Riverbank Erosion
Riverbank Erosion an endemic and recurrent natural hazard in Bangladesh. When rivers enter the mature stage (as in the case with the three mighty rivers, ganges, brahmaputra and meghna) they become sluggish and meander or braid. These oscillations cause massive riverbank erosion. Every year, millions of people are affected by erosion that destroys standing crops, farmland and homestead land. It is estimated that about 5% of the total floodplain of Bangladesh is directly affected by erosion. Some researchers have reported that bank erosion is taking place in about 94 out of 489 upazilas of the country. A few other researchers have identified 56 upazilas with incidence of erosion. At present, bank erosion and flood hazards in nearly 100 upazilas have become almost a regular feature. Of these, 35 are severely affected.
Some rivers cause erosion in large scale and high frequency due to their unstable character. These rivers assume a braided pattern consisting of several channels separated by small islands in their courses. During the last 200 years or so, the channels have been swinging between the main valley walls. During the monsoon, extensive overbank spills, bank erosion and bankline shifts are typical. The gradual migration or shifting of channels of the major rivers in Bangladesh amount to anywhere between 60m to 1,600m annually. In a typical year, about 2,400 km of the bank line experiences major erosion. The unpredictable shifting behaviour of the rivers and their encroachments not only affect the rural floodplain population but also urban growth centres and infrastructures.
No systemic pattern has yet been observed of the erosion hazards because of the involvement of a large number of variables in the process. The intensity of bank erosion varies widely from river to river as it depends on such characteristics as bank material, water level variations, nearbank flow velocities, planform of the river and the supply of water and sediment into the river. For example, loosely packed, recently deposited bank materials, consisting of silt and fine sand, are highly susceptible to erosion. Rapid recession of floods accelerates the rates of bank erosion in such materials.
The Jamuna is a braided river with bank materials that are highly susceptible to erosion. Since the Brahmaputra switched to the course of the jamuna at the western side of the madhupur tract, the average width of the river has fluctuated substantially. The recorded minimum average width of the Jamuna was 5.6 km in 1914. Locally, the maximum width has often exceeded 15 km, while the recorded local minimum width was about 1.1km. The rate of widening of the river within the period 1973 to 2000 is 128m/year (68m for the left bank and 60m for the right bank). The annual rate of widening has been as high as 184m during 1984-92, of which 100m occurred along the left and 84m along the right bank (Table 1). In this period, the average width of the river increased from 9.7 to 11.2 km (Table 2). The maximum bank erosion during 1984-92 occurred at the left bank, just upstream of Aricha. Both rotation and extension bank erosion mechanisms do occur.
The Jamuna has persistently widened from 1973 to the early nineties, but the yearly rate seems to have gone down significantly in the late nineties. The widening of the river in a 28-year period resulted in a loss of floodplain of 70,000 ha over the total 220 km length of the river in Bangladesh (average about 2,600 ha/year). Within the 1984-92 period, the river has eroded 40,150 ha of floodplain and accreted 7,140 ha, corresponding to an erosion rate of about 5,000 ha/year, and an accretion rate of about 900 ha/year.
In recent years, human interventions in the Jamuna are growing. Construction of the Bangabandhu Jamuna Bridge, and bank protection structures at Sirajganj, Sariakandi and Bahadurabad will doubtless have some influence on the changes of the width of the river. These types of structures are reducing the freedom of the river to widen through bank erosion.
The Ganges the bank material within the active corridor of the Ganges consists of loosely packed sand and silt. These materials are highly susceptible to erosion. The bank erosion process along the Ganges is controlled mainly by its wandering planform characteristics. In the braiding reaches, the river can erode along both banks, as can be seen in the reaches downstream of the hardinge bridge. Maximum bank erosion, however, occurs in the meandering reaches, where the outer bend can still migrate laterally within the corridor. In the period 1984-93, the maximum observed rate was 665 m/year. Along the right and left bank of the Ganges, erosion rates are 56m and 20m per year respectively, which is lower than the rates observed in the Jamuna. The width of the river varied from 1.7 to 10 km in 1984 and from 1.9 to 11.7 km in 1993. The average width of the river in 1984 was 4.37km, which increased to 4.69 km in 1993. The widening rate of the river is 36m/year, which is about one-fifth of the Jamuna. The widening of the Ganges is not considered significant.
