including unplanned dredging) have taken and are taking their toll. In this desperate situation a healthy year-round flow of rivers, including flood flow, has been playing a significant role in diluting and re-mediating the contaminants and flushing them out of the country to the Bay of Bengal. Any upstream control that will affect this sensitive situation of the flow of the rivers will have a tendency to affect the society’s culture, economics, emotions, and ecological aspects.
Background of Surma trough/haor basin
BENGAL Basin started to devolve during the late Jurassic to early Cretaceous period. The basin is a half graben Gondwana and moved to its present position after splitting from the Australia-Antarctica mass. As the Indian shield moved to its present position, it started colliding with the Euro-Asian Plate to create the Himalayas. It also interfered with the Burmese plate in the east. While in the north the collision gave rise to the Himalayas by thrust, in the eastern side, the Indian plate started subducting beneath the Burmese Plate. The thrust of Euro-Asian Plate created numerous thrust block faults and the east-west oriented mountain system in India and Tibetan Plateau. In the eastern portion, it created series of folded mountain system that are oriented north-south. The two complex movements together with the presence of Shilling Massif that started its ease-west orientation from northeast Naga Hills to continue to west had also created a series of complex faults, oriented mainly east-west in the north and north-south in the eastern portion. Arakan Youma, Desang and Dauki Faults in the transitional location of Burmese and Indian Plate are a few but very active faults that have produced catastrophic earthquakes in the past. Yarlong or Indus-Tsangpo Suture, Himalayan Main Thrust are a few but very active and dangerous east-west fault systems including Jianji Fault near the Namchi-Barwa area in eastern Himalaya. They are very active and determining their causative earthquake is also complex. In addition, no credible studies including collection of data on fault movement, or seismicity data are known. The subduction also created a fore-bay type depression in the north-eastern Bangladesh known as Surma trough which came to its present forms by receiving sediments from the Barak and other smaller rivers. As this land is locked in its present position due to the movement of the plate tectonics, the graben started filling with sediments shed from these mountain systems by the fluvial process.
One of the most important rivers that played and is playing the most significant economic, social and cultural shaping of the country is the Meghna. The Meghna is created by the joining of the Surma and the Kushiyara in greater Sylhet. These two rivers are the result of the bifurcation of the Barak at the Bangladesh-India border near a place called Amalshid. The Barak originated from the Lusai Hills of eastern Tripura and Manipur states of India and passes through the folded hills and densely-vegetated, long, narrow valleys within this area. The Barak receives other smaller rivers that originated from the Naga Hills and the hills of Assam. The upper watershed of the Barak receives more than 2,000 millimetres of rainfall in an average year with some areas receiving more than 6,000mm a year. The landscape consisted of dense tropical rainforest and scattered villages or communities of the original inhabitants. However, since World War II and since the British left the subcontinent, encroachment by the people of plain land have been migrating and settling in this area resulting in isolated urban sprawls and denudation of the forest to destroy the existing climax community to establish agricultural lands. This has created another unwanted concern that the sediment yields from this portion of the catchment area increased with increased runoff. However, as the leaf litters and the dead trees release essential nutrients to the runoff water and the forest floor, the destruction of the forests decreased the nutrient supply to the sediments to a minimum level only to generate sterile sediments. From some areas pollutants are also discharged from point and non-point sources such as agricultural practices, raw sewage, industries, manufacturing plants and cremations into the waters of the rivers.
The quality of water within the Barak-Meghna used to be one of the cleanest in the world. Natural cleaning of water would occur due to the presence of the Surma trough created by the plate tectonic of the area. The Surma trough, a fore-bay depression of the subduction zone of the Indian Plate into the Burmese Plate, houses a myriad of small and medium sized natural lakes locally known as haors. These haors allow varieties of phreatic plants, floating vegetation, periphytons, algal mats, fungi, bacteria and other micro-organism to grow which ultimately clean the river water. The haors also provide habitat for more than 53 resident water fowls with 160 species of migratory and resident birds, 260 species of fish. In addition, during lean months, as the haors start drying, the dried portions are utilised to grow boro rice. The topography of the lands in and around the haors is such that they have less than 0.001 per cent slope. The slopes are usually towards the deeper portions of haors and the banks of the rivers are slightly higher than the surrounding lands due to the formation of natural levee. This typical topography allows vast areas around the haors to be utilised to grow boro rice during winter season. Due to its almost flat slope, it is easy to irrigate the boro crops continuously by lifting water from the haors or the rivers without using massive lift. In fact, the irrigation used to be done manually by lifting water from the channels of the Surma, the Kushiyara, the Manu and other rivers that cross through these haors.
The entire haor basin and most of the Surma trough is inundated every year during the monsoon and flooding seasons. As such, people have built their homes in artificially filled higher ground along the natural levees and natural higher ground known as kandas. The haor system also united the people to a cohesive cultural society to utilise the resources. They face the natural calamities such as high floods, erosion of their villages as a one unit and do farming as a unit giving rise to especial social order which is very unique for this area.
According to the Bangladesh Bureau of Statistics, in 2008-2009, approximately 17 million tonnes of boro rice was grown in Bangladesh. Out of this 30 per cent of the boro rice was grown in Surma trough and along the floodplain of the Meghna which equates to about 5 million tonnes. A loss of 25 per cent of the boro crops due to inundation of the haor basins will cause loss of more than 1.25 million tonnes of rice in Bangladesh. In present-day market price of $500 per ton, total direct economic impact on food could be as high as $625 million. With increasing population or more mouths to feed coupled with loss of arable land due to increase in population will devastate the country. It may require a few billion dollars to be allocated to import rice to feed future projected population. In addition, the existing economic condition of the world, floods in Thailand, draught in Africa, and the unyielding mentality of rice exporting countries have created a volatile market of food grain. Anything but price of commodities are stable in international market. Surma Trough also grows winter crop such as mastered, beats, cabbages, and other crops.