Ganges Dependent Area
Ganges Dependent Area (GDA) the area directly influenced by the ganges river in terms of historic linkages, current direct impact and potentially commanded land. The GDA encompasses the whole of the southwest area of the country (with the exception of Bhola Island) plus a strip of land along the left banks of the Ganges and the padma, which relates directly to these two rivers. For water resource planning, Bangladesh has been divided into regions based on hydrological conditions. The GDA is located mainly in the Southwest Region (SWR) and the South Central Region (SCR), which together make up the Southwest Area (SWA), with small parts of the Northwest and Northcentral Regions making up the balance.
The rivers, lands and tides are interdependent. Half of the area lies less than 3m above sea level and the tidal range at the coast is about 3m. The dynamic equilibrium between the saline waters of the sea and the freshwater rivers draining the hinterlands follows the diurnal and fortnightly pattern of the tides, which take up to twelve hours to propagate into the heartland of the region. At the height of the rains, the freshwater boundary lies close to the coast, but as the rains cease, the front advances, penetrating further and further into the region over the dry months, until even the relatively small dry season flows are unable to resist its progress. Behind the front, brackish water ecosystems such as the sundarbans, which are adapted to this regime, develop, while ahead of it, freshwater wetland systems flourish. Throughout the region the people have evolved appropriate strategies to exploit the opportunities offered by each ecosystem.
The Ganges Dependent Area is generally low-lying with gentle slopes with few elevations exceeding 14m above sea level, even at 250 km from the coastline. While the northern part is characterised by a rolling topography which drops at a slope of 1:7500 to about 6m above sea level along a line through jessore to faridpur, the topography south and east of this line becomes much flatter and the number of beels and depressions increases. The tidal limit is approximately coincident with the 6m contour, with average tides at the coast of 2.7m, greater in the west than in the east.
As elsewhere in Bangladesh, the causes of flooding and inundation in the SWA are various and multiple. In 1987, widespread flooding made 24 million people homeless and in 1988, two-thirds of the country was submerged, destroying an estimated US $2 billion worth of property. In each case, the cause was different. The 1987 flood was mainly due to drainage congestion because of heavy rainfall in the upper catchment areas of the main rivers (ie Brahmaputra and Meghna). The 1988 floods were mainly due to over-bank spill from the Ganges, Brahmaputra and Meghna rivers, due to synchronisation or peaking of the high inflows in the same time and which affected about 6,500 sq km of the country, mostly in the South Central Region.
The river system As a result of high local rainfall and the historically eastwards progression of the mouth of the Ganges, the entire river system of GDA has evolved. Only two rivers of significance remain connected to the Ganges-Padma, the gorai-madhumati serving the SWR and the arial khan serving the SCR. Over time, the smaller distributaries have been separated from the Ganges and the fear now is that the Gorai, the one remaining river flowing through the southwest, may soon follow.
Since 1976 the dry season flow of the Ganges has been reduced by half. Consequently, minimum water levels measured at the confluence with the Gorai mouth have fallen some 2m and bed levels have risen by 1.5m, severely constraining dry season flows entering the Gorai and impeding abstraction by the ganges-kobadak irrigation project (G-K Project). The increasingly heavy deposit of sediments threatens to choke the Gorai-Ganges connection and substantial decreases are evident in the Gorai dry-season flows.
With delta accretion and the impact of man-made interventions, the inland rivers of the southwest are continuing to adjust to physical changes. Recent morphological studies indicate that substantial siltation is occurring in the Gorai, Madhumati, kobadak and ichamati rivers, medium siltation in the Pasur-Shibsa system and slow siltation in the Baleshwar. The siltation affects distribution of freshwater flows throughout the river system, reduces drainage capacity, impedes navigation and increases salinity intrusion.
The coastal area includes the delta region of the Ganges-Padma and the estuary of the lower meghna, with its numerous islands. The coastline has changed little along the western part, but has accreted on the seaward island shores to the east. Apart from the Sundarbans, and plantations on new accreted land, the coastal and estuarine areas have been cleared for agriculture, and are heavily populated. To provide flood protection for agriculture, a series of coastal embankments were started in the 1960s and 1970s under the Coastal Embankment Project (CEP). The CEP polders provided the means to enhance crop production through security from flooding, saline intrusion and soil leaching.
After initial successes, intervention in channels and tidal flooding led to problems of sedimentation, some of which have caused blockage of drainage channels and structures. These effects are causing channel silting and reduced navigation, poor drainage and loss of river and khal areas, formerly important habitats forfishand shrimps. Rather than enhancing crop yields, the effect has been to reduce them.