The Padma bank erosion in the padma is governed by the planform characteristics of a wandering river. The braided reach of the river is eroding along both banks, while the meandering reaches erode only the outer banks. The width of the river varied from 3.7 to 8.5 km during 1984 and from 2.7 to 10.7 km during 1993. The narrowest sections of 3.7 and 2.7km remain at Baruria, downstream of the confluence with the Jamuna for the period 1984-93. The average width of the river varied from 5.7 km in 1984 to 7.1 km in 1993, corresponding to a widening rate of 159 m/year; left and right bank erosion rates were 121 m and 38m per year respectably. Though less than what was observed in the case of the Jamuna, the widening of the Padma during the period 1984-93 was quite significant.
The part of the Padma between its confluence with the Jamuna and the Meghna has been undergoing significant physical changes in recent decades. The Padma has widened between 1973 and 1998 by the order of several kilometres. Banklines of the river are very unstable.
The Padma, traditionally considered a dominantly meandering system, is switching over into a braided system. The increased rate of bar development at the Padma may be due to increased suspended load, particularly the increase of sand fractions. This, in turn, may have accelerated the bank erosion by directing more and more of the finer flow towards the banks. Between Ramakrishnapur and Dohar the Padma is fairly wide with major erosion going on along the left bank.
The right bank of this reach near Faridpur moved to and fro while the left bank near Horirumpur showed a progressive retreat towards the northeast between 1973 and 1998. The left bank has moved several kilometres northeast near Horirumpur, where an acute erosion problem exists. This is also the point of the ichamati off-take, which is a small northward distributary of the river.
The part of the Padma between the downstream of Dohar and Mawa has the similar width as above but differs in terms of location of major bank erosion, which is taking place along its right bank. Due to the formation of a large bar near the mouth of the arial khan river, the main flow of this reach is diverted along the right bank and strikes along Mathbarer Char, which shows severe erosion. Between Mawa and Louhojong, the Padma is very narrow. Here, the absence of any major bar close to the bank allows relatively free flow of the water along the channel and does not cause any flow diversion or concentration towards the bank. Between the downstream of Louhojong and Munshiganj the river is wide and has many in-channel bars. This part has the distinction of including most parts of the confluence of the Padma and Meghna rivers. Some of the largest stable bars are located here. Therefore, the intensity of bank erosion is controlled by the development of mid-channel bars and by the diversion of the main channel towards the riverbanks.
Physical characteristics of the Padma and its distributaries are changing due to oscillation of the riverbanks, levee breaching and formation of mid-channel bars. River depths became shallower at places. Many of the offtakes were closed due to depositions of huge sediments at their mouths. Unstable riverbanks are a threat to the development of infrastructures, like the proposed Padma bridge, which are essential for the development of the entire southwestern region of the country.
The Upper Meghna unlike the other main rivers of Bangladesh, the Upper Meghna is a stable and seemingly inert river. The width of the river varied from 1 km to 11.5 km in 1984, while the range of variation was 1 to 11.3 km in 1993. The high spatial variation of width is due to the presence of a permanent char, with a width of more than 9 km. The average width of the river in 1984 was 3.41 km while in 1993 it was 3.39 km. Average bank erosion rates along the right and left banks of the river were found to be 9m and 7m per year respectively.
The Lower Meghna after the Padma broke through the Chandina alluvium and joined the Meghna at the present confluence about 150 years ago, the Lower Meghna had to adjust to carry the combined discharge of the Jamuna, Ganges and Meghna rivers. The confluence of the Upper Meghna and the Padma can be considered as part of the Lower Meghna. The confluence consists of a wide cluster of chars bounded by two channels. The highest width of the confluence is 14.4 km. Its average width varied from 7.98 in 1984 to 9.01 km in l993, reflecting a widening rate of about 100 m/year.