Water resource management Over the past fifty years, the water resources of the Ganges Dependent Area has been the subject of a number of studies, initially of the Ganges corridor and the adjacent land, more recently expanding to cover the whole of the Southwest Area. For example, in 1952 by the FAO for irrigation; and in 1963 by Tippetts, Abbett, McCarthy and Stratton (TAMS), who were contracted to undertake investigations and studies leading to the determination of a preliminary location, design and cost of a barrage on the Ganges in the vicinity of hardinge bridge. The designing phase was completed properly, but no engineering structure was constructed.
The early plans called for largescale irrigation, using water from the Ganges diverted by a barrage. The irrigation needs would have been met by the new supply, but it is unclear whether irrigation return flows would have balanced the effects of abstraction on natural flows into the region. The sudden change in flow regime which accompanied the start of largescale diversion of the Ganges water at farakka barrage in 1975 and the lack of assurance of continued supplies, terminated any plan for a barrage. Other development projects were formulated which were much less sensitive to the environment, including flood protection and drainage projects, some with small irrigation components where freshwater could be made available, and coastal protection projects. The issues all point back to the need to restore the freshwater balance which prevailed before the Farakka diversions first took place.
The first (1955-70) and second (1970-83) phases of G-K Project were developed on the right bank of the Ganges to provide supplementary irrigation to a net irrigable area of 125,000 ha. A large part of the Southwest area is subject to tidal movement which can penetrate as far inland as 180 km in the dry season. The Coastal Embankment Project (CEP) of 1960 was an immense undertaking designed to prevent tidal flooding of vast areas of low-lying land and increase agricultural yields. This involved a polder area of 860,000 ha with 3,700 km of new and reconstructed embankments. Following construction of the CEP, the volume of tidal over-bank spill was greatly reduced, with the result that the tidal ebb discharge could not sustain the original channel cross-sections, which have been silted up.
Polder construction and the associated siltation of the rivers has led to a congestion of drainage in places, particularly around satkhirato the west of khulna and to the north of bagerhat. Tidal effects propagate further inland as a result of the polder construction and this promotes further sedimentation. The diminution in both dry and wet season flows from the Ganges over recent years has reduced the capacity of the river to hold back silting.
Over the course of the last 50 years, water resource managers have attempted to develop the land and water resources of the Ganges delta through irrigation, drainage, flood control and flood proofing. Structural measures such as large and small-scale pumped irrigation, flood control with river embankments, polders, sluices, and cyclone shelters, and drainage improvements with new drains plus dredging of khals and rivers have all been tried. Non-structural measures have also been introduced, with policies to encourage small-scale irrigation using treadle pumps and small diesel or electric pumps, flood warning and cyclone warning systems. As a matter of fact, it is necessary to construct embankments which protect life and property from catastrophic floods, but not those which prevent lands from inundation by the normal annual floods.
Transportation Road and rail network in the southwest favours north-south movement in corridors parallel to the major rivers such as the Gorai. In addition, with decreasing dry season flows, ferry crossings which maintain East-West links are becoming increasingly difficult. The consideration now being given to the Asian Highway by the Asian Land Infrastructure Development (ALTID) project will link Dhaka with Calcutta through the southwest, and eastern part of Nepal with Dhaka or Mongla.
With the decline in the potential of river navigation, a shift to road and rail transport is required. However, construction of new roads to areas currently served by navigation would require considerable capital expenditure and result in loss of land available for agriculture. Navigation along the Gorai river is possible with improved inflows from the Ganges, but these flows would have to be substantial to be effective. Mongla Port is expected to benefit from the growth in traffic originating in eastern Nepal and northern West Bengal through Bangladesh. The government of Nepal has expressed interest in using the port of Mongla and in constructing a navigable waterway northwest to the Ganges to link up with Mongla. The revitalisation of Mongla will be key to the re-energising of the economy in the southwest. At present, Mongla is seen as a port plagued by siltation, but if this problem can be solved, the productivity and the economy of the entire region will be benefited.
Irrigation Minor irrigation has grown remarkably in recent years. This growth is set to continue over the next 10-15 years primarily in response to the growing market for boro. The net irrigated area of GDA under major irrigation has grown from 0.5 million ha in 1973 to 3.37 million ha in 1996. It is now over 90% of the national irrigated area and 42% of the net cultivated area.