In 1984, the river downstream of Chandpur consisted of a single thread channel with a large meandering wavelength. But in 1993, the planform of the river changed to a multi-channeled braided river. The width of the Lower Meghna downstream of Chandpur varied spatially from 3.82 km to 7.87 km in 1984 and from 5.03 km to 13.00 km in 1993. The average width of the river in the downstream reach was 5.74 km in 1984 and 8.82 km in 1993.
The river has eroded both its banks, causing formation of medial bars. Downstream of Chandpur, the westward bank erosion during the period 1984-1993 was severe, with the rate of erosion at times getting as high as 824 m/year. During the same period, the average bank erosion along the right and left banks were 182 and 66 m/year, respectively, the sum of which (248 m/year) is higher than the widening rate of the Jamuna during the same period (184 m/year).
Socio-economic impact riverbank erosion has disastrous socio-economic effects. The majority of the affected people perceive riverbank erosion as a natural phenomenon but in many cases the people believe erosion to be the 'will of God'. However, these days, riverbank erosion is seen as one of the major causes for national poverty. The degree of economic loss and vulnerability of population due to bank erosion has dramatically increased in recent years. The impact of land loss involves primarily the loss of homestead land, housing structures, crops, cattle, trees and household utensils. Loss of homesteads forces people to move to new places without any option and puts them in disastrous situations. About one million people are directly affected each year by bank erosion in the country. The total monetary loss is estimated to be approximately $500 million a year. An estimated 300,000 displaced persons usually take shelter on roads, embankments and government-requisitioned lands. Bank erosion affects people, irrespective of farm sizes. Riverbank erosion causes setback for village agriculture. Along with homestead settlements, it erodes farmland, infrastructure and the communication system. It affects the crop income of vulnerable groups. The big farmers are the worst affected, followed by medium farmers, and marginal groups. The affected people lose their assets and are forced to draw on savings and often fall into further debt. Researchers found that the land lost is much more than the land that rises out of riverbed through accretion. This erosion-accretion phenomenon is a characteristic feature of the courses of the rivers in Bangladesh and gives rise to a lot of tension in local politics.
Displacement is the immediate impact of riverbank erosion. The displaced usually move to nearby areas but migration to distant places are not uncommon. In erosion-prone areas, most families have witnessed a displacement in their lifetime. This involuntary movement can go up to 10 times or even more. A survey conducted in two Dhaka slums has revealed that they consist of migrants who mostly originated from the districts of Faridpur (34%), Barisal (25.6%), Comilla (24.3%) and Dhaka (14.3%). A closer examination of this distribution further revealed that most of the migrants came from an area consisting of only a few upazilas mostly located around the Ganges-Padma and the Meghna and their combined estuaries. The displacement caused by erosion, mostly involve displacement of whole families. On an average, a household experienced riverbank erosion 2.33 times in the life of its members. Some of them experienced displacement 4-5 times or more. Most of the environment-induced refugees turn mainly into labourers or rickshaw pullers. A large proportion of the victims remain unemployed due to lack of work oppurtunities. Moreover, women head many of these families. The female-headed households displaced by riverbank erosion and residing on embankments are the most deprived group. Fortunately, nowadays, social workers are focusing on these problems and also suggesting strategies of survival to these people. [Sifatul Quader Chowdhury]
See map in flood.
Bibliography M Aminul Islam, Environment Land Use and Natural Hazards in Bangladesh, University of Dhaka, 1995; Muhammad Zahir Mamun and ATM Nurul Amin, Densification, The University Press Limited, Dhaka, 1999; Environment and GIS Support Project for Water Sector Planning (EGIS), Riverine Chars in Bangladesh, The University Press Limited, 2000.