Shallow tubewells remain an affordable and profitable choice where possible. According to NWP, the overall potential for groundwater development in SWR and SCR was sufficient to irrigate the boro crop on 336,500 ha. The 1996-97 census, however, reveals that some 501,000 ha were irrigated by groundwater. But the rapid expansion of groundwater irrigation lowered the water table (typically by 12 cm/year at the time of lowest levels) and progressively raised the cost of water. It also creates a need to replace hand-suction tubewells for village water supplies by more expensive Tara pumps.
After its initial success, the growth of Low Lift Pump (LLP) irrigation has failed to keep pace with groundwater irrigation, despite its lower cost. With LLP irrigation the land has to be adjacent to a reliable water source whereas groundwater can usually be abstracted anywhere. Largescale irrigation projects such as the Ganges-Kobadak and Barisal Irrigation Project (BIP) have achieved only modest results for rather different reasons.
As a result of low flows in the Ganges in recent years, large sandbars have been formed across the Gorai mouth cutting off the river during the dry season. It is highly likely that this process will continue until the Gorai becomes separated from the Ganges in the wet season as well, with catastrophic consequences. To restore the dry season flows of the Gorai and to derive some benefit from the ganges water sharing Treaty at the earliest opportunity, the government has taken up the Gorai Restoration Project.
Farmers in the Southwest have steadily replaced local varieties with HYVs for all three rice crops. The proportion under aman has risen from 8% to 38%, under aus from 12% to 21%, and under boro from 58% to 90%. The suitability of areas, of necessity and flood-free land limit increases its adoption. There is also a strong consumer preference for the traditional varieties, considered much tastier, which may well sustain the market for these varieties.
Fishery At all stages in the cycle but particularly during the monsoon, riverine fish populations have traditionally been exploited by the rural community through capture fisheries. Indigenous fish species are adapted to the seasonal river flows and freshwater balance and naturally divided into two groups those which spawn in the rivers (eg, the major carps such as Rui, Catla and Mrigal) whose fries enter the floodplains to feed, and those which spawn in the water bodies in the floodplains.
With the lowering of water levels during the dry season and increase in salinisation, natural fish populations have declined, some species have been forced far upstream and fish populations have disappeared from large stretches of rivers and beels. Coupled with the impacts of flood control schemes and a lack of systematic management, capture fisheries have experienced almost total collapse, with dire consequences for fishermen and the poor.
Ponds for fish culture are also influenced by dry season salinity. The viability of ponds is dependent on their elevation in relation to groundwater and soil type, and whether they can be supplied with additional water. Salinity in the rivers prevents this and is damaging to ponds in contact with saline groundwater. The lowering of groundwater levels by excessive pumping has also had adverse effects on the culture ponds which are hydraulically linked to the surface aquifer. As groundwater levels are reduced, infiltration through the bed of the ponds increases, and the ponds drain faster than they can be replenished.
The shrimp industry relies mainly on the Penaeid shrimp and has grown into a nationally important resource, contributing $330 million to export earnings. In 1996/1997, shrimp exports were 28,000 tons and the target for 2002 is 70,000 tons. The industry relies on the tidal waters of the Sundarbans and surrounding areas for the supply of shrimp larvae which are netted and then transferred to inland brackish water lagoons and ponds.
Environment Reduced flows via the Ganges during the dry season have increased tidal incursion, allowing saline water to penetrate and threatening the ecological balance of the Southwest. Flushing with fresh water during the dry season can arrest the northwards movement of salinity. For the purpose the flask water should be distributed across the region via the main rivers such as the Gorai and Arial Khan.
The halt to saline incursion during the dry season can be achieved by restoring the pre- 1974 flow in the Gorai. This will reduce damage to aquatic habitats, fish populations, human health and agriculture and help restore a more natural salinity regime throughout the Sundarbans, ameliorating the habitat of both the sundari mangrove forests and the wildlife species that live there. Restored flows will aid in safeguarding water quality of the rivers through dilution of pollutants, facilitating access to potable, agricultural and industrial supplies and stimulation the re-growth of former fish populations.
Water supply despite the strides achieved in providing tubewells for drinking water, the future of drinking water supplies in the Southwest is now in jeopardy. The threat of arsenic contamination, lowering of the groundwater table and in some areas, salinisation of groundwater, has created a need to scrutinise options for unproven water supply. Until environmental conditions are restored, options for safe drinking water in the Ganges Dependent Area will have to consider a range of alternative sources, both for current generations as well as the projected population of 40 to 45 million in 2017.
Ganges Dependent Area or GDA is playing a vital role in respect to the physiography, hydrology, morphology, economy, natural resources and ecology of the whole country. This area needs more attention for proper development planning, so that it can continue its contribution to national development. [Masud Hasan Chowdhury]